OSPF external routes

Unanswered Question
Jun 1st, 2008

hello there guys all the routes which ASBR learns and flood using type 5 lasa are called extrnal routes(plz correct me if i am wrong). All teh roues which are redritibuted ether by other routing protocol, or connected or static is called external type 1 or 2. right? now whats the difference between type 1 and type 2 which routes would be called type 1 and which would be called type 2....regards and thnaks in advance

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lamav Sun, 06/01/2008 - 05:57

When you redistribute one routing protocol into another, a seed metric (default metric) must be defined, either by the administrator or by the router's IOS, as occurs when redistributing into OSPF. In that case, the default seed metric is 20, unless defined as some other quantity by the administrator.

A seed metric is required so that the receiving IGP can allocate a metric that it can understand. OSPF uses cost as a metric.

A type 2 (E2) external route is one in which the initial cost of the route is introduced into the receiving IGP and does NOT get incremented, regardless of the path a packet takes to get to the destination network. In other words, the redistributing router is saying, "Hello, everyone. I have a route to with a cost of 50." That type 2 cost will not change. It will always be 50. So, whether a packet takes 8 hops and traverses 8 interfaces (each with its own cost) to get to the advertising router, or whether a packet traverses 2 interfaces to get to the advertising router, the cost to the destination network will always be 50. A Type 1 (E1) external route has an initial cost that is incremented by the cost of each interface that the packet must traverse -- so, its additive. By default, routes redistributed into OSPF are tagged as E2.



Edison Ortiz Sun, 06/01/2008 - 06:01

now whats the difference between type 1 and type 2 which routes would be called type 1 and which would be called type 2

A Type 2 External OSPF route is the default route type when redistributing external routes into OSPF without specifying the metric type.

A Type 2 External OSPF route does not take into account the cost of the link after the redistribution has been made.

For instance, if you have a router sitting 2 hops away from the ASBR and each hop cost is 5, the total cost of the link towards the ASBR is 10.

A Type 2 won't take into account that cost. By default it will use the cost of 20 and maintain that cost throughout the OSPF topology.

Note, you can modify the cost with the metric option during the redistribution

Then, you have the Type 1 External OSPF route. Taking the example above, it will take into account the cost of the link, therefore from the router mentioned above - the cost towards the ASBR will be 10 instead of 20.




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Giuseppe Larosa Tue, 06/03/2008 - 11:11


just a little addition to what Victor and Edi have written about the OSPF order of preference of external routes.

If for a prefix exists an O E1 route and a O E2 route the O E1 route is always preferred without comparing the respective seed metric values.

This can have a great impact for example for the default route that is an external route itself.

So if you configure an ASBR to inject an O E2 default route into the OSPF domain and another ASBR to inject an O E1 default route all nodes will use the second one.

hope to help



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