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ip drp authentication key-chain
ip icmp rate-limit unreachable
IP Services Commands
Use the commands in this chapter to configure various IP services. For configuration information and examples on IP services, refer to the "Configuring IP Services" chapter of the Cisco IOS IP Configuration Guide.
access-class
To restrict incoming and outgoing connections between a particular vty (into a Cisco device) and the addresses in an access list, use the access-class command in line configuration mode. To remove access restrictions, use the no form of this command.
access-class access-list-number {in [vrf-also] | out}
no access-class access-list-number {in | out}
Syntax Description
Defaults
No access lists are defined.
Command Modes
Line configuration
Command History
Usage Guidelines
Remember to set identical restrictions on all the virtual terminal lines because a user can connect to any of them.
To display the access lists for a particular terminal line, use the show line EXEC command and specify the line number.
If you do not specify the vrf-also keyword, incoming Telnet connections from interfaces that are part of a VRF are rejected.
Examples
The following example defines an access list that permits only hosts on network 192.89.55.0 to connect to the virtual terminal ports on the router:
access-list 12 permit 192.89.55.0 0.0.0.255line 1 5access-class 12 inThe following example defines an access list that denies connections to networks other than network 36.0.0.0 on terminal lines 1 through 5:
access-list 10 permit 36.0.0.0 0.255.255.255line 1 5access-class 10 outRelated Commands
access-list (IP extended)
To define an extended IP access list, use the extended version of the access-list command in global configuration mode. To remove the access list, use the no form of this command.
access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [log | log-input] [time-range time-range-name] [fragments]
no access-list access-list-number
Internet Control Message Protocol (ICMP)
For ICMP, you can also use the following syntax:
access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} icmp source source-wildcard destination destination-wildcard [icmp-type [icmp-code] | icmp-message] [precedence precedence] [tos tos] [log | log-input] [time-range time-range-name] [fragments]
Internet Group Management Protocol (IGMP)
For IGMP, you can also use the following syntax:
access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} igmp source source-wildcard destination destination-wildcard [igmp-type] [precedence precedence] [tos tos] [log | log-input] [time-range time-range-name] [fragments]
Transmission Control Protocol (TCP)
For TCP, you can also use the following syntax:
access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} tcp source source-wildcard [operator [port]] destination destination-wildcard [operator [port]] [established] [precedence precedence] [tos tos] [log | log-input] [time-range time-range-name] [fragments]
User Datagram Protocol (UDP)
For UDP, you can also use the following syntax:
access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} udp source source-wildcard [operator [port]] destination destination-wildcard [operator [port]] [precedence precedence] [tos tos] [log | log-input] [time-range time-range-name] [fragments]
Syntax Description
access-list-number
Number of an access list. This is a decimal number from 100 to 199 or from 2000 to 2699.
dynamic dynamic-name
(Optional) Identifies this access list as a dynamic access list. Refer to lock-and-key access documented in the "Configuring Lock-and-Key Security (Dynamic Access Lists)" chapter in the Cisco IOS Security Configuration Guide.
timeout minutes
(Optional) Specifies the absolute length of time, in minutes, that a temporary access list entry can remain in a dynamic access list. The default is an infinite length of time and allows an entry to remain permanently. Refer to lock-and-key access documented in the "Configuring Lock-and-Key Security (Dynamic Access Lists)" chapter in the Cisco IOS Security Configuration Guide.
deny
Denies access if the conditions are matched.
permit
Permits access if the conditions are matched.
protocol
Name or number of an Internet protocol. It can be one of the keywords eigrp, gre, icmp, igmp, igrp, ip, ipinip, nos, ospf, pim, tcp, or udp, or an integer in the range from 0 to 255 representing an Internet protocol number. To match any Internet protocol (including ICMP, TCP, and UDP) use the ip keyword. Some protocols allow further qualifiers described below.
source
Number of the network or host from which the packet is being sent. There are three alternative ways to specify the source:
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Use a 32-bit quantity in four-part, dotted-decimal format.
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source-wildcard
Wildcard bits to be applied to source. Each wildcard bit 0 indicates the corresponding bit position in the source. Each wildcard bit set to 1 indicates that both a 0 bit and a 1 bit in the corresponding position of the IP address of the packet will be considered a match to this access list entry.
There are three alternative ways to specify the source wildcard:
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Wildcard bits set to 1 need not be contiguous in the source wildcard. For example, a source wildcard of 0.255.0.64 would be valid.
destination
Number of the network or host to which the packet is being sent. There are three alternative ways to specify the destination:
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destination-wildcard
Wildcard bits to be applied to the destination. There are three alternative ways to specify the destination wildcard:
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precedence precedence
(Optional) Packets can be filtered by precedence level, as specified by a number from 0 to 7, or by name as listed in the section "Usage Guidelines."
tos tos
(Optional) Packets can be filtered by type of service level, as specified by a number from 0 to 15, or by name as listed in the section "Usage Guidelines."
log
(Optional) Causes an informational logging message about the packet that matches the entry to be sent to the console. (The level of messages logged to the console is controlled by the logging console command.)
The message includes the access list number, whether the packet was permitted or denied; the protocol, whether it was TCP, UDP, ICMP, or a number; and, if appropriate, the source and destination addresses and source and destination port numbers. By default, the message is generated for the first packet that matches, and then at 5-minute intervals, including the number of packets permitted or denied in the prior 5-minute interval.
Use the ip access-list log-update command to generate logging messages when the number of matches reaches a configurable threshold (rather than waiting for a 5-minute interval). See the ip access-list log-update command for more information.
The logging facility might drop some logging message packets if there are too many to be handled or if there is more than one logging message to be handled in 1 second. This behavior prevents the router from crashing due to too many logging packets. Therefore, the logging facility should not be used as a billing tool or an accurate source of the number of matches to an access list.
If you enable CEF and then create an access list that uses the log keyword, the packets that match the access list are not CEF switched. They are fast switched. Logging disables CEF.
log-input
(Optional) Includes the input interface and source MAC address or VC in the logging output.
time-range time-range-name
(Optional) Name of the time range that applies to this statement. The name of the time range and its restrictions are specified by the time-range command.
icmp-type
(Optional) ICMP packets can be filtered by ICMP message type. The type is a number from 0 to 255.
icmp-code
(Optional) ICMP packets that are filtered by ICMP message type can also be filtered by the ICMP message code. The code is a number from 0 to 255.
icmp-message
(Optional) ICMP packets can be filtered by an ICMP message type name or ICMP message type and code name. The possible names are listed in the section "Usage Guidelines."
igmp-type
(Optional) IGMP packets can be filtered by IGMP message type or message name. A message type is a number from 0 to 15. IGMP message names are listed in the section "Usage Guidelines."
operator
(Optional) Compares source or destination ports. Possible operands include lt (less than), gt (greater than), eq (equal), neq (not equal), and range (inclusive range).
If the operator is positioned after the source and source-wildcard, it must match the source port.
If the operator is positioned after the destination and destination-wildcard, it must match the destination port.
The range operator requires two port numbers. All other operators require one port number.
port
(Optional) The decimal number or name of a TCP or UDP port. A port number is a number from 0 to 65535. TCP and UDP port names are listed in the section "Usage Guidelines." TCP port names can only be used when filtering TCP. UDP port names can only be used when filtering UDP.
TCP port names can only be used when filtering TCP. UDP port names can only be used when filtering UDP.
established
(Optional) For the TCP protocol only: Indicates an established connection. A match occurs if the TCP datagram has the ACK or RST control bits set. The nonmatching case is that of the initial TCP datagram to form a connection.
fragments
(Optional) The access list entry applies to noninitial fragments of packets; the fragment is either permitted or denied accordingly. For more details about the fragments keyword, see the "Access List Processing of Fragments" and "Fragments and Policy Routing" sections in the "Usage Guidelines" section.
Defaults
An extended access list defaults to a list that denies everything. An extended access list is terminated by an implicit deny statement.
Command Modes
Global configuration
Command History
Usage Guidelines
You can use access lists to control the transmission of packets on an interface, control vty access, and restrict the contents of routing updates. The Cisco IOS software stops checking the extended access list after a match occurs.
Fragmented IP packets, other than the initial fragment, are immediately accepted by any extended IP access list. Extended access lists used to control vty access or restrict the contents of routing updates must not match against the TCP source port, the type of service (ToS) value, or the precedence of the packet.
Note
The following is a list of precedence names:
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The following is a list of ToS names:
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The following is a list of ICMP message type names and ICMP message type and code names:
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The following is a list of IGMP message names:
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The following is a list of TCP port names that can be used instead of port numbers. Refer to the current assigned numbers RFC to find a reference to these protocols. Port numbers corresponding to these protocols can also be found if you type a ? in the place of a port number.
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The following is a list of UDP port names that can be used instead of port numbers. Refer to the current assigned numbers RFC to find a reference to these protocols. Port numbers corresponding to these protocols can also be found if you type a ? in the place of a port number.
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Access List Processing of Fragments
The behavior of access-list entries regarding the use or lack of the fragments keyword can be summarized as follows:
Be aware that you should not simply add the fragments keyword to every access list entry because the first fragment of the IP packet is considered a nonfragment and is treated independently of the subsequent fragments. An initial fragment will not match an access list permit or deny entry that contains the fragments keyword, the packet is compared to the next access list entry, and so on, until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword, and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases where there are multiple deny access list entries for the same host but with different Layer 4 ports, a single deny access-list entry with the fragments keyword for that host is all that needs to be added. Thus all the fragments of a packet are handled in the same manner by the access list.
Packet fragments of IP datagrams are considered individual packets and each counts individually as a packet in access list accounting and access list violation counts.
Note
Fragments and Policy Routing
Fragmentation and the fragment control feature affect policy routing if the policy routing is based on the match ip address command and the access list had entries that match on Layer 4 through 7 information. It is possible that noninitial fragments pass the access list and are policy routed, even if the first fragment was not policy routed or the reverse.
By using the fragments keyword in access list entries as described earlier, a better match between the action taken for initial and noninitial fragments can be made and it is more likely policy routing will occur as intended.
Examples
In the following example, serial interface 0 is part of a Class B network with the address 128.88.0.0, and the address of the mail host is 128.88.1.2. The established keyword is used only for the TCP protocol to indicate an established connection. A match occurs if the TCP datagram has the ACK or RST bits set, which indicates that the packet belongs to an existing connection.
access-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.0.0 0.0.255.255 establishedaccess-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.1.2 0.0.0.0 eq 25interface serial 0ip access-group 102 inThe following example permits Domain Naming System (DNS) packets and ICMP echo and echo reply packets:
access-list 102 permit tcp any 128.88.0.0 0.0.255.255 establishedaccess-list 102 permit tcp any host 128.88.1.2 eq smtpaccess-list 102 permit tcp any any eq domainaccess-list 102 permit udp any any eq domainaccess-list 102 permit icmp any any echoaccess-list 102 permit icmp any any echo-replyThe following examples show how wildcard bits are used to indicate the bits of the prefix or mask that are relevant. Wildcard bits are similar to the bitmasks that are used with normal access lists. Prefix or mask bits corresponding to wildcard bits set to 1 are ignored during comparisons and prefix or mask bits corresponding to wildcard bits set to 0 are used in comparison.
The following example permits 192.108.0.0 255.255.0.0 but denies any more specific routes of 192.108.0.0 (including 192.108.0.0 255.255.255.0):
access-list 101 permit ip 192.108.0.0 0.0.0.0 255.255.0.0 0.0.0.0 access-list 101 deny ip 192.108.0.0 0.0.255.255 255.255.0.0 0.0.255.255The following example permits 131.108.0/24 but denies 131.108/16 and all other subnets of 131.108.0.0:
access-list 101 permit ip 131.108.0.0 0.0.0.0 255.255.255.0 0.0.0.0 access-list 101 deny ip 131.108.0.0 0.0.255.255 255.255.0.0 0.0.255.255The following example uses a time range to deny HTTP traffic on Monday through Friday from 8:00 a.m. to 6:00 p.m.:
time-range no-httpperiodic weekdays 8:00 to 18:00!access-list 101 deny tcp any any eq http time-range no-http!interface ethernet 0ip access-group 101 inRelated Commands
access-list (IP standard)
To define a standard IP access list, use the standard version of the access-list command in global configuration mode. To remove a standard access list, use the no form of this command.
access-list access-list-number {deny | permit} source [source-wildcard] [log]
no access-list access-list-number
Caution
Syntax Description
access-list-number
Number of an access list. This is a decimal number from 1 to 99 or from 1300 to 1999.
deny
Denies access if the conditions are matched.
permit
Permits access if the conditions are matched.
source
Number of the network or host from which the packet is being sent. There are two alternative ways to specify the source:
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source-wildcard
(Optional) Wildcard bits to be applied to the source. There are two alternative ways to specify the source wildcard:
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log
(Optional) Causes an informational logging message about the packet that matches the entry to be sent to the console. (The level of messages logged to the console is controlled by the logging console command.)
The message includes the access list number, whether the packet was permitted or denied, the source address, and the number of packets. The message is generated for the first packet that matches, and then at 5-minute intervals, including the number of packets permitted or denied in the prior 5-minute interval.
Use the ip access-list log-update command to generate the logging messages to appear when the number of matches reaches a configurable threshold (rather than waiting for a 5-minute interval). See the ip access-list log-update command for more information.
The logging facility might drop some logging message packets if there are too many to be handled or if there is more than one logging message to be handled in 1 second. This behavior prevents the router from crashing due to too many logging packets. Therefore, the logging facility should not be used as a billing tool or an accurate source of the number of matches to an access list.
If you enable CEF and then create an access list that uses the log keyword, the packets that match the access list are not CEF switched. They are fast switched. Logging disables CEF.
Defaults
The access list defaults to an implicit deny statement for everything. The access list is always terminated by an implicit deny statement for everything.
Command Modes
Global configuration
Command History
Usage Guidelines
Plan your access conditions carefully and be aware of the implicit deny statement at the end of the access list.
You can use access lists to control the transmission of packets on an interface, control vty access, and restrict the contents of routing updates.
Use the show access-lists EXEC command to display the contents of all access lists.
Use the show ip access-list EXEC command to display the contents of one access list.
Examples
The following example of a standard access list allows access for only those hosts on the three specified networks. The wildcard bits apply to the host portions of the network addresses. Any host with a source address that does not match the access list statements will be rejected.
access-list 1 permit 192.5.34.0 0.0.0.255access-list 1 permit 128.88.0.0 0.0.255.255access-list 1 permit 36.0.0.0 0.255.255.255! (Note: all other access implicitly denied)The following example of a standard access list allows access for devices with IP addresses in the range from 10.29.2.64 to 10.29.2.127. All packets with a source address not in this range will be rejected.
access-list 1 permit 10.29.2.64 0.0.0.63! (Note: all other access implicitly denied)To specify a large number of individual addresses more easily, you can omit the wildcard if it is all zeros. Thus, the following two configuration commands are identical in effect:
access-list 2 permit 36.48.0.3access-list 2 permit 36.48.0.3 0.0.0.0Related Commands
access-list compiled
To enable the Turbo Access Control Lists (Turbo ACL) feature, use the access-list compiled command in global configuration mode. To disable the Turbo ACL feature, use the no form of this command.
access-list compiled
no access-list compiled
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Global configuration
Command History
Usage Guidelines
By default, the Turbo ACL feature is disabled. When Turbo ACL is disabled, normal ACL processing is enabled, and no ACL acceleration occurs.
When the Turbo ACL feature is enabled using the access-list compiled command, the ACLs in the configuration are scanned and, if suitable, compiled for Turbo ACL acceleration. This scanning and compilation may take a few seconds when the system is processing large and complex ACLs, or when the system is processing a configuration that contains a large number of ACLs.
Any configuration change to an ACL that is being accelerated, such as the addition of new ACL entries or the deletion of the ACL, triggers a recompilation of that ACL.
When Turbo ACL tables are being built (or rebuilt) for a particular ACL, the normal sequential ACL search is used until the new tables are ready for installation.
Examples
The following example enables the Turbo ACL feature:
access-list compiledaccess-list remark
To write a helpful comment (remark) for an entry in a numbered IP access list, use the access-list remark command in global configuration mode. To remove the remark, use the no form of this command.
access-list access-list-number remark remark
no access-list access-list-number remark remark
Syntax Description
access-list-number
Number of an IP access list.
remark
Comment that describes the access list entry, up to 100 characters long.
Defaults
The access list entries have no remarks.
Command Modes
Global configuration
Command History
Usage Guidelines
The remark can be up to 100 characters long; anything longer is truncated.
If you want to write a comment about an entry in a named access list, use the remark command.
Examples
In the following example, the workstation belonging to Jones is allowed access, and the workstation belonging to Smith is not allowed access:
access-list 1 remark Permit only Jones workstation throughaccess-list 1 permit 171.69.2.88access-list 1 remark Do not allow Smith workstation throughaccess-list 1 deny 171.69.3.13Related Commands
clear access-list counters
To clear the counters of an access list, use the clear access-list counters command in EXEC mode.
clear access-list counters {access-list-number | access-list-name}
Syntax Description
Command Modes
EXEC
Command History
Usage Guidelines
Some access lists keep counters that count the number of packets that pass each line of an access list. The show access-lists command displays the counters as a number of matches. Use the clear access-list counters command to restart the counters for a particular access list to 0.
Examples
The following example clears the counters for access list 101:
Router> clear access-list counters 101Related Commands
show access-lists
Displays the contents of current IP and rate-limit access lists.
clear ip accounting
To clear the active or checkpointed database when IP accounting is enabled, use the clear ip accounting command in EXEC mode.
clear ip accounting [checkpoint]
Syntax Description
Command Modes
EXEC
Command History
Usage Guidelines
You can also clear the checkpointed database by issuing the clear ip accounting command twice in succession.
Examples
The following example clears the active database when IP accounting is enabled:
Router> clear ip accountingRelated Commands
clear ip drp
To clear all statistics being collected on Director Response Protocol (DRP) requests and replies, use the clear ip drp command in EXEC mode.
clear ip drp
Syntax Description
This command has no arguments or keywords.
Command Modes
EXEC
Command History
Examples
The following example clears all DRP statistics:
Router> clear ip drpRelated Commands
Controls the sources of DRP queries to the DRP Server Agent.
Configures authentication on the DRP Server Agent for DistributedDirector.
clear tcp statistics
To clear TCP statistics, use the clear tcp statistics command in privileged EXEC mode.
clear tcp statistics
Syntax Description
This command has no arguments or keywords.
Command Modes
Privileged EXEC
Command History
Examples
The following example clears all TCP statistics:
Router# clear tcp statisticsRelated Commands
deny (IP)
To set conditions for a named IP access list, use the deny command in access-list configuration mode.To remove a deny condition from an access list, use the no form of this command.
deny source [source-wildcard]
no deny source [source-wildcard]
deny protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
no deny protocol source source-wildcard destination destination-wildcard
Internet Control Message Protocol (ICMP)
For ICMP, you can also use the following syntax:
deny icmp source source-wildcard destination destination-wildcard [icmp-type [icmp-code] | icmp-message] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
Internet Group Management Protocol (IGMP)
For IGMP, you can also use the following syntax:
deny igmp source source-wildcard destination destination-wildcard [igmp-type] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
Transmission Control Protocol (TCP)
For TCP, you can also use the following syntax:
deny tcp source source-wildcard [operator port [port]] destination destination-wildcard [operator [port]] [established] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
User Datagram Protocol (UDP)
For UDP, you can also use the following syntax:
deny udp source source-wildcard [operator port [port]] destination destination-wildcard [operator [port]] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
Syntax Description
source
Number of the network or host from which the packet is being sent. There are three alternative ways to specify the source:
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source-wildcard
Wildcard bits to be applied to the source. There are three alternative ways to specify the source wildcard:
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protocol
Name or number of an Internet protocol. It can be one of the keywords eigrp, gre, icmp, igmp, igrp, ip, ipinip, nos, ospf, tcp, or udp, or an integer in the range from 0 to 255 representing an Internet protocol number. To match any Internet protocol (including ICMP, TCP, and UDP), use the ip keyword. Some protocols allow further qualifiers described later.
destination
Number of the network or host to which the packet is being sent. There are three alternative ways to specify the destination:
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destination-wildcard
Wildcard bits to be applied to the destination. There are three alternative ways to specify the destination wildcard:
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precedence precedence
(Optional) Packets can be filtered by precedence level, as specified by a number from 0 to 7 or by name as listed in the section "Usage Guidelines."
tos tos
(Optional) Packets can be filtered by type of service (ToS) level, as specified by a number from 0 to 15, or by name as listed in the section "Usage Guidelines" of the access-list (IP extended) command.
log
(Optional) Causes an informational logging message about the packet that matches the entry to be sent to the console. (The level of messages logged to the console is controlled by the logging console command.)
The message includes the access list number, whether the packet was permitted or denied; the protocol, whether it was TCP, UDP, ICMP, or a number; and, if appropriate, the source and destination addresses and source and destination port numbers. The message is generated for the first packet that matches, and then at 5-minute intervals, including the number of packets permitted or denied in the prior 5-minute interval.
Use the ip access-list log-update command to generate logging messages when the number of matches reaches a configurable threshold (rather than waiting for a 5-minute interval). See the ip access-list log-update command for more information.
The logging facility might drop some logging message packets if there are too many to be handled or if there is more than one logging message to be handled in 1 second. This behavior prevents the router from crashing due to too many logging packets. Therefore, the logging facility should not be used as a billing tool or an accurate source of the number of matches to an access list.
If you enable CEF and then create an access list that uses the log keyword, the packets that match the access list are not CEF switched. They are fast switched. Logging disables CEF.
time-range time-range-name
(Optional) Name of the time range that applies to this deny statement. The name of the time range and its restrictions are specified by the time-range and absolute or periodic commands, respectively.
icmp-type
(Optional) ICMP packets can be filtered by ICMP message type. The type is a number from 0 to 255.
icmp-code
(Optional) ICMP packets that are filtered by ICMP message type can also be filtered by the ICMP message code. The code is a number from 0 to 255.
icmp-message
(Optional) ICMP packets can be filtered by an ICMP message type name or ICMP message type and code name. The possible names are listed in the section "Usage Guidelines" of the access-list (IP extended) command.
igmp-type
(Optional) IGMP packets can be filtered by IGMP message type or message name. A message type is a number from 0 to 15. IGMP message names are listed in the section "Usage Guidelines" of the access-list (IP extended) command.
operator
(Optional) Compares source or destination ports. Possible operands include lt (less than), gt (greater than), eq (equal), neq (not equal), and range (inclusive range).
If the operator is positioned after the source and source-wildcard, it must match the source port.
If the operator is positioned after the destination and destination-wildcard, it must match the destination port.
The range operator requires two port numbers. All other operators require one port number.
port
(Optional) The decimal number or name of a TCP or UDP port. A port number is a number from 0 to 65535. TCP and UDP port names are listed in the section "Usage Guidelines" of the access-list (IP extended) command.
TCP port names can only be used when filtering TCP. UDP port names can only be used when filtering UDP.
established
(Optional) For the TCP protocol only: Indicates an established connection. A match occurs if the TCP datagram has the ACK or RST bits set. The nonmatching case is that of the initial TCP datagram to form a connection.
fragments
(Optional) The access list entry applies to noninitial fragments of packets; the fragment is either permitted or denied accordingly. For more details about the fragments keyword, see the "Access List Processing of Fragments" and "Fragments and Policy Routing" sections in the "Usage Guidelines" section.
Defaults
There is no specific condition under which a packet is denied passing the named access list.
Command Modes
Access-list configuration
Command History
11.2
This command was introduced.
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The fragments keyword was added.
Usage Guidelines
Use this command following the ip access-list command to specify conditions under which a packet cannot pass the named access list.
The time-range option allows you to identify a time range by name. The time-range, absolute, and periodic commands specify when this deny statement is in effect.
Access List Processing of Fragments
The behavior of access-list entries regarding the use or lack of the fragments keyword can be summarized as follows:
Be aware that you should not simply add the fragments keyword to every access list entry because the first fragment of the IP packet is considered a nonfragment and is treated independently of the subsequent fragments. An initial fragment will not match an access list permit or deny entry that contains the fragments keyword, the packet is compared to the next access list entry, and so on, until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword, and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases where there are multiple deny access list entries for the same host but with different Layer 4 ports, a single deny access-list entry with the fragments keyword for that host is all that needs to be added. Thus all the fragments of a packet are handled in the same manner by the access list.
Packet fragments of IP datagrams are considered individual packets and each counts individually as a packet in access list accounting and access list violation counts.
Note
Fragments and Policy Routing
Fragmentation and the fragment control feature affect policy routing if the policy routing is based on the match ip address command and the access list had entries that match on Layer 4 through 7 information. It is possible that noninitial fragments pass the access list and are policy routed, even if the first fragment was not policy routed or the reverse.
By using the fragments keyword in access list entries as described earlier, a better match between the action taken for initial and noninitial fragments can be made and it is more likely policy routing will occur as intended.
Examples
The following example sets a deny condition for a standard access list named Internetfilter:
ip access-list standard Internetfilterdeny 192.5.34.0 0.0.0.255permit 128.88.0.0 0.0.255.255permit 36.0.0.0 0.255.255.255! (Note: all other access implicitly denied)The following example denies HTTP traffic on Monday through Friday from 8:00 a.m. to 6:00 p.m.:
time-range no-httpperiodic weekdays 8:00 to 18:00!ip access-list extended strictdeny tcp any any eq http time-range no-http!interface ethernet 0ip access-group strict inRelated Commands
dynamic
To define a named dynamic IP access list, use the dynamic access-list configuration command. To remove the access lists, use the no form of this command.
dynamic dynamic-name [timeout minutes] {deny | permit} protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [log] [fragments]
no dynamic dynamic-name
Internet Control Message Protocol (ICMP)
For ICMP, you can also use the following syntax:
dynamic dynamic-name [timeout minutes] {deny | permit} icmp source source-wildcard destination destination-wildcard [icmp-type [icmp-code] | icmp-message] [precedence precedence] [tos tos] [log] [fragments]
Internet Group Management Protocol (IGMP)
For IGMP, you can also use the following syntax:
dynamic dynamic-name [timeout minutes] {deny | permit} igmp source source-wildcard destination destination-wildcard [igmp-type] [precedence precedence] [tos tos] [log] [fragments]
Transmission Control Protocol (TCP)
For TCP, you can also use the following syntax:
dynamic dynamic-name [timeout minutes] {deny | permit} tcp source source-wildcard [operator [port]] destination destination-wildcard [operator [port]] [established] [precedence precedence] [tos tos] [log] [fragments]
User Datagram Protocol (UDP)
For UDP, you can also use the following syntax:
dynamic dynamic-name [timeout minutes] {deny | permit} udp source source-wildcard [operator [port]] destination destination-wildcard [operator [port]] [precedence precedence] [tos tos] [log] [fragments]
Caution
Syntax Description
dynamic-name
Identifies this access list as a dynamic access list. Refer to lock-and-key access documented in the "Configuring Lock-and-Key Security (Dynamic Access Lists)" chapter in the Cisco IOS Security Configuration Guide.
timeout minutes
(Optional) Specifies the absolute length of time (in minutes) that a temporary access list entry can remain in a dynamic access list. The default is an infinite length of time and allows an entry to remain permanently. Refer to lock-and-key access documented in the "Configuring Lock-and-Key Security (Dynamic Access Lists)" chapter in the Cisco IOS Security Configuration Guide.
deny
Denies access if the conditions are matched.
permit
Permits access if the conditions are matched.
protocol
Name or number of an Internet protocol. It can be one of the keywords eigrp, gre, icmp, igmp, igrp, ip, ipinip, nos, ospf, tcp, or udp, or an integer in the range from 0 to 255 representing an Internet protocol number. To match any Internet protocol (including ICMP, TCP, and UDP), use the ip keyword. Some protocols allow further qualifiers described later.
source
Number of the network or host from which the packet is being sent. There are three alternative ways to specify the source:
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source-wildcard
Wildcard bits to be applied to source. There are three alternative ways to specify the source wildcard:
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destination
Number of the network or host to which the packet is being sent. There are three alternative ways to specify the destination:
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destination-wildcard
Wildcard bits to be applied to the destination. There are three alternative ways to specify the destination wildcard:
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precedence precedence
(Optional) Packets can be filtered by precedence level, as specified by a number from 0 to 7, or by name as listed in the section "Usage Guidelines."
tos tos
(Optional) Packets can be filtered by type of service (ToS) level, as specified by a number from 0 to 15, or by name as listed in the section "Usage Guidelines."
log
(Optional) Causes an informational logging message about the packet that matches the entry to be sent to the console. (The level of messages logged to the console is controlled by the logging console command.)
The message includes the access list number, whether the packet was permitted or denied; the protocol, whether it was TCP, UDP, ICMP, or a number; and, if appropriate, the source and destination addresses and source and destination port numbers. The message is generated for the first packet that matches, and then at 5-minute intervals, including the number of packets permitted or denied in the prior 5-minute interval.
Use the ip access-list log-update command to generate logging messages when the number of matches reaches a configurable threshold (rather than waiting for a 5-minute interval). See the ip access-list log-update command for more information.
The logging facility might drop some logging message packets if there are too many to be handled or if there is more than one logging message to be handled in 1 second. This behavior prevents the router from crashing due to too many logging packets. Therefore, the logging facility should not be used as a billing tool or an accurate source of the number of matches to an access list.
icmp-type
(Optional) ICMP packets can be filtered by ICMP message type. The type is a number from 0 to 255.
icmp-code
(Optional) ICMP packets that are filtered by ICMP message type can also be filtered by the ICMP message code. The code is a number from 0 to 255.
icmp-message
(Optional) ICMP packets can be filtered by an ICMP message type name or ICMP message type and code name. The possible names are found in the section "Usage Guidelines."
igmp-type
(Optional) IGMP packets can be filtered by IGMP message type or message name. A message type is a number from 0 to 15. IGMP message names are listed in the section "Usage Guidelines."
operator
(Optional) Compares source or destination ports. Possible operands include lt (less than), gt (greater than), eq (equal), neq (not equal), and range (inclusive range).
If the operator is positioned after the source and source-wildcard, it must match the source port.
If the operator is positioned after the destination and destination-wildcard, it must match the destination port.
The range operator requires two port numbers. All other operators require one port number.
port
(Optional) The decimal number or name of a TCP or UDP port. A port number is a number from 0 to 65535. TCP and UDP port names are listed in the section "Usage Guidelines" of the access-list (IP extended) command. TCP port names can only be used when filtering TCP. UDP port names can only be used when filtering UDP.
established
(Optional) For the TCP protocol only: Indicates an established connection. A match occurs if the TCP datagram has the ACK or RST bits set. The nonmatching case is that of the initial TCP datagram to form a connection.
fragments
(Optional) The access list entry applies to noninitial fragments of packets; the fragment is either permitted or denied accordingly. For more details about the fragments keyword, see the "Access List Processing of Fragments" and "Fragments and Policy Routing" sections in the "Usage Guidelines" section.
Defaults
An extended access list defaults to a list that denies everything. An extended access list is terminated by an implicit deny statement.
Command Modes
Access-list configuration
Command History
11.2
This command was introduced.
12.0(11) and 12.1(2)
The fragments keyword was added.
Usage Guidelines
You can use named access lists to control the transmission of packets on an interface and restrict contents of routing updates. The Cisco IOS software stops checking the extended access list after a match occurs.
Fragmented IP packets, other than the initial fragment, are immediately accepted by any extended IP access list. Extended access lists used to control vty access or restrict the contents of routing updates must not match against the TCP source port, the ToS value, or the precedence of the packet.
Note
The following is a list of precedence names:
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The following is a list of ICMP message type names and ICMP message type and code names:
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The following is a list of IGMP message names:
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The following is a list of TCP port names that can be used instead of port numbers. Refer to the current assigned numbers RFC to find a reference to these protocols. Port numbers corresponding to these protocols can also be found if you type a ? in the place of a port number.
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The following is a list of UDP port names that can be used instead of port numbers. Refer to the current assigned numbers RFC to find a reference to these protocols. Port numbers corresponding to these protocols can also be found if you type a ? in the place of a port number.
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Access List Processing of Fragments
The behavior of access-list entries regarding the use or lack of the fragments keyword can be summarized as follows:
Be aware that you should not simply add the fragments keyword to every access list entry because the first fragment of the IP packet is considered a nonfragment and is treated independently of the subsequent fragments. An initial fragment will not match an access list permit or deny entry that contains the fragments keyword, the packet is compared to the next access list entry, and so on, until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword, and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases where there are multiple deny access list entries for the same host but with different Layer 4 ports, a single deny access-list entry with the fragments keyword for that host is all that needs to be added. Thus all the fragments of a packet are handled in the same manner by the access list.
Packet fragments of IP datagrams are considered individual packets and each counts individually as a packet in access list accounting and access list violation counts.
Note
Fragments and Policy Routing
Fragmentation and the fragment control feature affect policy routing if the policy routing is based on the match ip address command and the access list had entries that match on Layer 4 through 7 information. It is possible that noninitial fragments pass the access list and are policy routed, even if the first fragment was not policy routed or the reverse.
By using the fragments keyword in access list entries as described earlier, a better match between the action taken for initial and noninitial fragments can be made and it is more likely policy routing will occur as intended.
Examples
The following example defines a dynamic access list named washington:
ip access-group washington in!ip access-list extended washingtondynamic testlist timeout 5permit ip any anypermit tcp any host 185.302.21.2 eq 23Related Commands
forwarding-agent
To specify the port on which the Forwarding Agent will listen for wildcard and fixed affinities, use the forwarding-agent command in CASA-port configuration mode. To disable listening on that port, use the no form of the command.
forwarding-agent port-number [password [timeout]]
no forwarding-agent
Syntax Description
Defaults
The default password timeout is 180 seconds.
The default port for the services manager is 1637.
Command Modes
CASA-port configuration
Command History
Examples
The following example specifies that the Forwarding Agent will listen for wildcard and fixed affinities on port 1637:
forwarding-agent 1637Related Commands
ip access-group
To control access to an interface, use the ip access-group command in interface configuration mode. To remove the specified access group, use the no form of this command.
ip access-group {access-list-number | access-list-name}{in | out}
no ip access-group {access-list-number | access-list-name}{in | out}
Syntax Description
Defaults
No access list is applied to the interface.
Command Modes
Interface configuration
Command History
10.0
This command was introduced.
11.2
The access-list-name argument was added.
Usage Guidelines
Access lists are applied on either outbound or inbound interfaces. For standard inbound access lists, after receiving a packet, the Cisco IOS software checks the source address of the packet against the access list. For extended access lists, the router also checks the destination access list. If the access list permits the address, the software continues to process the packet. If the access list rejects the address, the software discards the packet and returns an ICMP host unreachable message.
For standard outbound access lists, after receiving and routing a packet to a controlled interface, the software checks the source address of the packet against the access list. For extended access lists, the router also checks the destination access list. If the access list permits the address, the software sends the packet. If the access list rejects the address, the software discards the packet and returns an ICMP host unreachable message.
If the specified access list does not exist, all packets are passed.
When you enable outbound access lists, you automatically disable autonomous switching for that interface. When you enable input access lists on any CBus or CxBus interface, you automatically disable autonomous switching for all interfaces (with one exception—an SSE configured with simple access lists can still switch packets, on output only).
Examples
The following example applies list 101 on packets outbound from Ethernet interface 0:
interface ethernet 0ip access-group 101 outRelated Commands
ip access-list
To define an IP access list by name, use the ip access-list command in global configuration mode. To remove a named IP access list, use the no form of this command.
ip access-list {standard | extended} access-list-name
no ip access-list {standard | extended} access-list-name
Caution
Syntax Description
Defaults
No named IP access list is defined.
Command Modes
Global configuration
Command History
Usage Guidelines
Use this command to configure a named IP access list as opposed to a numbered IP access list. This command will take you into access-list configuration mode, where you must define the denied or permitted access conditions with the deny and permit commands.
Specifying the standard or extended keyword with the ip access-list command determines the prompt you get when you enter access-list configuration mode.
Use the ip access-group command to apply the access list to an interface.
Named access lists are not compatible with Cisco IOS releases prior to Release 11.2.
Examples
The following example defines a standard access list named Internetfilter:
ip access-list standard Internetfilterpermit 192.5.34.0 0.0.0.255permit 128.88.0.0 0.0.255.255permit 36.0.0.0 0.255.255.255! (Note: all other access implicitly denied)Related Commands
ip access-list log-update
To set the threshold number of packets that generate a log message if they match an access list, use the ip access-list log-update command in global configuration mode. To remove the threshold, use the no form of this command.
ip access-list log-update threshold number-of-matches
no ip access-list log-update
Syntax Description
number-of-matches
Threshold number of packets necessary to match an access list before a log message is generated. The range is 0 to 2147483647. There is no default number of matches.
Defaults
Log messages are sent at the first matching packet and at 5-minute intervals after that.
Command Modes
Global configuration
Command History
Usage Guidelines
Log messages are generated if you have specified the log keyword in the access-list (IP standard), access-list (IP extended), deny (IP), dynamic, or permit command.
Log messages provide information about the packets that are permitted or denied by an access list. By default, log messages appear at the console. (The level of messages logged to the console is controlled by the logging console command.) The log message includes the access list number, whether the packet was permitted or denied, and other information.
By default, the log messages are sent at the first matching packet and after that, identical messages are accumulated for 5-minute intervals, with a single message being sent with the number of packets permitted and denied during that interval. However, you can use the ip access-list log-update command to set the number of packets that, when match an access list (and are permitted or denied), cause the system to generate a log message. You might want to do this to receive log messages more frequently than at 5-minute intervals.
Caution
Even if you use the ip access-list log-update command, the 5-minute timer remains in effect, so the cache is emptied at the end of 5 minutes, regardless of the count of messages in the cache. Regardless of when the log message is sent, the cache is flushed and the count reset to 0 for that message the same way it is when a threshold is not specified.
If the syslog server is not directly connected to a LAN that the router shares, any intermediate router might drop the log messages because they are UDP (unreliable) messages.
Examples
The following example enables logging whenever the 1000th packet matches an access list entry:
ip access-list log-update threshold 1000Related Commands
ip accounting
To enable IP accounting on an interface, use the ip accounting command in interface configuration mode. To disable IP accounting, use the no form of this command.
ip accounting [access-violations] [output-packets]
no ip accounting [access-violations] [output-packets]
Syntax Description
Defaults
Disabled
Command Modes
Interface configuration
Command History
10.0
This command was introduced.
10.3
The access-violations keyword was added.
Usage Guidelines
The ip accounting command records the number of bytes (IP header and data) and packets switched through the system on a source and destination IP address basis. Only transit IP traffic is measured and only on an outbound basis; traffic generated by the router access server or terminating in this device is not included in the accounting statistics. Traffic coming from a remote site and transiting through a router is also recorded.
If you specify the access-violations keyword, the ip accounting command provides information identifying IP traffic that fails IP access lists. Identifying IP source addresses that violate IP access lists alerts you to possible attempts to breach security. The data might also indicate that you should verify IP access list configurations.
To receive a logging message on the console when an extended access list entry denies a packet access (to log violations), you must include the log keyword in the access-list (IP extended) or access-list (IP standard) command.
Statistics are accurate even if IP fast switching or IP access lists are being used on the interface.
IP accounting disables autonomous switching, SSE switching, and distributed switching (dCEF) on the interface. IP accounting will cause packets to be switched on the Route Switch Processor (RSP) instead of the Versatile Interface Processor (VIP), which can cause performance degradation.
Examples
The following example enables IP accounting on Ethernet interface 0:
interface ethernet 0ip accountingRelated Commands
ip accounting-list
To define filters to control the hosts for which IP accounting information is kept, use the ip accounting-list command in global configuration mode. To remove a filter definition, use the no form of this command.
ip accounting-list ip-address wildcard
no ip accounting-list ip-address wildcard
Syntax Description
ip-address
IP address in dotted decimal format.
wildcard
Wildcard bits to be applied to the ip-address argument.
Defaults
No filters are defined.
Command Modes
Global configuration
Command History
Usage Guidelines
The wildcard argument is a 32-bit quantity written in dotted-decimal format. Address bits corresponding to wildcard bits set to 1 are ignored in comparisons; address bits corresponding to wildcard bits set to zero are used in comparisons.
Examples
The following example adds all hosts with IP addresses beginning with 192.31 to the list of hosts for which accounting information will be kept:
ip accounting-list 192.31.0.0 0.0.255.255Related Commands
ip accounting-threshold
To set the maximum number of accounting entries to be created, use the ip accounting-threshold command in global configuration mode. To restore the default number of entries, use the no form of this command.
ip accounting-threshold threshold
no ip accounting-threshold threshold
Syntax Description
threshold
Maximum number of entries (source and destination address pairs) that the Cisco IOS software accumulates.
Defaults
The default maximum number of accounting entries is 512 entries.
Command Modes
Global configuration
Command History
Usage Guidelines
The accounting threshold defines the maximum number of entries (source and destination address pairs) that the software accumulates, preventing IP accounting from possibly consuming all available free memory. This level of memory consumption could occur in a router that is switching traffic for many hosts. Overflows will be recorded; see the monitoring commands for display formats.
The default accounting threshold of 512 entries results in a maximum table size of 12,928 bytes. Active and checkpointed tables can reach this size independently.
Examples
The following example sets the IP accounting threshold to only 500 entries:
ip accounting-threshold 500Related Commands
ip accounting-transits
To control the number of transit records that are stored in the IP accounting database, use the ip accounting-transits command in global configuration mode. To return to the default number of records, use the no form of this command.
ip accounting-transits count
no ip accounting-transits
Syntax Description
Defaults
The default number of transit records that are stored in the IP accounting database is 0.
Command Modes
Global configuration
Command History
Usage Guidelines
Transit entries are those that do not match any of the filters specified by ip accounting-list global configuration commands. If no filters are defined, no transit entries are possible.
To maintain accurate accounting totals, the Cisco IOS software maintains two accounting databases: an active and a checkpointed database.
Examples
The following example specifies that no more than 100 transit records are stored:
ip accounting-transits 100Related Commands
ip accounting mac-address
To enable IP accounting on a LAN interface based on the source and destination MAC address, use the ip accounting mac-address command in interface configuration mode. To disable IP accounting based on the source and destination MAC address, use the no form of this command.
ip accounting mac-address {input | output]
no ip accounting mac-address {input | output]
Syntax Description
input
Performs accounting based on the source MAC address on received packets.
output
Performs accounting based on the destination MAC address on transmitted packets.
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
This feature is supported on Ethernet, FastEthernet, and FDDI interfaces.
To display the MAC accounting information, use the show interface mac EXEC command.
MAC address accounting provides accounting information for IP traffic based on the source and destination MAC address on LAN interfaces. This calculates the total packet and byte counts for a LAN interface that receives or sends IP packets to or from a unique MAC address. It also records a timestamp for the last packet received or sent. With MAC address accounting, you can determine how much traffic is being sent to and/or received from various peers at NAPS/peering points.
Examples
The following example enables IP accounting based on the source and destination MAC address for received and transmitted packets:
interface ethernet 4/0/0ip accounting mac-address inputip accounting mac-address outputRelated Commands
show interface mac
Displays MAC accounting information for interfaces configured for MAC accounting.
ip accounting precedence
To enable IP accounting on any interface based on IP precedence, use the ip accounting precedence command in interface configuration mode. To disable IP accounting based on IP precedence, use the no form of this command.
ip accounting precedence {input | output]
no ip accounting precedence {input | output]
Syntax Description
input
Performs accounting based on IP precedence on received packets.
output
Performs accounting based on IP precedence on transmitted packets.
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
To display IP precedence accounting information, use the show interface precedence EXEC command.
The precedence accounting feature provides accounting information for IP traffic, summarized by IP precedence value(s). This feature calculates the total packet and byte counts for an interface that receives or sends IP packets and sorts the results based on IP precedence. This feature is supported on all interfaces and subinterfaces and supports CEF, dCEF, flow, and optimum switching.
Examples
The following example enables IP accounting based on IP precedence for received and transmitted packets:
interface ethernet 4/0/0ip accounting precedence inputip accounting precedence outputRelated Commands
show interface precedence
Displays precedence accounting information for an interface configured for precedence accounting.
ip casa
To configure the router to function as a forwarding agent, use the ip casa command in global configuration mode. To disable the forwarding agent, use the no form of this command.
ip casa control-address igmp-address
no ip casa
Syntax Description
Defaults
No default behavior or values.
Command Modes
Global configuration
Command History
Examples
The following example specifies the Internet address (10.10.4.1) and IGMP address (224.0.1.2) for the Forwarding Agent:
ip-casa 10.10.4.1 224.0.1.2Related Commands
Specifies the port on which the Forwarding Agent will listen for wildcard and fixed affinities.
ip drp access-group
To control the sources of Director Response Protocol (DRP) queries to the DRP Server Agent, use the ip drp access-group command in global configuration mode. To remove the access list, use the no form of this command.
ip drp access-group access-list-number
no ip drp access-group access-list-number
Syntax Description
access-list-number
Number of a standard IP access list in the range from 1 to 99 or from 1300 to 1999.
Defaults
The DRP Server Agent will answer all queries.
Command Modes
Global configuration
Command History
Usage Guidelines
This command applies an access list to the interface, thereby controlling which devices can send queries to the DRP Server Agent.
If both an authentication key chain and an access group have been specified, both security measures must permit access before a request is processed.
Examples
The following example configures access list 1, which permits only queries from the host at 33.45.12.4:
access-list 1 permit 33.45.12.4ip drp access-group 1Related Commands
Configures authentication on the DRP Server Agent for DistributedDirector.
show ip drp
Displays information about the DRP Server Agent for DistributedDirector.
ip drp authentication key-chain
To configure authentication on the Director Response Protocol (DRP) Server Agent for DistributedDirector, use the ip drp authentication key-chain command in global configuration mode. To remove the key chain, use the no form of this command.
ip drp authentication key-chain name-of-chain
no ip drp authentication key-chain name-of-chain
Syntax Description
Defaults
No authentication is configured for the DRP Server Agent.
Command Modes
Global configuration
Command History
Usage Guidelines
When a key chain and key are configured, the key is used to authenticate all DRP requests and responses. The active key on the DRP Server Agent must match the active key on the primary agent. Use the key and key-string commands to configure the key.
Examples
The following example configures a key chain named ddchain:
ip drp authentication key-chain ddchainRelated Commands
ip drp server
To enable the Director Response Protocol (DRP) Server Agent that works with DistributedDirector, use the ip drp server command in global configuration mode. To disable the DRP Server Agent, use the no form of this command.
ip drp server
no ip drp server
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Global configuration
Command History
Examples
The following example enables the DRP Server Agent:
ip drp serverRelated Commands
ip icmp rate-limit unreachable
To have the Cisco IOS software limit the rate that Internet Control Message Protocol (ICMP) destination unreachable messages are generated, use the ip icmp rate-limit unreachable command in global configuration mode. To remove the rate limit, use the no form of this command.
ip icmp rate-limit unreachable [df] milliseconds
no ip icmp rate-limit unreachable [df]
Syntax Description
Defaults
The default value is one ICMP destination unreachable message per 500 milliseconds.
Command Modes
Global configuration
Command History
Usage Guidelines
The no ip icmp rate-limit unreachable command turns off the previously configured rate limit. To re-set the rate limit to its default value, use the default ip icmp rate-limit unreachable command.
The Cisco IOS software maintains two timers: one for general destination unreachable messages and one for DF destination unreachable messages. Both share the same time limits and defaults. If the df option is not configured, the ip icmp rate-limit unreachable command sets the time values for DF destination unreachable messages. If the df option is configured, its time values remain independent from those of general destination unreachable messages.
Examples
The following example sets the rate of the ICMP destination unreachable message to one message every 10 milliseconds:
ip icmp rate-limit unreachable 10The following example turns off the previously configured rate limit:
no ip icmp rate-limit unreachableThe following example sets the rate limit back to the default:
default ip icmp rate-limit unreachableip icmp redirect
To control the type of Internet Control Message Protocol (ICMP) redirect message that is sent by the Cisco IOS software, use the ip icmp redirect command in global configuration mode. To set the value back to the default, use the no form of this command.
ip icmp redirect [host | subnet]
no ip icmp redirect [host | subnet]
Syntax Description
Defaults
The router will send ICMP subnet redirect messages.
Because the ip icmp redirect subnet command is the default, the command will not be displayed in the configuration.
Command Modes
Global configuration
Command History
Usage Guidelines
An ICMP redirect message can be generated by a router when a packet is received and transmitted on the same interface. In this situation, the router will forward the original packet and send a ICMP redirect message back to the sender of the original packet. This behavior allows the sender to bypass the router and forward future packets directly to the destination (or a router closer to the destination).
There are two types of ICMP redirect messages: redirect for a host address or redirect for an entire subnet.
The ip icmp redirect command determines the type of ICMP redirects sent by the system and is configured on a per system basis. Some hosts do not understand ICMP subnet redirects and need the router to send out ICMP host redirects. Use the ip icmp redirect host command to have the router send out ICMP host redirects. Use the ip icmp redirect subnet command to set the value back to the default, which is to send subnet redirects.
To prevent the router from sending ICMP redirects, use the no ip redirects interface configuration command.
Examples
The following example enables the router to send out ICMP host redirects:
ip icmp redirect hostsThe following example sets the value back to the default, which is subnet redirects:
ip icmp redirect subnetRelated Commands
ip mask-reply
To have the Cisco IOS software respond to Internet Control Message Protocol (ICMP) mask requests by sending ICMP mask reply messages, use the ip mask-reply command in interface configuration mode. To disable this function, use the no form of this command.
ip mask-reply
no ip mask-reply
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Interface configuration
Command History
Examples
The following example enables the sending of ICMP mask reply messages on Ethernet interface 0:
interface ethernet 0ip address 131.108.1.0 255.255.255.0ip mask-reply
