Configuring OSPF Local RIB

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 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

  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:


Device(config-router)# end

Returns to privileged EXEC mode.

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.

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

  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.

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.

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