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OSPF Route Selection

If I have two OSPF learnt routes

Inter Area - Cost 10

Intra Area - Cost 100

Which one would OSPF select, the lowest cost route or the Inra Area route ?

2 REPLIES
Silver

Re: OSPF Route Selection

OSPF performs route selection based on the type of route, the order is

Intra Area

Inter Area

External Type 1

External Type 2

If there are multiple routes with the same route type then the route with the lowest cost wins.

In your example the Intra Area - Cost 100 route would be selected

See Section 11 and Apendix of RFC 2328:

11 Path-type

There are four possible types of paths used to route traffic to

the destination, listed here in decreasing order of preference:

intra-area, inter-area, type 1 external or type 2 external.

Intra-area paths indicate destinations belonging to one of the

router's attached areas. Inter-area paths are paths to

destinations in other OSPF areas. These are discovered through

the examination of received summary-LSAs. AS external paths are

paths to destinations external to the AS. These are detected

through the examination of received AS-external-LSAs.

G.4 Routing table lookup

The routing table lookup algorithm in Section 11.1 has been

modified to reflect current practice. The "best match" routing

table entry is now always selected to be the one providing the

most specific (longest) match. Suppose for example a router is

forwarding packets to the destination 192.9.1.1. A routing table

entry for 192.9.1/24 will always be a better match than the

routing table entry for 192.9/16, regardless of the routing

table entries' path-types. Note however that when multiple paths

are available for a given routing table entry, the calculations

in Sections 16.1, 16.2, and 16.4 always yield the paths having

the most preferential path-type. (Intra-area paths are the most

preferred, followed in order by inter-area, type 1 external and

type 2 external paths; see Section 11).

Bronze

Re: OSPF Route Selection

Hi

OSPF performs route selection based on the type of route.

Following the order

Intra Area

Inter Area

External Type 1

External Type 2

If there are more than on route from the same type then the route with the lowest cost wins.

Hope that helps you

Roger

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