Wednesday, April 12, 2006


Strange OSPF Router ID (Question #53)

Consider the following output from a router running OSPF

R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
Serial2 YES NVRAM up up
Serial3 YES NVRAM up up
Loopback0 YES manual up up

R1#sh running-config | begin ospf
router ospf 100
network area 0
network area 0
network area 0
ip classless
no ip http server
line con 0
line aux 0
line vty 0 4

R1#sh ip protocols
Routing Protocol is "ospf 100"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks: area 0 area 0 area 0
Routing Information Sources:
Gateway Distance Last Update 110 00:00:38
Distance: (default is 110)


Note that the OSPF router ID is seen to be even though the only loopback interface in the system does not have that IP address and the router-id is not explicitly set under the router ospf command. How could it be possibly be that given this configuration the OSPF router id is

The loopback address probably existed before when the router ospf 100 was created but has since disappeared. This router-id will remain until ospf process 100 is restarted.
Yes, that's right Rich.
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