IS-IS

IS-IS
Like OSPF, IS-IS does not require you to set a metric. If a metric is not defined, the default will
be 0. IS-IS does understand 0 as a valid metric. You should still define a metric, however. Unlike
the other routing protocols, IS-IS does not support the default-metric command, which
requires you to set the metric on the redistribution line.
When redistributing into IS-IS, it is suggested that you specify whether the routes are internal or
external and the level of the routes being redistributed. If you do not specify these things, IS-IS will
default redistributed routes to internal Level 2 routes. This in turn will represent the metrics of these
connections the same way it would represent the metrics of any internal IS-IS route. So it is advisable
if you are redistributing routes into IS-IS to set them as external routes. This way, the metric will be
higher and will better represent that the route is not an internal route to the IS-IS process.
It is important to note that IS-IS is a classless routing protocol and doesn’t require extra configuration
in order to redistribute classless routes. Let’s refer to Figure 10.10.
In this example, we’re going to configure R1 so that the routes from EIGRP AS 100 are
redistributed into IS-IS. We will set the metric of these routes to 10 and set them to external
Level 2 routes:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router isis
R1(config-router)#redistribute eigrp 100 metric 10 metric-type external
R1(config-router)#^Z
R1#
Other routing protocols do not require you to specify anything about them when redistributing
their routes into another routing protocol, but IS-IS does. When redistributing IS-IS into
another routing protocol, you must specify whether the routes you want to redistribute are
Level 1 (level-1 keyword), Level 2 (level-2 keyword), or Level 1/Level 2 (level-1-2 keyword)
routes. If you specify Level 1 routes, no Level 2 routes will be redistributed, and vice
versa. So in order to redistribute all routes from IS-IS into another routing protocol, you must
specify Level 1 and Level 2 routes.