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show ip ospf neighbor detail

Hello, regarding the output of this 'show ip ospf neighbor detail' command which has been run from the router 172.16.51.3 (I have attached an image of the link etc).

I am confused by the neighbor IP address and the interface address.

I presume the neighbor address is the router ID ?

If so, how could that be, as the interface address is higher, and I know OSPF chooses the highest active address as router ID.

Should not the router ID be 172.16.51.1 ?

Thanks for any help.

1 ACCEPTED SOLUTION

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Hello Philip,

> thanks, I have no explicit configuration of router address and no loopback addresses configured, so how could 172.16.2.1 become the router ID ?

When OSPF process started to run, after its configuration it picked the highest IP address in the box in that moment, if you have configured the serial interfaces later, then the OSPF RID was taken from the fastethernet interface.

IF you execute a

clear ospf process

the OSPF process will restart and will choice the highest IP address in the box as router-id.

This is why the router-id is manually configured to provide consistency in a network.

Use of loopbacks is also good practice to provide router-id IP address.

Hope to help

Giuseppe

View solution in original post

5 REPLIES 5
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Rising star

Hello,

The (172.16.2.1) is the Router-id of the Neighbor, the (172.16.51.1) is the neighbor Interface address.  and the reason they are not the same because OSPF has special criteria to select its Router-id, the Router-id is chossen based on explicitly configured (router-id) command under router OSPF configuration, if not, then its choosen based on the highest Loopback interface , if its not, then the highest Physical address.

HTH

Mohamed

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thanks, I have no explicit configuration of router address and no loopback addresses configured, so how could 172.16.2.1 become the router ID ?

172.16.51.1 is the higher address so surely it should be the router ID ?

Thanks for any help.

Highlighted

This shouldnt be the case! These information is exchanged with the OSPF Hello Packets before establishing the djacency.

Try to manually reset the neighbors and check back.

Regards,

Mohamed

Highlighted

Now I am thinking perhaps when I was playing around with the network, I reloaded the config and then activated the interfaces one by one. Perhaps I activated 172.16.2.1 first. Seeing as that was the first activated on the router, OSPF grabbed it for it's router ID.

Perhaps that could explain things.

Highlighted

Hello Philip,

> thanks, I have no explicit configuration of router address and no loopback addresses configured, so how could 172.16.2.1 become the router ID ?

When OSPF process started to run, after its configuration it picked the highest IP address in the box in that moment, if you have configured the serial interfaces later, then the OSPF RID was taken from the fastethernet interface.

IF you execute a

clear ospf process

the OSPF process will restart and will choice the highest IP address in the box as router-id.

This is why the router-id is manually configured to provide consistency in a network.

Use of loopbacks is also good practice to provide router-id IP address.

Hope to help

Giuseppe

View solution in original post

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