OSPF is a major topic on both the CCNA and CCNP exams, and it’s
also the topic that requires the most attention to detail. Where
dynamic routing protocols such as RIP and IGRP have only one
router type, a look at a Cisco routing table shows several
different OSPF route types.
R1#show ip route
Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile,
B – BGP D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF
inter area N1 – OSPF NSSA external type 1, N2 – OSPF NSSA
external type 2 E1 – OSPF external type 1, E2 – OSPF external
type 2, E – EGP
In this tutorial, we’ll take a look at the difference between
two of these route types, E1 and E2.
Route redistribution is the process of taking routes learned via
one routing protocol and injecting those routes into another
routing domain. (Static and connected routes can also be
redistributed.) When a router running OSPF takes routes learned
by another routing protocol and makes them available to the
other OSPF-enabled routers it’s communicating with, that router
becomes an Autonomous System Border Router (ASBR).
Let’s work with an example where R1 is running both OSPF and
RIP. R4 is in the same OSPF domain as R1, and we want R4 to
learn the routes that R1 is learning via RIP. This means we have
to perform route redistribution on the ASBR. The routes that are
being redistributed from RIP into OSPF will appear as E2 routes
on R4:
R4#show ip route ospf
O E2 5.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E2 6.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
O E2 172.12.21.0/30 [110/20] via 172.34.34.3, 00:33:32, Ethernet0
O E2 7.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
15.0.0.0/24 is subnetted, 1 subnets
O E2 15.1.1.0 [110/20] via 172.34.34.3, 00:33:32, Ethernet0
E2 is the default route type for routes learned via
redistribution. The key with E2 routes is that the cost of these
routes reflects only the cost of the path from the ASBR to the
final destination; the cost of the path from R4 to R1 is not
reflected in this cost. (Remember that OSPF’s metric for a path
is referred to as “cost”.) In this example, we want the cost of
the routes to reflect the entire path, not just the path between
the ASBR and the destination network. To do so, the routes must
be redistributed into OSPF as E1 routes on the ASBR, as shown
here.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#redistribute rip subnets metric-type 1
Now on R4, the routes appear as E1 routes and have a larger
metric, since the entire path cost is now reflected in the
routing table.
O E1 5.1.1.1 [110/94] via 172.34.34.3, 00:33:21, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E1 6.1.1.1 [110/100] via 172.34.34.3, 00:33:21, Ethernet0
172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
O E1 172.12.21.0/30 [110/94] via 172.34.34.3, 00:33:32, Ethernet0
O E1 7.1.1.1 [110/94] via 172.34.34.3, 00:33:21, Ethernet0
15.0.0.0/24 is subnetted, 1 subnets
O E1 15.1.1.0 [110/94] via 172.34.34.3, 00:33:32, Ethernet0
About the author:
Chris Bryant, CCIE #12933, owns The Bryant Advantage
(http://www.thebryantadvantage.com), home of free CCNA and CCNP
tutorials, Pass the CCNA exam with Chris Bryant!