Glen, One transition scenario you noted below is often a use case. I have seen networks move from OSPF to IS-IS (more cases then the reverse). In those cases, the overlap period may not be very short (years vs. weeks/months). I have also seen some use one protocol (which I think was mentioned in another response) used for IPv4 and another used for IPv6. The cases I am familiar, tended to be IPv6 with IS-IS and IPv4 with OSPFv2. I guess the reasoning here was that if you are running dual stack, with OSPF you will need to run two protocols anyway, so running OSPFv2(IPv3) and OSPFv3(IPv6) may not be that different then running OSPFv2(IPv4) with IS-IS(IPv6). This dual stack option has run longer or is semi-permanent at times. A sub-case to the above may also be that one (operator) may want to leverage some of capabilities of IS-IS and may not be willing to get off OSPF for some reason. The Multi-topology option in IS-IS may be quite useful if you have some functions which are non-congruent in your network and you want to maintain topology variations (multicast being one, or in-band management which I believe was alluded to in your OOB use case) Regards, Victor K On 2013-05-12 4:41 AM, "Glen Kent" <glen.kent@gmail.com> wrote:
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