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ISIS and OSPF together


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.

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)


Victor K


On 2013-05-12 4:41 AM, "Glen Kent" <glen.kent at gmail.com> wrote:

>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
>and the ISIS is used for network reachability.
>Is there any other scenario?