Cisco IOS XE: Настройка локального RIB OSPF - руководство по эксплуатации (EN)

Information About OSPF Local RIB

Configuring OSPF Local RIB Prerequisite for OSPF Local RIB, on page 1 Restriction for OSPF Local RIB, on page 1 Information About OSPF Local RIB, on page 1 How to Configure OSPF Local RIB, on page 2 Configuration Examples for OSPF Local RIB, on page 4 Feature History for OSPF Local RIB, on page 8 Prerequisite for OSPF Local RIB Before configuring this feature, ensure the OSPF routing protocol is configured. Restriction for OSPF Local RIB This feature is available only for IP Version 4 networks. Information About OSPF Local RIB A router that is running OSPF 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. 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 null routes. It is recommended that you not change the default values because they are conservative and preserve the current global RIB behavior.

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 Configuring OSPF Local RIB 1

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Changing the Default Local RIB Criteria

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.

OSPF Local RIB Path Limit The OSPF Local RIB Path Limit feature allows network administrators to control the number of paths OSPF installs in its Local RIB for a specific prefix. How to Configure OSPF Local RIB Although it is recommended to keep the default settings for the commands described in the following sections, it is optional to change the defaults settings. Changing the Default Local RIB Criteria Procedure Configuring OSPF Local RIB How to Configure OSPF Local RIB Command or Action Purpose Step 1 enable Example: Device> enable Enables privileged EXEC mode. Enter your password, if prompted. Step 2 configure terminal Example:

Device# configure terminal Enters global configuration mode. Step 3 router ospf process-id [vrf vpn-name] Example: Device(config)# router ospf 23 Configures an OSPFv2 routing process and enters router configuration mode. Step 4 local-rib-criteria [forwarding-address] [inter-area-summary] [nssa-translation] Example: Device(config-router)# local-rib-criteria forwarding-address Specifies that the OSPF local RIB will be used for route validation. Step 5 end Example: Returns to privileged EXEC mode. Configuring OSPF Local RIB 2

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Changing the Administrative Distance for Discard Routes

Changing the Administrative Distance for Discard Routes It is recommended that you keep the default settings. However, you can follow the steps in this section to change the administrative distance for discard routes. Procedure Configuring OSPF Local RIB Changing the Administrative Distance for Discard Routes Command or Action Purpose Device(config-router)# end Step 6 show ip ospf process-id rib [redistribution] [network-prefix] [network-mask] [detail] Example: Device# show ip ospf 23 rib Displays information for the OSPF local RIB or locally redistributed routes. Command or Action Purpose Step 1 enable Example: Device> enable Enables privileged EXEC mode. • Enter your password if prompted.

Step 2 configure terminal Example: Device# configure terminal Enters global configuration mode. Step 3 router ospf process-id [vrf vpn-name] Example: Device(config)# router ospf 23 Configures an OSPFv2 routing process and enters router configuration mode. Step 4 discard-route [external [distance]] [internal [distance]] Example: Device(config-router)# discard-route external 150 Specifies the administrative distance to be used for internal and external discard routes. Note You can now specify the administrative distance for internal and external discard routes. Step 5 end Example: Device(config-router)# end Returns to privileged EXEC mode. Configuring OSPF Local RIB 3

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Configuration Examples for OSPF Local RIB

Example The sample output displayed for the show ip route command confirms that the administrative distance for the IP route 192.168.0.0/24 is 110. Device# 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 Configuration Examples for OSPF Local RIB Example: Configuring OSPF Local RIB In the following example, when the maximum-paths parameter is set to four in the default configuration mode, RIB has successfully installed four paths.

Device# show ip route ospf 45.1.1.0 [110/30] via 13.1.20.2, 00:03:03, Ethernet0/2.20 [110/30] via 13.1.19.2, 00:03:03, Ethernet0/2.19 [110/30] via 13.1.18.2, 00:03:03, Ethernet0/2.18 [110/30] via 13.1.17.2, 00:03:03, Ethernet0/2.17 The following is a sample output of the show ip ospf rib command. In the given example, the default value is set to four for maximum-paths. Within the OSPF Local RIB, it is visible that the first four paths have been successfully installed into RIB and the last two paths have been rejected because of exceeding maximum-path limit.

Configuring OSPF Local RIB Configuration Examples for OSPF Local RIB Command or Action Purpose Step 6 show ip route [ip-address [mask] [longer-prefixes] | protocol [process-id] | list [access-list-number | access-list-name] | static download] Example: Device# show ip route ospf 23 Displays the current state of the routing table. Note Entering the show ip route command will verify the changed administrative distance values for external and internal discard routes. Configuring OSPF Local RIB 4

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