OSPF Related

Unanswered Question
Jan 11th, 2010

Dear Netpros,

My Question is related about OSPF router ID.

We were trying a scenerio on OSPF. We have configured OSPF on router without loopback interface. By default OSPF was taking router's highest IP address as router ID. Then we have configured a loopback interface "Loopback 0" on router with IP address .

After which OSPF router ID change to (i.e Loopback interface ID). then again we have configured a another Loopback interface on same router "Loopback 1" with IP address But this time the router ID remain unchanged i.e it was only. But as we know the Highest IP address of router should be router ID(in case if loopback interface is not created).

But if we having two loopback address (or more)  then also the Highest IP address out all loopback should be the Router ID. but in our case is still showing as router ID even after presence of Higher loopback Address it is like this. Kindly clear.


Another Question :-

We have a server say with IP placed at Branch-A.

Reachability to that server from our LAN of other branch (Branch-B) on BGP Path. BGP is configured on the L3 Switch in Branch-B and connection between this L3 Switch and Branch-B router(Router-1) is on OSPF protocol.

From one more our another branch (Branch-C). we don’t have the reachability to the server placed at Branch-A and we have OSPF running from Branch-B(Router-1) to Branch-C(Router-2).

When adding a static route for /32 of the destination on Branch-C router (router-2) . the packet is getting looped between Branch-A router (router-1) and L3 switch. But it is working when we add /24 i.e. whole network …Branch-A server reachable properly.

Kindly clear the cause for this….


I have this problem too.
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Giuseppe Larosa Mon, 01/11/2010 - 09:54

Hello cisco.net,

OSPF router-id is chosen at OSPF process startup:

if later a new loopback is configured with an higher IP address the OSPF router-id not changes until:

or the OSPF process is restarted with clear ip ospf process-id

or the whole router is restarted with a reload.

best practice to ensure consistency over time is to use router-id command under router ospf process.

router-(config-router)#router-id ?
  A.B.C.D  OSPF router-id in IP address format

to be noted the IP address given here doesn't need to exist on the router.

OSPF router-id has to be unique in OSPF domain.

About second question:

more details are needed provide configurations of OSPF and BGP processes on involved devices and sh ip route of Branch A LAN IP subnet where the server is connected taken from all devices with and without that static route that fixed the issue.

There are chances that something has gone wrong with redistribution between OSPF and BGP that probably happen on branch WAN routers.

Hope to help



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