Route Distribution of OSPF Protocol

Redistributing into OSPF

Example syntax:

R1(config)#router ospf 1 R1(config-router)#redistribute connected % Only classful networks will be redistributed R1(config-router)#redistribute connected subnets

Notice there is no seed metric required for redistribution into OSPF. OSPF has a default seed metric of 20, so none has to be specified with the redistribute command.

What OSPF does require is the subnets option to be enabled — if you want subnets to be redistributed into OSPF.

OSPF Defaults with Route Redistribution

• The route has a metric of 20, OSPF’s default seed metric
• The route is marked as “E2″, short for External-Type 2. This is the default routing code for a route redistributed into OSPF. An E2 metric reflects only the cost from the ASBR to the external destination. It does not reflect the cost from the local router to the ASBR itself. This would be covered by an E1 metric which is not the default for route redistribution with OSPF.

Default Seed Metrics

RIP and EIGRP have a seed metric of “infinity”, and you know no routes with a metric of “infinity” are ever going to be placed into a routing table. Both RIP and EIGRP require a default seed metric to be defined during the redistribution process.

OSPF has a default seed metric of 20, as well as defaulting to a route type of E2. There’s always an exception, and the exception here is that BGP routes are given a metric of 1 by OSPF. Be really careful when redistributing BGP into OSPF.

Another link-state protocol, ISIS has a default seed metric of zero, and it does allow routes to be redistributed without a specified seed metric.

OSPF’s default seed metric can be changed in the redistribute command itself.

Example:R2(config)#router ospf 1 R2(config-router)#redistribute eigrp 100 ? metric Metric for redistributed routes metric-type OSPF/IS-IS exterior metric type for redistributed routes route-map Route map reference subnets Consider subnets for redistribution into OSPF tag Set tag for routes redistributed into OSPF <cr> R2(config-router)#redistribute eigrp 100 metric ? <0-16777214> OSPF default metric R2(config-router)#redistribute eigrp 100 metric 30 % Only classful networks will be redistributed R2(config-router)#redistribute eigrp 100 metric 30 subnets

*Remember to add the word subnets to the end to ensure classless networks can be advertised and not the default of classful.