OSPF E1 route

Answered Question
Jul 14th, 2008
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Can I know what the does the following statement means?


For OSPF E1 router, it is external to the autonomous system. E1 includes the internal cost to the ASBR added to the

external cost.


Does it mean that the cost to reach a other AS is the internal cost + external advertised cost?


Please help to explain. Thanks


Correct Answer by lee.reade about 8 years 9 months ago

Hi,


E1 means that you can advertise the route by the ASBR, ie the router advertising the non ospf route, and specify the metric.


Then the routers will add the metric to the ASBR to the advertised metric for the external routes.


This is used when you want to make sure that internal routers take their OWN best path to the ASBR. Whereas E2 you just advertise the metric on the ASBR and it is hard set throughout the ospf domain.


For E1 the metric will change the further away from the ASBR that the routes propogates. Whereas E2 it is the same value on all ospf routers in the ospf domain.


HTH


LR

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Correct Answer
lee.reade Mon, 07/14/2008 - 06:36
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Hi,


E1 means that you can advertise the route by the ASBR, ie the router advertising the non ospf route, and specify the metric.


Then the routers will add the metric to the ASBR to the advertised metric for the external routes.


This is used when you want to make sure that internal routers take their OWN best path to the ASBR. Whereas E2 you just advertise the metric on the ASBR and it is hard set throughout the ospf domain.


For E1 the metric will change the further away from the ASBR that the routes propogates. Whereas E2 it is the same value on all ospf routers in the ospf domain.


HTH


LR

chrislau388 Tue, 07/15/2008 - 04:32
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Thanks for your information.


Can I know how does ospf calculate the metric value? Is it using the same method to calculate ospf cost?


cost = 100000000 /ref BW bps



lee.reade Tue, 07/15/2008 - 05:42
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Hi,


That is correct, a router will use its interface ospf cost to determine what the real metric is for a route, for E1.


HTH


LR

rsgamage1 Tue, 07/15/2008 - 05:44
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Cost is the OSPF metric, expressed as an unsigned 16-bit integer in the range of 1 to 65535.


Cisco default 10^8/BW

Here 10^8 is the reference bandwidth, thus for an interface having a configured bandwidth of 512K,


the Cost is 10^8/512K ~ 195


HTH


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