Discussion

Solution
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# set preference 175
Discussion
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 strategy. The first step is to configure IS-IS on the same interfaces that are
running OSPF (see Recipe 11.1), then verify that IS-IS adjacencies are established on
the same interfaces that have OSPF adjacencies:
aviva@Router3> show isis adjacency
Interface System L State Hold (secs) SNPA
so-0/2/0.0 R1 2 Up 20
so-0/2/1.0 R2 2 Up 21
so-0/2/2.0 R4 2 Up 19
so-0/2/3.0 R10 2 Up 19
aviva@Router3> show ospf neighbor
Address Interface State ID Pri Dead
10.1.2.1 so-0/2/0.0 Full 10.10.255.1 128 36
10.1.6.1 so-0/2/1.0 Full 10.10.255.2 128 38
10.1.7.2 so-0/2/2.0 Full 10.10.255.4 128 34
10.1.8.2 so-0/2/3.0 Full 10.10.255.10 128 38
You then need to ensure that IS-IS calculates the same paths as OSPF. The default IS-IS
metrics must be changed so that they give the same relative cost to each path as does
OSPF. You can do this by manually assigning appropriate interface metrics or by configuring
a reference bandwidth. (See Recipe 11.10 for IS-IS and Recipe 12.11 for OSPF.)
Changing Route Preferences to Migrate to Another IGP | 275
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