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 20.20.20.1 YES NVRAM up up
Serial3 30.30.30.1 YES NVRAM up up
Loopback0 1.1.1.1 YES manual up up

R1#sh running-config | begin ospf
router ospf 100
log-adjacency-changes
network 1.1.1.1 0.0.0.0 area 0
network 20.20.20.0 0.0.0.255 area 0
network 30.30.30.0 0.0.0.255 area 0
!
ip classless
no ip http server
!
line con 0
line aux 0
line vty 0 4
login
!
end

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 223.223.223.223
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
1.1.1.1 0.0.0.0 area 0
20.20.20.0 0.0.0.255 area 0
30.30.30.0 0.0.0.255 area 0
Routing Information Sources:
Gateway Distance Last Update
223.223.223.223 110 00:00:38
Distance: (default is 110)

R1#


Note that the OSPF router ID is seen to be 223.223.223.223 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 223.223.223.223?

Comments:
The loopback address 223.223.223.223 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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