OSPF

[sunil0144]

hi Guys,

please help to get answer of below query .

1. two router with same router id in ospf become neighbor or not ??

[GRANT3779]

Hi,
Please see below from the following document. It makes sense if you think about the hello exchange.

Router IDs are used in order to identify each router in an OSPF network. Routers with the same Router ID will ignore HELLOs sent by each other, which prevents them from forming adjacency.

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13699-29.html

[Georg Pauwen]

Hello,

no. You will get the error below. Each router needs a unique router ID.

*Jun 30 09:19:22.472: %OSPF-4-DUP_RTRID_NBR: OSPF detected duplicate router-id 1.1.1.1 from 192.168.1.1 on interface GigabitEthernet0/0

[Giuseppe Larosa]

Hello Sunil0144,

be aware that OSPF is able to deal with duplicated OSPF RIDs only in the case they are directly connected as noted by other colleagues.

OSPF duplicated RIDs between distant OSPF routers cause more difficult to troubleshoot issues:

While all other routers may be able to work the two routers with same OSPF RID will not accept the Router LSA and any other LSAs (network. summary, ASBR summary or external) generated by the other device.

This can create problems of connectivity only for selected IP subnets that are not easy to troubleshoot.

Best practice is to use domain wide unique OSPF RIDs, even if technically two internal routers in two different areas could re-use the same OSPF router-id. But again for easy of troubleshooting and management this should not be done.

Other possible effects of introducing a new router with a duplicated RID are possible like: the tearing down of iBGP session destined to the OSPF RID when OSPF RID = loopback address.

To be noted the concept of Router-id has been introduced in EIGRP last implementation for all kinds of routes.

In previous EIGRP implementation only  EIGRP external routes had an EIGRP RID field.

Hope to help

Giuseppe