You are migrating the IGP on your network from OSPF to IS-IS and you want to do this in a controlled manner.
After configuring both OSPF and IS-IS on the routers you are migrating, change the route preference on the OSPF routes to be higher than the IS-IS preference:
[edit protocols ospf] aviva@Router3#
While you can change the preference values for all dynamic routing protocols and user-configured routes using the
set preference and
set external-preference commands when configuring the protocols, it is generally not a good idea. The changes you make affect only the router you are configuring, and the local router will end up with a different idea of relative route preferences than the other routers on the network. Also,
changing route preferences could affect which routes become active, which, in turn, would affect which routes are used to forward traffic.
However, one situation where you might want to change route preferences is when migrating IGPs. In this recipe, we are migrating from using OSPF as the IGP to using IS-IS. A preference of 175 for OSPF is greater than that of any IS-IS routes. From Table 8-2, you see that IS-IS internal routes have a preference of either 15 (for Level 1) or 18 (for Level 2), and external routes have a preference of 160 (for Level 1) or 165 (for Level 2).
The command shown in this recipe is actually the last step in an OSPF-to-IS-IS migration ...