Advanced OSPF Design Concepts

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The following is the output of show ip route on Router E:

    Router E# 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    E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP    i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate    default    Gateway of last resort is not set    203.250.15.0 255.255.255.252 is subnetted, 1 subnets    C 203.250.15.0 is directly connected, Serial0    O IA 203.250.14.0 [110/74] via 203.250.15.1, 00:02:31, Serial0    O E2 128.213.0.0 [110/20] via 203.250.15.1, 00:02:32, Serial0 

Notice that the only external route that has appeared is 128.213.0.0, because you did not use the subnets keyword in Router C’s configuration. If the subnets keyword is not used, only routes that are not subnetted, will be redistributed. In this case, 16.16.16.0 is a class A route that is subnetted and it did not get redistributed. Because the metric keyword was not used (or a default-metric statement under router OSPF), the cost allocated to the external route is 20 (the default is 1 for bgp). Watch what happens when you use the redistribute static metric 50 subnets command in Router C’s configuration:

    Router E# 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    E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP    i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate    default    Gateway of last resort is not set    16.0.0.0 255.255.255.0 is subnetted, 1 subnets    O E2 16.16.16.0 [110/50] via 203.250.15.1, 00:00:02, Serial0    203.250.15.0 255.255.255.252 is subnetted, 1 subnets    C 203.250.15.0 is directly connected, Serial0    O IA 203.250.14.0 [110/74] via 203.250.15.1, 00:00:02, Serial0    O E2 128.213.0.0 [110/50] via 203.250.15.1, 00:00:02, Serial0 

Notice that network 16.16.16.0 has shown up now, and the cost to external routes is 50. Because the external routes are of type 2 (E2), the internal cost has not been added. Suppose now, you change the external route type to E1 through the use of the redistribute static metric 50 metric-type 1 subnets command, which is added to Router C’s configuration:

    Router E# 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    E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP    i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate    default    Gateway of last resort is not set    16.0.0.0 255.255.255.0 is subnetted, 1 subnets    O E1 16.16.16.0 [110/114] via 203.250.15.1, 00:04:20, Serial0    203.250.15.0 255.255.255.252 is subnetted, 1 subnets    C 203.250.15.0 is directly connected, Serial0    O IA 203.250.14.0 [110/74] via 203.250.15.1, 00:09:41, Serial0    O E1 128.213.0.0 [110/114] via 203.250.15.1, 00:04:21, Serial0 

Notice that the external route type for network 16.16.16.0 has changed to E1, and the cost has been incremented by the internal cost of S0, which is 64. The total cost is 64+50=114. Assuming that you add a route map to Router C’s configuration, you will get the following:

    Router C#      interface Ethernet0        ip address 203.250.14.2 255.255.255.0      interface Serial1        ip address 203.250.15.1 255.255.255.252      router ospf 10        redistribute static metric 50 metric-type 1 subnets route-map        network 203.250.15.0 0.0.0.255 area 2        network 203.250.14.0 0.0.0.255 area 0        ip route 16.16.16.0 255.255.255.0 Ethernet0        ip route 128.213.0.0 255.255.0.0 Ethernet0        access-list 1 permit 128.213.0.0 0.0.255.255        route-map permit 10        match ip address 1 

The preceding route map will only permit 128.213.0.0 to be redistributed into OSPF and will deny the rest. This is why 16.16.16.0 does not show up in Router E’s routing table anymore, as shown in the following:

    Router E# 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    E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP    i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate    default    Gateway of last resort is not set    203.250.15.0 255.255.255.252 is subnetted, 1 subnets    C 203.250.15.0 is directly connected, Serial0    O IA 203.250.14.0 [110/74] via 203.250.15.1, 00:00:04, Serial0    O E1 128.213.0.0 [110/114] via 203.250.15.1, 00:00:05, Serial0 

Redistributing OSPF into Other Protocols

The most important rule when redistributing OSPF into other protocols is that you respect the rules of operation for those other protocols. In particular, the metric applied should match the metric used by that protocol.

For example, the RIP metric is a hop count ranging between 1 and 16, where 1 indicates that a network is one hop away and 16 indicates that the network is unreachable. On the other hand IGRP and EIGRP require a series of metrics as demonstrated in the command:

    Router eigrp    default-metric bandwidth delay reliability loading mtu 

Variable-Length Subnet Masking (VLSM)

Another issue to consider is VLSM. OSPF can carry multiple subnet information for the same major network, but other protocols such as RIP and IGRP (EIGRP is okay with VLSM) cannot.

If the same major network crosses the boundaries of an OSPF and RIP domain, VLSM information redistributed into RIP or IGRP will be lost and static routes will have to be configured in the RIP or IGRP domains as illustrated in Figure 6-7.


Figure 6-7  Losing VLSM information.

In Figure 6-7, Router E is running OSPF and Router A is running RIP. Router C is doing the redistribution between the two protocols.


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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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