ISIS and OSPF together

Sun May 12 08:41:37 UTC 2013

Hi,

I would like to understand the scenarios wherein the service
provider/network admin might run both ISIS and OSPF together inside their
network. Is this something that really happens out there?

One scenario that i can think of when somebody might run the 2 protocols
ISIS and OSPF together for a brief period is when the admin is migrating
from one IGP to the other. This, i understand never happens in steady
state. The only time this can happen is if an AS gets merged into another
AS (due to mergers and acquisitions) and the two ASes happen to run ISIS
and OSPF respectively. In such instances, there is a brief period when two
protocols might run together before one gets turned off and there is only
one left.

The other instance would be when say OSPF is used to manage the OOB network
and the ISIS is used for network reachability.

Is there any other scenario?

Glen