OSPF is a topic on both CCNA and CCNP exams, and it’s also the subject that needs the attention to detail. Check out a Cisco routing table shows many different OSPF path types where advanced routing protocols such as IGRP and RIP have only one router type.
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
Within this tutorial, we’ll take a look at the difference between two of these route types, E1 and E2.
Route redistribution is the procedure of injecting people paths and accepting routes learned routing protocol name. (Static and related paths may also be redistributed.) If a router running OSPF takes paths learned through another routing protocol also makes them accessible to another OSPF-enabled routers it’s communication with, this router becomes an Autonomous System Border Router (ASBR).
In which both RIP and OSPF is running, let’s work with an example. R4 is in precisely the OSPF domain name as R1, and also now we need R4 to understand the paths that R1 is learning through RIP. This means we must do course redistribution about the ASBR. The paths that are being redistributed from RIP into OSPF will look as E2 paths 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 your default path type for routes. The main with E2 paths is that the cost of those paths reflects the cost of the path by the ASBR to the final destination; the total cost of the path from R4 to R1 is not reflected in this cost. (Recall that OSPF’s metric for a route is known as”cost”.)
In this example, we need the expense of the paths to reflect the path, not only the path between the destination community and the ASBR. The paths must be redistributed into OSPF as shown here to accomplish this.
R1#conf t
Enter configuration commands, one per line. Finish with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#redistribute tear subnets metric-type 1
Now as E1 paths, the paths appear on R4 and have a metric, since the path cost is reflected in the routing table.
O E1 5.1.1.1 [110/94] through 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] through 172.34.34.3, 00:33:32, Ethernet0
O E1 7.1.1.1 [110/94] through 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] through 172.34.34.3, 00:33:32, Ethernet0
Knowing the difference between E1 and E2 paths is crucial for understanding a production router’s routing table, in addition to CCNP exam success. Fantastic luck on your research!
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