OSPF redistribution

FIGURE 1 0 . 9 OSPF redistribution
We will need to configure redistribution from EIGRP AS 100 into OSPF 1 on R1. We will
assign the metric of 100 to the redistributed routes. We also need to make sure that the redistributed
routes are in their classless form:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#redistribute eigrp 100 metric 100 subnets
R1(config-router)#^Z
R1#
OSPF also allows you to define what type of external routes the redistributed routes are:
Type 1 or Type 2. To accomplish this, you need to add the metric-type keyword to the redistribution
line specifying the type of route.
Refer back to the network in Figure 10.9. The same redistribution needs to occur, except this
time we need to make sure that all routes coming from EIGRP AS 100 are marked as Type 2
external routes:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#redistribute eigrp 100 metric 100 subnets metric-type 2
R1(config-router)#^Z
R1#
If you would like to set a default metric for all routes redistributed into OSPF, you would use
the default-metric command with the value of the metric.
An important item to note is that you can run multiple OSPF processes on the same router.
These different OSPF processes do not automatically redistribute between themselves. You need
to configure redistribution in order for the different OSPF processes to share routing information
with one another.
EIGRP AS 100
R1
OSPF 1
Area 0
10.10.100.0 /24 10.10.200.0 /24 10.10.20.0 /24 10.10.10.0 /24
172.16.24.0 /24 172.16.100.0 /24
Configuring Redistribution 343
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.
The Two Metric Types of OSPF
As mentioned in this section, OSPF defines two external metric types: Type 1 and Type 2. By
default, routes are redistributed into OSPF as Type 2. From the perspective of the router computing
the cost to a route external to the OSPF autonomous system, a Type 1 external route
(marked with E1 in the IP routing table) has a metric that is the sum of the internal OSPF cost,
all the way back to the ASBR, and the external redistributed cost. From the viewpoint of the
same router, a Type 2 external route (marked with E2 in the IP routing table) has a metric equal
only to the redistributed cost that the ASBR originally advertised, with no premium for pathways
back internal to the ASBR