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Table Of Contents
Related Features and Technologies
Configuration Tasks for Multiarea IS-IS
Verifying Multiarea IS-IS Configuration
Monitoring and Maintaining Multiarea IS-IS
IS-IS Multiarea Support
Feature History
Feature Overview
This feature module describes multiarea Intermediate System-to-Intermediate System (IS-IS) support for ISO Connectionless Network Service (CLNS). It also describes how to use the modified isis mesh-group command to limit link-state packet (LSP) flooding, a technique that is related to IS-IS configuration in general and is not limited to configuring multiarea IS-IS networks. Small IS-IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.
Routers establish Level 1 adjacencies to perform routing within a local area (intra-area routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (interarea routing).
Some networks use legacy equipment that supports only Level 1 routing. These devices are typically organized into many small areas that cannot be aggregated due to performance limitations. Cisco routers are used to interconnect each area to the Level 2 backbone.
Previously, each Cisco router could participate in only one area, performing Level 1 (intra-area) routing locally, and Level 2 (interarea) routing to other areas in the network. This limitation meant that when the network was divided into a large number of Level 1 areas, a correspondingly large number of Level 1/2 routers was required to route between all areas, as shown in the example in Figure 1. The need for redundancy increases the number of routers needed.
With this feature release, Cisco IOS software supports configuration of multiple Level 1 areas within a single router.
Figure 1 represents a telco network used to monitor the status of the switching equipment in multiple remote central offices (where the telco equipment resides) from a central monitoring point.
In this example, the CLNS network of the telco is being used to monitor the status of the switching equipment. The CLNS network is not used to perform actual circuit switching (end users making voice or data calls).
Figure 1
IS-IS Network for Monitoring Remote Equipment Status
Figure 2 shows the same network reconfigured using multiarea IS-IS. The number of local access routers has been reduced. Each router continues to provide access to the backbone, but also participates in multiple Level 1 areas.
A more powerful area router platform may be required to handle the increased load, but fewer routers are required at the access level. In this example a 3:1 reduction in the number of routers required is shown. However, if you configure more Level 1 areas on each multiarea router, further reduction in the number of routers required is possible.
Figure 2
Multiarea IS-IS Network for Monitoring Remote Equipment Status
Redistribution
IP routes learned via Level 1 routing are advertised by default into Level 2. Even when multiple Level 1 processes are configured on the same unit, this fact is still true. No additional configuration is required to redistribute all Level 1 IP routes into the Level 2 process.
In CLNS routing there is no redistribution of Level 1 host routes into Level 2. Only Level 1 addresses are advertised into Level 2. Redistribution of all area addresses of all Level 1 areas into Level 2 is implicit in IS-IS, and no additional configuration is required for this redistribution.
Redistribution of other protocols will continue to work with conventional IS-IS configurations as it does in Release 12.0.
TARP
For a conventional IS-IS configuration with a single Level 1 and a Level 2 area (or configuration with a single Level 1 area or a Level 2 area), Target Address Resolution Protocol (TARP) continues to work without any change in previous behavior.
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If multiple Level 1 areas are defined, the router resolves addresses using TARP in the following way:
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Benefits
Fewer Routers Required to Support the Same Number of Areas
The IS-IS Multiarea Support feature makes it possible for one Cisco router to support multiple Level 1 areas, as opposed to the single Level 1 area previously supported by each router.
Network Scaling
A single Cisco router can now participate in routing in up to 29 areas, as well as perform Level 2 (interarea) routing in the backbone. Expansion of an IS-IS network consisting of many small areas is simpler that previously, because you can configure multiple Level 1 areas in the same Cisco unit, without needing to add and configure physical units for each additional local area.
Connectivity for Local Level 1 Areas on the Same Router
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Restrictions
Configuration Limitations
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You can configure only one process to perform Level 2 (interarea) routing. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1. You can configure this process to perform intra-area (Level 1) routing at the same time. You can configure up to 29 additional processes as Level 1-only processes.
Redistribution
Explicit redistribution between IS-IS areas cannot be configured. Redistribution from any other routing protocol into a particular area is possible, and is configured per router instance, as in Release 12.0 of the Cisco IOS software, using the redistribute and route map commands. By default, redistribution is into Level 2.
External Area Merge
The IS-IS protocol allows you to merge areas by configuring multiple network entity titles (NETs) on a router. If these NETs define a router to be in both Area A and Area B, for example, the router can potentially merge areas A and B. The result of the merge will be one Level 1 area with two area addresses: A and B. All routers in this merged area must have NSAPs with unique system IDs.
Distinct areas defined in a multiarea router cannot share a common area address. This means that all routers in a single area must have unique system IDs. However, it is possible for two areas to be "merged" external to the router, such as when a common area number is introduced by other routers in the two areas. When this happens, the areas that now have a common area number are said to be "merged" into a single area.
If this happens, the multiarea router will see its own area data from two different areas on a single interface, and the router will display the CLNS-DUPSYSTEM warning message.
The error is a configuration error and must be corrected manually.
Related Features and Technologies
The IS-IS multiarea feature is an extension of integrated IS-IS and ISO CLNS. Integrated IS-IS is documented in Network Protocols Configuration Guide, Part 1, and the Network Protocols Command Reference, Part 1. ISO CLNS is documented in Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3, and the Cisco IOS Release 12.0 Network Protocols Command Reference, Part 3.
Related Documents
For related information on this feature, refer to the following documents:
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Supported Platforms
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Supported MIBs and RFCs
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Configuration Tasks
There is no change to the configuration process for configuring conventional IS-IS with ISO CLNS:
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Unlike other routing protocols, enabling IS-IS requires that you create an IS-IS routing process and assign it to a specific interface, rather than to a network. You can specify more than one IS-IS routing process per Cisco unit, using the multiarea IS-IS configuration syntax. You then configure the parameters for each instance of the IS-IS routing process.
In general, each routing process corresponds to an area. By default, the first instance of the routing process configured performs both intra-area (Level 1) and Level 2 (interarea) routing. You can configure additional router instances, which are automatically treated as Level 1 areas. You must configure the parameters for each instance of the IS-IS routing process individually.
You can configure at most only one IS-IS routing process to perform Level 2 (interarea) routing. If Level 2 routing is not desired for a router instance, remove the Level 2 capability using the is-type command. Use the is-type command to configure a different router instance as a Level 2 router.
Configuration Tasks for Multiarea IS-IS
To configure multiarea IS-IS in integrated mode, or ISO CLNS, complete the tasks in the following sections. Enabling IS-IS and ISO CLNS is required; the remainder of the tasks in the following list are optional (although you might need to perform them, depending upon your specific application):
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Limiting LSP flooding is described in this document. The other tasks are documented in Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1 and Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3.
Enabling IS-IS and Assigning Areas
To enable IS-IS and specify the area for each instance of the IS-IS routing process, use the following commands, starting in global configuration mode:
Enabling CLNS Routing for an Area on an Interface
To enable CLNS routing and specify the area for each instance of the IS-IS routing process, use the following commands, starting in global configuration mode:
Modifying the Output of show Commands
To customize display output when the multiarea feature is used, making the display easier to read, use the following EXEC command:
cnt |char cnt]Specifies the delimiter to be used to separate displays of information about individual IS-IS areas.
For example, the following command causes information about individual areas to be separated by 14 dashes (-) in the display:
isis display delimiter - 14The output for a configuration with two Level 1 areas and one Level 2 area configured is as follows:
dtp-5#show clns neighbors--------------Area L2BB:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0009 Tu529 172.21.39.9 Up 25 L1L2 IS-IS--------------Area A3253-01:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0053 Et1 0060.3e58.ccdb Up 22 L1 IS-IS0000.0000.0003 Et1 0000.0c03.6944 Up 20 L1 IS-IS--------------Area A3253-02:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0002 Et2 0000.0c03.6bc5 Up 27 L1 IS-IS0000.0000.0053 Et2 0060.3e58.ccde Up 24 L1 IS-ISEnabling Partitioning Avoidance
In ISO CLNS networks using a redundant topology, it is possible for an area to become "partitioned" when full connectivity is lost between a Level 1-2 border router, all adjacent Level 1 routers, and end hosts. In such a case, multiple Level 1-2 border routers advertise the Level 1 area prefix into the backbone area, even though any one router can reach only a subset of the end hosts in the Level 1 area.
When enabled, the partition avoidance command prevents this partitioning by causing the border router to stop advertising the Level 1 area prefix into the Level 2 backbone.
Other cases of connectivity loss within the Level 1 area itself are not detected or corrected by the border router, and this command has no effect.
To enable partitioning avoidance, use the following router configuration command:
Changing the Routing Level for an Area
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Limiting LSP Flooding
Limiting LSP flooding is important to IS-IS networks in general, and is not limited to configuring multiarea IS-IS networks. In a network with a high degree of redundancy, such as a fully meshed set of point-to-point links over a non-broadcast multiAccess (NBMA) transport, flooding of LSPs can limit network scalability. You can reduce LSP flooding in two ways:
Blocking Flooding on Specific Interfaces
The advantage of full blocking over mesh groups is that it is easier to configure and understand, and fewer LSPs are flooded. Blocking flooding on all links permits the best scaling performance, but results in a less robust network structure. Permitting flooding on all links results in poor scaling performance.
Configuring Mesh Groups
The advantage of mesh groups over full blocking is that mesh groups allow LSPs to be flooded over one hop to all routers on the mesh, while full blocking allows some routers to receive LSPs over multiple hops. This relatively small delay in flooding can have an impact on convergence times, but the delay is negligible compared to overall convergence times.
Blocking Flooding on Specific Interfaces
You can completely block flooding (full blocking) on specific interfaces, so that new LSPs will not be flooded out over those interfaces. However, if flooding is blocked on a large number of links, and all remaining links go down, routers cannot synchronize their link-state databases even though there is connectivity to the rest of the network. When the link-state database is no longer updated, routing loops usually result.
To use Complete Sequence Number PDUs (CSNPs) on selected point-to-point links to synchronize the link-state database, configure a CSNP interval using the isis csnp-interval command on selected point-to-point links over which normal flooding is blocked. You should use CSNPs for this purpose only as a last resort.
Configuring Mesh Groups
Configuring mesh groups (a set of interfaces on a router) can help to limit redundant flooding. All routers reachable over the interfaces in a particular mesh group are assumed to be densely connected (each router has many links to other routers), where many links can fail without isolating one or more routers from the network.
Normally, when a new LSP is received on an interface, it is flooded out over all other interfaces on the router. When the new LSP is received over an interface that is part of a mesh group, the new LSP will not be flooded out over the other interfaces that are part of that same mesh group.
Mesh groups rely on a full mesh of links between a group of routers. If one or more links in the full mesh goes down, the full mesh is broken, and some routers might miss new LSPs, even though there is connectivity to the rest of the network. When you configure mesh groups to optimize or limit LSP flooding, be sure to select alternative paths over which to flood in case interfaces in the mesh group go down.
To minimize the possibility of incomplete flooding, you should allow unrestricted flooding over at least a minimal set of links in the mesh. Selecting the smallest set of logical links that covers all physical paths results in very low flooding, but less robustness. Ideally you should select only enough links to ensure that LSP flooding is not detrimental to scaling performance, but enough links to ensure that under most failure scenarios no router will be logically disconnected from the rest of the network.
Configuring Other IS-IS Parameters
You can now configure other IS-IS parameters for each area. The procedures and commands are described in the "Configuring Integrated IS-IS" chapter of the Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1, or the "Configuring ISO CLNS" chapter of the Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 3.
Verifying Multiarea IS-IS Configuration
Output for all of the show commands used to verify IS-IS configuration is "per area" when multiarea IS-IS is configured.
The following example shows how to use the show commands to verify a dual CLNS-IP multiarea network.
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The following is sample output from the show clns command in a dual CLNS-IP network for Area 49:
dtp-5#show clnsGlobal CLNS Information:3 Interfaces Enabled for CLNSNET: 49.2222.0000.0000.0005.00NET: 49.0553.0001.0000.0000.0005.00NET: 49.0553.0002.0000.0000.0005.00Configuration Timer: 60, Default Holding Timer: 300, Packet Lifetime 64ERPDU's requested on locally generated packetsIntermediate system operation enabled (CLNS forwarding allowed)IS-IS level-1-2 Router: L2BBRouting for Area: 49.2222IS-IS level-1 Router: A3253-01Routing for Area: 49.0553.0001IS-IS level-1 Router: A3253-02Routing for Area: 49.0553.00022.
The following is sample output from the show clns protocol command:
dtp-5# show clns protocolIS-IS Router: L2BBSystem Id: 0000.0000.0005.00 IS-Type: level-1-2Manual area address(es):49.2222Routing for area address(es):49.2222Interfaces supported by IS-IS:Tunnel529 - OSI - IPRedistribute:static (on by default)Distance for L2 CLNS routes: 110IS-IS Router: A3253-01System Id: 0000.0000.0005.00 IS-Type: level-1Manual area address(es):49.0553.0001Routing for area address(es):49.0553.0001Interfaces supported by IS-IS:Ethernet1 - OSI - IPRedistribute:static (on by default)Distance for L2 CLNS routes: 110IS-IS Router: A3253-02System Id: 0000.0000.0005.00 IS-Type: level-1Manual area address(es):49.0553.0002Routing for area address(es):49.0553.0002Interfaces supported by IS-IS:Ethernet2 - OSI - IPRedistribute:static (on by default)Distance for L2 CLNS routes: 1103.
The following is sample output from the show clns neighbors command in a dual CLNS-IP network:
dtp-5# show clns neighborsArea L2BB:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0009 Tu529 172.21.39.9 Up 27 L1L2 IS-ISArea A3253-01:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0053 Et1 0060.3e58.ccdb Up 24 L1 IS-IS0000.0000.0003 Et1 0000.0c03.6944 Up 27 L1 IS-ISArea A3253-02:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0053 Et2 0060.3e58.ccde Up 25 L1 IS-IS0000.0000.0002 Et2 0000.0c03.6bc5 Up 27 L1 IS-IS4.
The following is sample output from the show isis topology command in a dual CLNS-IP network:
dtp-5# show isis topologyArea L2BB:IS-IS paths to level-1 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0005 --0000.0000.0009 10 0000.0000.0009 Tu529 *Tunnel*0000.0000.0017 20 0000.0000.0009 Tu529 *Tunnel*0000.0000.0053 30 0000.0000.0009 Tu529 *Tunnel*0000.0000.0068 20 0000.0000.0009 Tu529 *Tunnel*IS-IS paths to level-2 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0005 --0000.0000.0009 10 0000.0000.0009 Tu529 *Tunnel*0000.0000.0017 20 0000.0000.0009 Tu529 *Tunnel*0000.0000.0053 30 0000.0000.0009 Tu529 *Tunnel*0000.0000.0068 20 0000.0000.0009 Tu529 *Tunnel*Area A3253-01:IS-IS paths to level-1 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0003 10 0000.0000.0003 Et1 0000.0c03.69440000.0000.0005 --0000.0000.0053 10 0000.0000.0053 Et1 0060.3e58.ccdbArea A3253-02:IS-IS paths to level-1 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0002 10 0000.0000.0002 Et2 0000.0c03.6bc50000.0000.0005 --0000.0000.0053 10 0000.0000.0053 Et2 0060.3e58.ccde5.
These commands, used with ping, verify connectivity to all desired end hosts in the network.
As the display shows, neighbors are not included in the show clns route output.
The following is sample output from the show clns route command:
dtp-5# show clns routeCodes: C - connected, S - static, d - DecnetIVI - ISO-IGRP, i - IS-IS, e - ES-ISC 49.0553.0002.0000.0000.0005.00 [1/0], Local IS-IS NETC 49.0553.0001.0000.0000.0005.00 [1/0], Local IS-IS NETC 49.2222.0000.0000.0005.00 [1/0], Local IS-IS NETC 49.0553.0002 [2/0], Local IS-IS AreaC 49.0553.0001 [2/0], Local IS-IS AreaC 49.2222 [2/0], Local IS-IS AreaThe following is sample output from the show ip route isis command:
dtp-5# show ip route isis11.0.0.0/24 is subnetted, 1 subnetsi L1 11.0.0.0 [115/20] via 10.0.0.9, Tunnel529, L2BB, from LSP 3i*L1 0.0.0.0/0 [115/10] via 10.0.0.9, Tunnel529, L2BB, from LSP 0[115/10] via 10.2.2.53, Ethernet2, A3253-02, from LSP 0[115/10] via 10.1.1.53, Ethernet1, A3253-01, from LSP 06.
Monitoring and Maintaining Multiarea IS-IS
Configuration Example
This section provides the following configuration example:
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Multiarea IS-IS Configuration
The following output shows an IS-IS configuration with two Level 1 areas and one Level 1-2 area. Figure 3 illustrates this configuration.
clns routing...interface Tunnel529ip address 10.0.0.5 255.255.255.0ip router isis BBclns router isis BBinterface Ethernet1ip address 10.1.1.5 255.255.255.0ip router isis A3253-01clns router isis A3253-01!interface Ethernet2ip address 10.2.2.5 255.255.255.0ip router isis A3253-02clns router isis A3253-02...router isis BB ! Defaults to "is-type level-1-2"net 49.2222.0000.0000.0005.00!router isis A3253-01net 49.0553.0001.0000.0000.0005.00is-type level-1!router isis A3253-02net 49.0553.0002.0000.0000.0005.00is-type level-1Figure 3 Multiarea IS-IS Configuration with Three Level 1 Areas and One Level 2 Area
Command Reference
This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.
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clns router isis
To configure an Intermediate System-to-Intermediate System (IS-IS) routing process for International Standards Organization (ISO) Connectionless Network Service Protocol (CLNS) on an interface and to attach an area designator to the routing process, use the clns router isis interface configuration command. Use the no form of the command to disable IS-IS for ISO.
clns router isis [area-tag]
no clns router isis [area-tag]
Syntax Description
DefaultsDefault
No routing processes are specified.
Command Modes
Interface configuration
Command History
10.0
This command was introduced.
12.0(5)T
Multiarea functionality was added, changing the way the tag argument (now area-tag) is used.
Usage Guidelines
Before the IS-IS router process is useful, a network entity title (NET) must be assigned with the net command and some interfaces must be enabled with IS-IS.
If you have IS-IS running and at least one ISO-IGRP process, the IS-IS process and the ISO-IGRP process cannot both be configured without an area tag. The null tag can be used by only one process. If you run ISO-IGRP and IS-IS, a null tag can be used for IS-IS, but not for ISO-Interior Gateway Routing Protocol (IGRP) at the same time. However, each area in an IS-IS multiarea configuration should have a nonnull area tag to facilitate identification of the area.
You can configure only one process to perform Level 2 (interarea) routing. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1. You can configure this process to perform intra-area (Level 1) routing at the same time. You can configure up to 29 additional processes as Level 1-only processes. Use the is-type command to remove Level 2 routing from a router instance. You can then use the is-type command to enable Level 2 routing on some other IS-IS router instance.
An interface cannot be part of more than one area, except in the case where the associated routing process is performing both Level 1 and Level 2 routing. On media (such as WAN media, for example) where subinterfaces are supported, different subinterfaces could be configured for different areas.
Examples
The following example enables IS-IS routing for ISO CLNS on Ethernet interface 0:
router isis cisconet 39.0001.0000.0c00.1111.00interface ethernet 0clns router isis ciscoThe following example shows an IS-IS configuration with two Level 1 areas and one Level 1-2 area:
clns routing...interface Tunnel529ip address 10.0.0.5 255.255.255.0ip router isis BBclns router isis BBinterface Ethernet1ip address 10.1.1.5 255.255.255.0ip router isis A3253-01clns router isis A3253-01!interface Ethernet2ip address 10.2.2.5 255.255.255.0ip router isis A3253-02clns router isis A3253-02...router isis BB ! Defaults to "is-type level-1-2"net 49.2222.0000.0000.0005.00!router isis A3253-01net 49.0553.0001.0000.0000.0005.00is-type level-1!router isis A3253-02net 49.0553.0002.0000.0000.0005.00is-type level-1Related Commands
ip router isis
To configure an Intermediate System-to-Intermediate System (IS-IS) routing process for IP on an interface and to attach an area designator to the routing process, use the ip router isis interface configuration command. Use the no form of the command to disable IS-IS for IP.
ip router isis [area-tag]
no ip router isis [area-tag]
Syntax Description
Defaults
No routing processes are specified.
Command Modes
Interface configuration
Command History
10.0
This command introduced.
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Multiarea functionality was added, changing the way the tag argument (now area-tag) is used.
Usage Guidelines
Before the IS-IS routing process is useful, a network entity title (NET) must be assigned with the net command and some interfaces must have IS-IS enabled.
If you have IS-IS running and at least one ISO-Interior Gateway Routing Protocol (IGRP) process, the IS-IS process and the ISO-IGRP process cannot both be configured without an area tag. The null tag can be used by only one process. If you run ISO-IGRP and IS-IS, a null tag can be used for IS-IS, but not for ISO-IGRP at the same time. However, each area in an IS-IS multiarea configuration should have a nonnull area tag to facilitate identification of the area.
You can configure only one process to perform Level 2 (interarea) routing. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1. You can configure this process to perform intra-area (Level 1) routing at the same time. You can configure up to 29 additional processes as Level 1-only processes. Use the is-type command to remove Level 2 routing from a router instance. You can then use the is-type command to enable Level 2 routing on some other IS-IS router instance.
An interface cannot be part of more than one area, except in the case where the associated routing process is performing both Level 1 and Level 2 routing. On media (such as WAN media, for example) where subinterfaces are supported, different subinterfaces could be configured for different areas.
Examples
The following example specifies IS-IS as an IP routing protocol for a process named Finance, and specifies that the Finance process will be routed on interfaces Ethernet 0 and serial 0:
router isis Financenet 49.0001.aaaa.aaaa.aaaa.00interface Ethernet 0ip router isis Financeinterface serial 0ip router isis FinanceThe following example shows an IS-IS configuration with two Level 1 areas and one Level 1-2 area:
ip routing...interface Tunnel529ip address 10.0.0.5 255.255.255.0ip router isis BBinterface Ethernet1ip address 10.1.1.5 255.255.255.0ip router isis A3253-011!interface Ethernet2ip address 10.2.2.5 255.255.255.0ip router isis A3253-02...router isis BB ! Defaults to "is-type level-1-2"net 49.2222.0000.0000.0005.00!router isis A3253-01net 49.0553.0001.0000.0000.0005.00is-type level-1!router isis A3253-02net 49.0553.0002.0000.0000.0005.00is-type level-1Related Commands
isis display delimiter
To make output from multiarea displays easier to read by specifying the delimiter to use to separate displays of information, use the isis display delimiter global configuration command. This command displays the output from different areas as a string or additional white space. Use the no form of the command to disable this output format.
isis display delimiter [return cnt | char cnt]
no isis display delimiter [return cnt | char cnt]
Syntax Description
return
(Optional) Delimit with carriage returns.
char
(Optional) Character to use for delimiter string.
cnt
(Optional) Number of carriage returns or length of string to use for delimiter.
Defaults
By default the isis display delimiter command is disabled.
Command Modes
Global configuration
Command History
Usage Guidelines
Use this command to customize display output when the IS-IS multiarea feature is used.
Examples
The following command causes different areas in multiarea displays (such as show command output) to be delimited by a string of dashes (-):
isis display delimiter - 14With three IS-IS neighbors configured, this command displays the following output from the show clns neighbors command:
dtp-5# show clns neighbors--------------Area L2BB:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0009 Tu529 172.21.39.9 Up 25 L1L2 IS-IS--------------Area A3253-01:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0053 Et1 0060.3e58.ccdb Up 22 L1 IS-IS0000.0000.0003 Et1 0000.0c03.6944 Up 20 L1 IS-IS--------------Area A3253-02:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0002 Et2 0000.0c03.6bc5 Up 27 L1 IS-IS0000.0000.0053 Et2 0060.3e58.ccde Up 24 L1 IS-ISRelated Commands
isis mesh-group
To optimize LSP flooding in nonbroadcast multiaccess (NBMA) networks with highly meshed, point-to-point topologies, use the isis mesh-group interface configuration command. To remove a (sub)interface from a mesh group, use the no form of this command.
isis mesh-group [num | blocked]
no isis mesh-group [num | blocked]
Syntax Description
Defaults
The interface performs normal flooding.
Command Modes
Interface configuration
Command History
12.0
This command was introduced.
12.0(5)T
Command was modified to include multiarea IS-IS routing.
Usage Guidelinescr
LSPs that are first received on (sub)interfaces that are not part of a mesh group are flooded to all other (sub)interfaces in the usual way.
LSPs that are first received on (sub)interfaces that are part of a mesh group are flooded to all interfaces except those in the same mesh-group. If the "blocked" keyword is configured on a (sub)interface, then a newly received LSP is not flooded out over that interface.
To minimize the possibility of incomplete flooding, you should allow unrestricted flooding over at least a minimal set of links in the mesh. Selecting the smallest set of logical links that covers all physical paths results in very low flooding, but less robustness. Ideally you should select only enough links to ensure that LSP flooding is not detrimental to scaling performance, but enough links to ensure that under most failure scenarios no router will be logically disconnected from the rest of the network. In other words, blocking flooding on all links permits the best scaling performance, but there is no flooding. Permitting flooding on all links results in very poor scaling performance.
Examples
In the following example six interfaces are configured in three mesh groups. LSPs received are handled as follows:
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interface atm 1/0.1
ip router isis
isis mesh-group 10interface atm 1/0.2ip router isis
isis mesh-group 10interface atm 1/1.1
ip router isis
isis mesh-group 11interface atm 1/1.2
ip router isis
isis mesh-group 11interface atm 1/2.1
ip router isisisis mesh-group blockedinterface atm 1/2.2
ip router isisRelated Commands
is-type
To configure the routing level for an instance of the Intermediate System-to-Intermediate System (IS-IS) routing process, use the is-type router configuration command. To reset the default value, use the no form of this command.
is-type {level-1 | level-1-2 | level-2-only}
no is-type {level-1 | level-1-2 | level-2-only}
Syntax Description
Defaults
In conventional IS-IS configurations, the router acts as both a Level 1 (intra-area) and a Level 2 (interarea) router.
In multiarea IS-IS configurations, the first instance of the IS-IS routing process configured is by default a Level 1-2 (intra-area and interarea) router. The remaining instances of the IS-IS process configured by default are Level 1 routers.
Command Modes
Router configuration
Command History
10.3
This command was introduced.
12.0(5)T
Multiarea IS-IS routing was included.
Usage Guidelines
It is highly recommended that you configure the type of an IS-IS routing process. If you are configuring multiarea IS-IS, you must configure the type of the router, or allow it to be configured by default. By default, the first instance of the IS-IS routing process that you configure using the router isis command is a Level 1-2 router.
If there is only one area in the network, there is no need to run both Level 1 and Level 2 routing algorithms. If IS-IS is used for Connectionless Network Service (CLNS) routing (and there is only one area), Level 1-only must be used everywhere. If IS-IS is used for IP routing only (and there is only one area), you can run Level 2-only everywhere. Areas you add after the Level 1-2 area exists are by default Level 1 areas.
If the router instance has been configured for Level 1-2 (the default for the first instance of the IS-IS routing process in a Cisco unit), you can remove Level 2 (interarea) routing for the area using the is-type command. You can also use is-type to configure Level 2 routing for an area, but it must be the only instance of the IS-IS routing process configured for Level 2 on the Cisco unit.
Examples
The following example specifies an area router:
router isisis-type level-2-onlyRelated Commands
net
To configure an Intermediate System-to-Intermediate System (IS-IS) network entity title (NET) for the routing process, use the net router configuration command. To remove a NET, use the no form of this command.
net network-entity-title
no net network-entity-title
Syntax Description
network-entity-title
NET that specifies the area address and the system ID for an IS-IS routing process. This argument can be either an address or a name.
Defaults
No NET is configured and the IS-IS process will not start. A NET is mandatory.
Command Modes
Router configuration
Command History
10.0
This command was introduced.
12.0(5)T
Command was modified to include multiarea IS-IS routing.
Usage Guidelines
Under most circumstances, one and only one NET must be configured.
A NET is an Network Service Access Point (NSAP) where the last byte is always zero. On a Cisco router running IS-IS, a NET can be 8 to 20 bytes. The last byte is always the n-selector and must be zero.
The six bytes directly in front of the n-selector are the system ID. The system ID length is a fixed size and cannot be changed. The system ID must be unique throughout each area (Level 1) and throughout the backbone (Level 2).
All bytes in front of the system ID are the area ID.
Even when IS-IS is used to perform IP routing only (no Connectionless Network Service (CLNS) routing enabled), a NET must still be configured to define the router system ID and area ID.
Multiple NETs per router are allowed, with a maximum of three. In rare circumstances, it is possible to configure two or three NETs. In such a case, the area this router is in will have three area addresses. There will still be only one area, but it will have more area addresses.
Configuring multiple NETs can be temporarily useful in the case of network reconfiguration where multiple areas are merged, or where one area is split into more areas. Multiple area addresses enable you to renumber an area individually as needed.
If you are configuring multiarea IS-IS, the area ID must be unique, but the system ID portion of the NET must be the same for all IS-IS routing process instances.
Examples
The following example configures a router with system ID 0000.0c11.1110 and area ID 47.0004.004d.0001:
router isis Pieintheskynet 47.0004.004d.0001.0001.0c11.1111.00The following example shows three IS-IS routing processes with three areas configured. Each area has a unique identifier, but the system ID is the same for all areas.
clns routing...interface Tunnel529ip address 10.0.0.5 255.255.255.0ip router isis BBclns router isis BBinterface Ethernet1ip address 10.1.1.5 255.255.255.0ip router isis A3253-01clns router isis A3253-01!interface Ethernet2ip address 10.2.2.5 255.255.255.0ip router isis A3253-02clns router isis A3253-02...router isis BB ! Defaults to "is-type level-1-2"net 49.2222.0000.0000.0005.00!router isis A3253-01net 49.0553.0001.0000.0000.0005.00is-type level-1!router isis A3253-02net 49.0553.0002.0000.0000.0005.00is-type level-1Related Commands
partition avoidance
To causes an Intermediate System-to-Intermediate System (IS-IS) Level 1-2 border router to stop advertising the Level 1 area prefix into the Level 2 backbone when full connectivity is lost between the border router, all adjacent Level 1 routers, and end hosts, use the partition avoidance router configuration command. This command displays the output from different areas as a string or additional white space. Use the no form of the command to disable this output format.
partition avoidance [area-tag]
no partition avoidance [area-tag]
Syntax Description
Defaults
By default the partition avoidance command is disabled.
Command Modes
Router configuration
Command History
Usage Guidelines
When the partition avoidance command is enabled, a multiarea router withdraws a Level 1 area prefix from the Level 2 backbone when it no longer has any active adjacencies to that Level 1 area. This withdrawal prevents the Level 1 area from appearing to be partitioned within the Level 2 backbone.
In International Standards Organization (ISO) CLNS networks using a redundant topology, it is possible for an area to become "partitioned" when full connectivity is lost between a Level 1-2 border router, all adjacent Level 1 routers, and end hosts. In such a case, multiple Level 1-2 border routers advertise the Level 1 area prefix into the backbone area, even though any one router can reach only a subset of the end hosts in the Level 1 area.
When enabled, the partition avoidance command prevents this partitioning by causing the border router to stop advertising the Level 1 area prefix into the Level 2 backbone.
Other cases of connectivity loss within the Level 1 area itself are not detected nor corrected by the border router, and this command will have no effect.
Examples
The following example causes the routing process Finance to stop advertising the prefix for the area named "area1" with the router no longer has any active adjacencies to area1.
router isis Financepartition avoidance area1Related Commands
router isis
To enable the Intermediate System-to-Intermediate System (IS-IS) routing protocol and to specify an IS-IS process, use the router isis global configuration command. This command identifies an area the router will work in and lets the router know that it will be routing dynamically rather than statically. Use the no form of the command with the appropriate tag to disable IS-IS routing for the area.
router isis [area-tag]
no router isis [area-tag]
Syntax Description
Defaults
By default the router isis command is disabled.
Command Modes
Global configuration
Command History
10.0
This command was introduced.
12.0(5)T
Multiarea functionality was added, changing the way the tag argument is used.
Usage Guidelines
This command is used to enable routing for an area. An appropriate network entity title (NET) must be configured to specify the area address of the area and system ID of the router. Routing must be enabled on one or more interfaces before adjacencies may be established and dynamic routing is possible.
If you have IS-IS running and at least one International Standards Organization Interior Gateway Routing Protocol (ISO-IGRP) process, the IS-IS process and the ISO-IGRP process cannot both be configured without an area tag. The null tag can be used by only one process. If you run ISO-IGRP and IS-IS, a null tag can be used for IS-IS, but not for ISO-IGRP at the same time. However, each area in an IS-IS multiarea configuration should have a nonnull area tag to facilitate identification of the area.
You can configure only one IS-IS routing process to perform Level 2 (interarea) routing. You can configure this process to perform Level 1 (intra-area) routing at the same time. You can configure up to 29 additional processes as Level 1-only processes. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1.
An interface cannot be part of more than one area, except in the case where the associated routing process is performing both Level 1 and Level 2 routing. On media (such as WAN media, for example) where subinterfaces are supported, different subinterfaces could be configured for different areas.
If Level 2 routing is not desired for a given area, use the is-type command to remove Level 2. Level 2 routing can then be enabled on some other router instance.
Explicit redistribution between IS-IS instances is prohibited (prevented by the parser). In other words, you cannot issue a redistribute isis tag command in the context of another IS-IS router instance (router isis area-tag). Redistribution from any other routing protocol into a particular area is possible, and is configured per router instance, as in Release 12.0 of the Cisco IOS software, using the redistribute and route map commands. By default, redistribution is into Level 2.
If multiple Level 1 areas are defined, the Target Address Resolution Protocol (TARP) behaves in the following way:
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Examples
The following example starts IS-IS routing with the optional tag argument, where Pieinthesky is the value for the tag argument:
router isis PieintheskyThe following example specifies IS-IS as an IP routing protocol for a process named Finance, and specifies that the Finance process will be routed on interfaces Ethernet 0 and serial 0:
router isis Financenet 49.0001.aaaa.aaaa.aaaa.00interface Ethernet 0ip router isis Financeinterface serial 0ip router isis FinanceRelated Commands
Show Commands
In Cisco IOS Release 12.0(1)T or later, you can search and filter the output for show and more commands. This functionality is useful when you need to sort through large amounts of output, or if you want to exclude output that you do not need to see.
To use this functionality, enter a show or more command followed by the "pipe" character (|), one of the keywords begin, include, or exclude, and an expression that you want to search or filter on:
command | {begin | include | exclude} regular-expression
Following is an example of the show atm vc command in which you want the command output to begin with the first line where the expression "PeakRate" appears:
show atm vc | begin PeakRate
For more information on the search and filter functionality, refer to the Cisco IOS Release 12.0(1)T feature module titled CLI String Search.
The syntax for the following list of show commands listed has been changed to include an area tag. For example, the old syntax for the command to show all neighbors was as follows:
show clns neighbors
The new syntax is as follows:
show clns area-tag neighbors
If the area-tag argument is null, the output shows all neighbors. If the area tag is specified, however, output is limited to the specified area. For example, the following output shows area A3253-02 neighbors:
dtp-5# show clns A3253-02 neighborsArea A3253-02:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0053 Et2 0060.3e58.ccde Up 21 L1 IS-IS0000.0000.0002 Et2 0000.0c03.6bc5 Up 28 L1 IS-ISThe following show commands (and their output) have been similarly changed to include the area-tag argument:
show clns neighbor areas
To display information about Intermediate System-to-Intermediate System (IS-IS) neighbors and the areas to which they belong, use the show clns neighbor areas EXEC command.
show clns area-tag neighbor areas
Syntax Description
Command Modes
EXEC
Command History
Usage Guidelines
Use the show clns neighbor areas command to verify that all expected adjacencies are up with all neighbors. If they are not, recheck the area addresses specified in both routers. If the router is running in dual CLNS-IP mode in an area, verify that a valid IP address is configured on each interface in the area. Consider using the debug isis adjacency command to gather additional information.
Examples
The following example shows output when two Level 1 and one Level 2 IS-IS areas are configured.
dtp-5# show clns neighbor areasSystem Id Interface Area Name State Holdtime Type Protocol0000.0000.0009 Tu529 L2BB Up 26 L1L2 IS-IS0000.0000.0053 Et1 A3253-01 Up 21 L1 IS-IS0000.0000.0003 Et1 A3253-01 Up 28 L1 IS-IS0000.0000.0002 Et2 A3253-02 Up 22 L1 IS-IS0000.0000.0053 Et2 A3253-02 Up 23 L1 IS-ISThe following table describes the fields shown in the display.
Related Commands
show clns route
To display one or all of the destinations to which this router knows how to route Connectionless Network Service (CLNS) packets, use the show clns route EXEC command.
show clns route nsap
Syntax Description
Command Modes
EXEC
Command History
10.0
This command was introduced.
12.0(5)T
Fields for displaying information about prefix routes were added or changed.
Usage Guidelines
The show clns route command shows the Intermediate System-to-Intermediate System (IS-IS) Level 2 routing table and static and Intermediate System-to-Intermediate System Interior Gateway Routing Protocol (ISO-IGRP) learned prefix routes. This table stores IS-IS area addresses and prefix routes.
Examples
The following example shows output from the show clns route command when the nsap argument is not used:
rips# show clns routeISO-IGRP Routing Table for Domain 49.0002, Area 0007System Id Next-Hop SNPA Interface Metric Statemilles milles *HDLC* Se1 8476 Up0000.0000.0007 milles *HDLC* Se1 10476 Uprips 0000.0000.0000 -- -- 0 UpISO-IGRP Routing Table for Domain 49.0002Area Id Next-Hop SNPA Interface Metric State0002 0000.0000.0000 -- -- 0 UpCodes: C - connected, S - static, d - DecnetIVI - ISO-IGRP, i - IS-IS, e - ES-ISC 49.0002 [2/0], Local ISO-IGRP DomainC 49.0001.0000.0000.0005.00 [1/0], Local IS-IS NETC 49.0002.0007.0000.0000.0005.00 [1/0], Local ISO-IGRP NETC 49.0001 [2/0], Local IS-IS Areai 33.3333.3333 [110/10]via bakel, Ethernet0S 50.1234 [10/0], Discard EntryI 55.5555.5555 [100/8476]via milles, Serial1S 77.7777.7777.7777 [10/0]via Serial0d 88.8888.8888.0007 [120/0], DecnetIV Entryi 33.4567.8901 [110/10]via bakel, Ethernet0rips#The following table describes the fields shown in the display.
The following table describes how information about prefix routes is displayed:
The following is an example of output showing a single CLNS route using the show clns route command with the nsap argument:
rips# show clns route 33.3333.3333Routing entry for 33.3333.3333Known via "isis", distance 110, metric 10, Dynamic EntryRouting Descriptor Blocks:via bakel, Ethernet0isis, route metric is 10, route version is 4The following table describes the fields shown in the display:
Related Commands
show isis topology
To display a list of all connected routers in all areas, use the show isis topology EXEC command.
show isis area-tag topology
Syntax Description
Command Modes
EXEC
Command History
Usage Guidelines
Use the show isis topology EXEC command to verify the presence and connectivity between all routers in all areas.
Examples
The following example shows output from the show isis topology command in a dual Connectionless Network Service (CLNS)-IP network:
dtp-5# show isis topologyArea L2BB:IS-IS paths to level-1 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0005 --0000.0000.0009 10 0000.0000.0009 Tu529 *Tunnel*0000.0000.0017 20 0000.0000.0009 Tu529 *Tunnel*0000.0000.0053 30 0000.0000.0009 Tu529 *Tunnel*0000.0000.0068 20 0000.0000.0009 Tu529 *Tunnel*IS-IS paths to level-2 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0005 --0000.0000.0009 10 0000.0000.0009 Tu529 *Tunnel*0000.0000.0017 20 0000.0000.0009 Tu529 *Tunnel*0000.0000.0053 30 0000.0000.0009 Tu529 *Tunnel*0000.0000.0068 20 0000.0000.0009 Tu529 *Tunnel*Area A3253-01:IS-IS paths to level-1 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0003 10 0000.0000.0003 Et1 0000.0c03.69440000.0000.0005 --0000.0000.0053 10 0000.0000.0053 Et1 0060.3e58.ccdbArea A3253-02:IS-IS paths to level-1 routersSystem Id Metric Next-Hop Interface SNPA0000.0000.0002 10 0000.0000.0002 Et2 0000.0c03.6bc50000.0000.0005 --0000.0000.0053 10 0000.0000.0053 Et2 0060.3e58.ccdeRelated Commands
Glossary
area—Group of connected routers and end systems in a routing domain (or autonomous system). All routers in the domain share topology and adjacency data. If Level 1 (intra-area) routing is enabled in the domain, all routers know how to reach all system IDs. If Level 2 routing is enabled in the domain, all routers know how to reach all other areas.
conventional IS-IS—In this feature module, a router configured to perform intra-area (Level 1) IS-IS routing in a single area is considered to be conventionally configured. That router can also be configured to perform Level 2 (interarea) routing.
Intermediate System-to-Intermediate System Routing—See IS-IS.
IP—Internet Protocol.
IS-IS—Intermediate System-to-Intermediate System Protocol as defined by ISO 10589 and RFC 1195.
Level 1—Routers that establish Level 1 adjacencies in order to form a Level 1 area and perform intra-area routing.
Level 2—Routers that establish Level 2 adjacencies in order to form a Level 2 area and perform interarea routing.
link-state packet—See LSP.
link-state packet database—See LSDB.
LSDB—link-state packet database. Database of all link-state packets from all routers in an area.
LSP—link-state packet. Packet containing information about the link state on a router instance.
Open Systems Interconnect (model)—See OSI.
OSI—Open Systems Interconnect (model). Protocol stack with ISO CLNS at Layer 3.
routing domain—Group of areas that share topology and adjacency data.
shortest path first—See SPF.
SPF—shortest path first (Dijkstra's) algorithm.
