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Beginner

BGP MED across iBGP to eBGP

Dear All,

I understand that MED can be used to influence the path taken by incoming traffic arriving from the adjacent AS. Could anyone tell me whether one iBGP neighbor can pass MED to another iBGP neighbour and then expect that neighbour to announce the MED into the adjacent AS? I'm assuming here that all routers support the MED attribute.

 

I have drawn a diagram to help explain my question.

 

Regards

 

James.

MED.png

 

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Hello
Yes you should be able to do this - i am mis-inturpeting the question so apologies for that,

Appending a RM with AS-prepending on a ingress route from R1 in ASN60001 should allow that route to be advertised to R4 with a longer as-sequence.



kind regards
Paul

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VIP Mentor

Hello
My understading is youll need to set the med on R3 so R4 receives the assigned value, the Med from R2 wont be transitive into another ASN as such when that update is sent to R4  by R3 (set from R2) it will be reset to 0

However If the connection between R2-R3 was IGP and the same route was then redistrubuted into the R3 bgp then when that route is advertised to R4 the redistributed metric should be advertised.



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
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I like your IGP suggestion, but I'd prefer to keep this BGP throughout.

 

Since I'm looking to influence the routing at R4 and I'd like to do this by configuring R2 only, would an AS-Prepend on the incoming prefix into R2 (from R1) be allowable? i.e. Can you prepend on the inbound route-map?

 

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Hello
No as-prepend wont work either from R2 , To influcene R4 on any of its recived routes from ASN65000 I would say you need to do that on either R3 or R4



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
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Are you sure the AS-Path can't be adjusted on an inbound route-map? My preference would be to do the prepend on R2 for the incoming eBGP advertisement from R1 -> R2.

 

According to the nx-os configuration guide:

 

" Appends the AS number to the autonomous system path of the route that is matched by the route map. Applies to both inbound and outbound BGP route maps."

 

I would have thought IOS would also support as-prepend on the incoming route-map.

Highlighted

Hello
Yes you should be able to do this - i am mis-inturpeting the question so apologies for that,

Appending a RM with AS-prepending on a ingress route from R1 in ASN60001 should allow that route to be advertised to R4 with a longer as-sequence.



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future

View solution in original post