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.

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.

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.

Note	It is recommended that you not change the default values because they are conservative and preserve the current global RIB behavior.

1.    enable

2.    configure terminal

3.    router ospf process-id [vrf vpn-name]

4.    local-rib-criteria [forwarding-address] [inter-area-summary] [nssa-translation]

5.    end

6.    show ip ospf process-id rib [redistribution] [network-prefix] [network-mask] [detail]

Note	It is recommended to keep the default settings, but you can follow the steps in this section to change the administrative distance for discard routes.

1.    enable

2.    configure terminal

3.    router ospf process-id [vrf vpn-name]

4.    discard-route [external [distance]] [internal [distance]]

5.    end

6.    show ip route [ip-address [mask] [longer-prefixes] | protocol [process-id] | list [access-list-number | access-list-name] | static download]

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