OSPF route redistribution is an important topic on the BSCI exam, and it's a topic full of details and defaults that you need to know for the exam room and the job. To help you pass the BSCI exam, here's a quick review of some of the OSPF route redistribution basics.
To see if a router is an ABR or ASBR, run show ip ospf. This also displays any routes being redistributed into OSPF on this router.
R1#show ip ospf
Routing Process "ospf 1" with ID 1.1.1.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an area border and autonomous system boundary router
Redistributing External Routes from,
connected, includes subnets in redistribution
rip, includes subnets in redistribution
When redistributing RIP into OSPF, the subnets" option is needed to include subnets in redistribution. When redistributing OSPF into RIP, a seed metric must be specified. (OSPF gives redistributed routes a default metric of 20 this can be changed, but a seed metric does not have to be set.)
R1(config)#router ospf 1
R1(config-router)#redistribute connected
% Only classful networks will be redistributed
R1(config-router)#redistribute connected subnets
R1(config-router)#redistribute rip subnets
R1(config-router)#router rip
R1(config-router)#redistribute connected metric 1
R1(config-router)#redistribute ospf 1 metric 1
By default, routes redistributed into OSPF are marked as E2 routes. The metric for these routes reflects only the cost of the path from the ASBR to the destination network and does not include the cost of the path from the local router to the ASBR. By contrast, E1 routes include the cost of the entire path from the local router to the destination network.
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
To redistribute routes into OSPF and mark them as E1 upon redistribution, use the metric-type option with the redistribution command.
R1(config)#router ospf 1
R1(config-router)#redistribute rip subnets metric-type ?
1 Set OSPF External Type 1 metrics
2 Set OSPF External Type 2 metrics
R1(config-router)#redistribute rip subnets metric-type 1
Look at the same two routes in R4's routing table, which are now displayed as E1 routes:
O E1 5.1.1.1 [110/94] via 172.34.34.3, 00:04:13, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E1 6.1.1.1 [110/94] via 172.34.34.3, 00:04:14, Ethernet0
BSCI exam success and earning your CCNP certification depends on knowing the details, and there are plenty of details involved in OSPF route redistribution! Keep studying, practice different scenarios in your CCNA / CCNP home lab or rack rental, and you'll master these details and pass the exam! |
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