Randy is correct. In most cases, the two protocols are running co-incident for a while so you can do your table validation and topology mapping and then you turn off OSPF. For vendors that aren't capable of supporting ISIS, this is a feature and not a bug. On Sun, May 12, 2013 at 1:57 AM, Randy Bush <randy@psg.com> wrote:
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.
no. some ops come to see the light and move their network from ospf to is-is. see vijay gill's nanog preso http://nanog.org/meetings/nanog29/presentations/aol-backbone.ram