Discussion

Unlike OSPF, for which the JUNOS software has an automatic costing algorithm
based on interface bandwidth, for IS-IS JUNOS software assigns a default metric of
10 to all IS-IS interfaces. If this default is left unchanged, the shortest paths calculated
by IS-IS are essentially shortest-hop paths.
With IS-IS enabled and metrics configured correctly, the routing table should have
the same number of IS-IS entries as OSPF entries for each destination, and they
should point to the same outgoing next hops. That is, if there is one OSPF route to a
prefix, there should be one IS-IS route to the same prefix and with the same outgoing
interface. If there are two OSPF routes to a prefix, there should be two IS-IS
routes, and so on.
aviva@Router3> show route
inet.0: 28 destinations, 48 routes (28 active, 0 holddown, 0 hidden)
Restart Complete
+ = Active Route, - = Last Active, * = Both
10.1.1.0/24 *[OSPF/10] 00:16:20, metric 65
via so-0/2/0.0
> via so-0/2/1.0
[IS-IS/18] 00:16:08, metric 126
to 10.1.2.1 via so-0/2/0.0
> to 10.1.6.1 via so-0/2/1.0
10.1.3.0/24 *[OSPF/10] 00:16:20, metric 65
via so-0/2/0.0
> via so-0/2/2.0
[IS-IS/18] 00:16:08, metric 126
> to 10.1.2.1 via so-0/2/0.0
to 10.1.7.2 via so-0/2/2.0
...
The output shows that for each destination prefix, OSPF and IS-IS have the same
number of routes and the same outgoing interfaces. The preferred route for each prefix
(marked with an *) is the OSPF route. If there are equal-cost paths, the next hop
chosen by OSPF (>) is not always the same as that chosen by IS-IS. This will not
cause routing problems as long as the equal-cost paths are consistent between the
protocols.
At this point, you change the route preference using the set protocols ospf
preference command shown in this recipe. Here, you increase the OSPF preference
to a value higher than any of the IS-IS preferences because OSPF is the protocol that
you are migrating away from. You could instead change the IS-IS route preference so
that its routes are more preferred than OSPF. However, leaving the IS-IS preference
at its default value simplifies any future addition of routers to the network. Another
alternative at this point would be to delete OSPF from the configuration altogether.
However, you should leave it in for a short period of time to test the migration. If
there are any problems and you need to back out of the migration, all you need to do
is return to the default OSPF preference value:
[edit protocols ospf]
aviva@Router3# delete preference
aviva@Router3# commit
276 | Chapter 8: IP Routing
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