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Table Of Contents
Prerequisites for OSPFv2 Local RIB
Restrictions for OSPFv2 Local RIB
Information About OSPFv2 Local RIB
Function of the OSPF Local RIB
How to Configure the OSPFv2 Local RIB Feature
Changing the Default Local RIB Criteria
Changing the Administrative Distance for Discard Routes
Configuration Examples for the OSPFv2 Local RIB Feature
Changing the Default Local RIB Criteria: Example
Changing the Administrative Distance for Discard Routes: Example
Feature Information for the OSPFv2 Local RIB Feature
OSPFv2 Local RIB
First Published: June 28, 2007Last Updated: June 28, 2007With the OSPFv2 Local RIB feature, each OSPF protocol instance has its own local Routing Information Base (RIB). The OSPF local RIB serves as the primary state for OSPF SPF route computation. The global RIB is not updated with intermediate results during the SPF. Instead, the global RIB is updated only when routes are added, deleted, or changed, thereby reducing global RIB computation. This reduced update activity may result in fewer dropped packets.
This feature is enabled by default and does not need to be configured. This document describes some optional configuration tasks to modify how the global and local RIBs function, although it is recommended to keep the default settings.
Finding Feature Information in This Module
Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for the OSPFv2 Local RIB Feature" section.
Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for OSPFv2 Local RIB
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Prerequisites for OSPFv2 Local RIB
Before this feature is configured, the OSPF routing protocol must be configured.
Restrictions for OSPFv2 Local RIB
This feature is available only for IP Version 4 networks.
Information About OSPFv2 Local RIB
Before you configure this feature, you should understand the following concept:
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Function of the OSPF Local RIB
A router that is running OSPFv2 maintains a local RIB in which it stores all routes to destinations that it has learned from its neighbors. At the end of each SPF, OSPF attempts to install the best (that is, the least-cost) routes to a destination present in the local RIB into the global IPv4 routing table. The global RIB will be updated only when routes are added, deleted, or changed. Routes in the local RIB and Forwarding Information Base (FIB) will not compute when intermediate results are computed during SPF, resulting in fewer dropped packets in some circumstances.
How to Configure the OSPFv2 Local RIB Feature
Although it is recommnded to keep the default settings for the commands described in the following sections, it is optional to change the defaults settings. This section describes the following otional tasks:
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Changing the Default Local RIB Criteria
By default, the contents of the global RIB are used to compute inter-area summaries, NSSA translation, and forwarding addresses for type-5 and type-7 LSAs. Each of these functions can be configured to use the contents of the OSPF local RIB instead of the global RIB for their computation. Using the local RIB for the computation may be slightly faster in some circumstances, but because the local RIB has information for only a particular instance of OSPF, using it for the computation may yield incorrect results. Potential problems that may occur include routing loops and black-hole routes. It is recommended that you not change the default values because they are conservative and preserve the current global RIB behavior.
SUMMARY STEPS
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DETAILED STEPS
Changing the Administrative Distance for Discard Routes
By default, OSPF installs discard routes to null0 for any area range (internal) or summary-address (external) prefixes that it advertises to other routers. Installation of a discard route can prevent routing loops in cases where portions of a summary do not have a more specific route in the RIB. Normally, internal discard routes are installed with an administrative distance of 110, while external discard routes have an administrative distance of 254.
There may be rare circumstances, however, when some other values are needed. For example, if one OSPF process installs a route that exactly matches an area range configured on another OSPF process, the internal discard routes for the second OSPF process could be given a higher (less desirable) administrative distance.
Although it is recommended to keep the default settings, you can follow the steps in this section to change the administrative distance for discard routes.
SUMMARY STEPS
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DETAILED STEPS
Examples
The sample output displayed for the show ip route command confirms that the administrative distance for the IP route 192.168.0.0 255.255.255.0 is 110.
Router# show ip route 192.168.0.0 255.255.255.0
Routing entry for 192.168.0.0/24
Known via "ospf 1", distance 110, metric 0, type intra area
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 0, traffic share count is 1
Troubleshooting Tips
You can research the output from the debug ip ospf rib command to learn about the function of the local RIB and the interaction between the route redistribution process and the global RIB. For example, you can learn why the routes that OSPF placed in the global RIB are not the same ones that you anticipated.
Configuration Examples for the OSPFv2 Local RIB Feature
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Changing the Default Local RIB Criteria: Example
In the following example, the local-rib-criteria command is entered without any keywords to specify that the local RIB will be used as criteria for all of the following options: forwarding address, inter-area summary, and NSSA translation.
router ospf 1router-id 10.0.0.6local-rib-criteriaChanging the Administrative Distance for Discard Routes: Example
In the following example, the administrative distance for external and internal discard routes is set to 25 and 30, respectively.
router ospf 1router-id 10.0.0.6log-adjacency-changesdiscard-route external 25 internal 30area 4 range 10.2.0.0 255.255.0.0summary-address 192.168.130.2 255.255.255.0redistribute static subnetsnetwork 192.168.129.2 0.255.255.255 area 0network 192.168.130.12 0.255.255.255 area 0The output from the show ip route command verifies that the administrative distance for the internal route 10.2.0.0/16 is set to 30.
Router# show ip route 10.2.0.0 255.255.0.0Routing entry for 10.2.0.0/16Known via "ospf 1", distance 30, metric 1, type intra area
Routing Descriptor Blocks:* directly connected, via Null0Route metric is 1, traffic share count is 1The output from the show ip route command verifies that the administrative distance for the external route 192.168.130.2/24 is set to 25.
Router# show ip route 192.168.130.2 255.255.255.0Routing entry for 192.168.130.2/24Known via "ospf 1", distance 25, metric 20, type intra areaRouting Descriptor Blocks:* directly connected, via Null0Route metric is 20, traffic share count is 1Additional References
The following sections provide references related to the OSPFv2 Local RIB feature.
Related Documents
Standards
MIBs
None
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
Command Reference
This section documents only commands that are new or modified.
New Commands
Modified Commands
debug ip ospf rib
To display debugging information for Open Shortest Path First (OSPF) Version 2 routes in the global or local Routing Information Base (RIB), use the debug ip ospf rib command in privileged EXEC mode. To disable the debugging of OSPF Version 2 routes, use the no form of this command.
debug ip ospf rib [local | [redistribution | global [access-list-number]]] [detail]
no debug ip ospf rib [local | [redistribution | global [access-list-number]]] [detail]
Syntax Description
Command Modes
Privileged EXEC (#)
Command History
Usage Guidelines
You can use the output from the debug ip ospf rib command to learn about the function of the local RIB and the interaction between the route redistribution process and the global RIB. For example, you can learn why the routes that OSPF placed in the global RIB are not the same ones that you anticipated.
A Cisco Technical Assistance Center representative may ask you to turn on debugging using the debug ip ospf rib command as part of troubleshooting a problem.
To monitor updates from the OSPF database to the OSPF local RIB, use the local keyword, and to monitor updates from the OSPF database to the OSPF global RIB, use the global keyword.
It is highly recommended that you limit the debugging output to information specific to the IP prefix that is associated with a specific access list by entering the access-list-number argument.
Examples
The following is sample output from the debug ip ospf rib command with the access-list-number argument used in order to limit the debugging output to information specific to the IP prefix that is associated with the specific access list 1:
Router# show running-config | include access-list 1access-list 112 permit 10.1.1.0 0.0.0.255! access-list 1 is configuredRouter# debug ip ospf rib local detail 1*May 31 21:28:17.331: OSPF-RIB-LOCAL: Delete intra-area connectedroute 192.168.130.2/255.255.255.0, area 1, dist 10, for interfaceEthernet0/0.1*May 31 21:28:17.331: OSPF-RIB-LOCAL: Local RIB process OSPF-1Router clear*May 31 21:28:17.331: OSPF-RIB-LOCAL: Add intra-area connectedroute 192.168.130.2/255.255.255.0, area 1, dist 10, for interfaceEthernet0/0.1...discard-route
To reinstall an external or internal discard route that was previously removed, use the discard-route command in router address family topology or router configuration mode. To remove an external or internal discard route, use the no form of this command.
discard-route [external [distance]] [internal [distance]]
no discard-route [external [distance]] [internal [distance]]
Syntax Description
Defaults
External and internal discard-route entries are installed.
Command Modes
Router address family topology configuration (config-router-af-topology)
Router configuration (config-router)Command History
Usage Guidelines
External and internal discard-route entries are installed in routing tables by default. During route summarization, routing loops may occur when data is sent to a nonexisting network that appears to be a part of the summary, and the router that is performing the summarization has a less specific route (pointing back to the sending router) for this network in its routing table. To prevent the routing loop, a discard route entry is installed in the routing table of the ABR or ASBR.
If for any reason you do not want to use the external or internal discard route, remove the discard route by entering the no discard-route command with the external or internal keyword.
Release 12.2(33)SRB
If you plan to configure the Multi-Topology Routing (MTR) feature, you need to enter the discard-route command in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.
Release 12.4(15)T
Prior to Cisco IOS Release 12.4(15)T, both external and internal discard routes were installed using the default OSPF intra-area administrative distance 110. You can now modify this default distance for discard routes by entering a new administrative distance for the distance argument of the discard-route command.
Examples
The following display shows the discard-route functionality installed by default. When external or internal routes are summarized, a summary route to Null0 will appear in the router output from the show ip route command. See the router output lines that appear in bold font:
Router# show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGPD - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter areaN1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGPi - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2ia - IS-IS inter area, * - candidate default, U - per-user static routeo - ODR, P - periodic downloaded static routeGateway of last resort is not set172.16.0.0/24 is variably subnetted, 3 subnets, 2 masksC 172.16.0.128/25 is directly connected, Loopback1O 172.16.0.0/24 is a summary, 00:00:14, Null0C 172.16.0.0/25 is directly connected, Loopback0172.31.0.0/24 is variably subnetted, 3 subnets, 2 masksC 172.31.0.128/25 is directly connected, Loopback3O 172.31.0.0/24 is a summary, 00:00:02, Null0C 172.31.0.0/25 is directly connected, Loopback2C 192.168.0.0/24 is directly connected, Ethernet0/0Router# show ip route ospf172.16.0.0/24 is variably subnetted, 3 subnets, 2 masksO 172.16.0.0/24 is a summary, 00:00:29, Null0172.16.0.0/24 is variably subnetted, 3 subnets, 2 masksO 172.16.0.0/24 is a summary, 00:00:17, Null0When the no discard-route command with the internal keyword is entered, notice the following route change, indicated by the router output lines that appear in bold font:
Router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Router(config)# router ospf 1Router(config-router)# no discard-route internalRouter(config-router)# endRouter# show ip route ospf172.31.0.0/24 is variably subnetted, 3 subnets, 2 masksO 172.16.0.0/24 is a summary, 00:04:14, Null0Next, the no discard-route command with the external keyword is entered to remove the external discard route entry:
Router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Router(config)# router ospf 1Router(config-router)# no discard-route externalRouter(config-router)# endThe following router output from the show running-config command confirms that both the external and internal discard routes have been removed from the routing table of the router. See the router output lines that appear in bold font.
Router# show running-configBuilding configuration...Current configuration : 1114 bytes!version 12.2service timestamps debug uptimeservice timestamps log uptimeno service password-encryption!hostname Router...router ospf 1log-adjacency-changesno discard-route externalno discard-route internalarea 1 range 172.16.0.0 255.255.255.0summary-address 172.31.0.0 255.255.255.0redistribute rip subnetsnetwork 192.168.0.0 0.0.0.255 area 0network 172.16.0.0 0.0.0.255 area 1!Related Commands
local-rib-criteria
To specify that the OSPF local Routing Information Base (RIB) will be used for route validation, use the local-rib-criteria command in router configuration mode. To remove local RIB route validation, use the no form of this command.
local-rib-criteria [forwarding-address] [inter-area-summary] [nssa-translation]
no local-rib-criteria [forwarding-address] [inter-area-summary] [nssa-translation]
Syntax Description
Command Default
The global RIB (not the local RIB) is used for route validation.
Command Modes
Router configuration (config-router)
Command History
Usage Guidelines
If the local-rib-criteria is command is entered without any optional keywords, the local RIB will be used as criteria for all of the options (forwarding address, inter-area summary, and NSSA translation).
You can enter the local-rib-criteria command with one or more of the optional keywords to configure the OSPF routing process to use the local RIB only for forwarding address, inter-area-summary, or NSSA translation route generation.
Note
Note
Examples
The following example specifies that the local RIB should be used as the criteria for NSSA translation:
router ospf 23local-rib-criteria nssa-translationThe following example enables all local RIB criteria options:
router ospf 1
local-rib-criteria
The following example specifies that the local RIB will be used only for inter-area summary route generation:
router ospf 1
local-rib-criteria inter-area-summary
show ip ospf rib
To display information for the OSPF local Routing Information Base (RIB) or locally redistributed routes, use the show ip ospf rib command in privileged EXEC mode.
show ip ospf process-id rib [redistribution] [network-prefix] [network-mask] [detail]
Syntax Description
Command Modes
Privileged EXEC (#)
Command History
Usage Guidelines
If the network-prefix and network-mask arguments are both entered, only the route that matches the network prefix and IP address mask is displayed. If only the prefix argument is entered, a longest prefix lookup is performed and the matching route is displayed.
Examples
The following example displays information about locally redistributed routes:
Router# show ip ospf 1 rib redistribution 192.168.240.0OSPF Redistribution for Process 1192.168.240/20, metric 0, tag 0, from OSPF Router 130Attributes 0x1000220, event 1via Ethernet0/0OSPF Redistribution Process 130Table 1 describes the significant fields shown in the display.
Feature Information for the OSPFv2 Local RIB Feature
Table 2 lists the release history for this feature.
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note
Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.
© 2007 Cisco Systems, Inc. All rights reserved.
