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New Member

MED not being accepted

Hi,

There r 3 router in two AS..

Router A in AS 100

Router B in AS 100

Router C in AS 200

No-sync and no auto-sum is configured on all. Router A sets some MED for all routes it advertises to Router C.

Topology is as follows..:

(A)AS 100-----(C)AS 200---- (B) AS 100

On router B in AS 100 , I have used <neighbor C_ip allow-as in> (to let routes of A be learnt by B)

..now B gets routes propogated by A via C but I cannot see any med when i do sh ip bgp.

I see Router C getting MED value and there is no attribute modification on C manually.. and it should propogate to B which is in same AS as A.

Also, I have heard that customer metric is carried transparently over MPLS , even if BGP is CE-PE protocol.. that ways my LAB router C should see MED of A routes.

AFAIK, MED id not propogated to third AS..

Plz comment..

G

7 REPLIES

Re: MED not being accepted

Hi,

are you using VRFs on C? If not it comes down to normal BGP behaviour. And MED in non transitive, thus should be removed when forwarded from C to B.

So I would not expect to see MED on C in an internet scenario.

If you talk about VRFs the MED is transported from PE to PE to allow to transparently transport the IGP metric of a customer. The MED is used in the VRFs during redistribution in and out of the customer IGP.

Hope this helps! Please rate all posts.

Regards, Martin

New Member

Re: MED not being accepted

Thx Martin, u r right.

1. Its plain BGP But when I do .. on router C , I see that router C is sending MED while I couldn't not see MED on B.

You had replied in one of the post that will show all that is being propogated out to the neighbor.

2. But In case of MPLS with BGP as CE-PE routing protocol on two sites. What is the mechanism that carries & forwards MED back to the same AS to which the two sites belong.

CE1(AS100)---PE1:MPLS(AS200):PE2--- CE2(AS100)

Thx,

Gaurav

Re: MED not being accepted

Hi Gaurav,

All routing protocol information received on a a PE-CE link is carried using MPBGP extended communities,

Now the mechanism how non transitive attribute is carried over from one side to another side, is that the received information always gets mapped into and carried in the MPLS cloud into extended communities.

And by default extended communities are transitive communities. and also they are carrying the MED information into extended communities and it gets preserved, and gets into the other side BGP table.

In Summary the intermediate MPLS routers are carrying MED transaprently through extended communities and hence dont touch it.

HTH-Cheers,

Swaroop

New Member

Re: MED not being accepted

MED is only transitive between to adjacent ASes.

New Member

Re: MED not being accepted

If router A is in AS100 and B is in AS 100, BGP will reject the route because of dual AS, this is a loop to BGP.

If router A is a CE site and Router B is a CE, then you need to use the AS Override command to get around have the same AS Path.

Such as:

address-family ipv4 vrf V28:Internet-VPN

redistribute connected

redistribute static

neighbor 192.168.202.18 remote-as 65100

neighbor 192.168.202.18 activate

neighbor 192.168.202.18 as-override

Also don't forget the MED is only passed between EBGP peers. If the Peers are in the same AS MED is not passed. To better influence outbound routing use Local Preference to better influence inbound routing use Prepending.

Hope that helps or gives you some ideas...

Karl Solie

New Member

Re: MED not being accepted

Thx all for replies..

Hi Karl,

Do u mean if MED is learnt over eBGP session by a router X , it will not be passed on iBGP session or it will not be passed over other eBGP ?

Till now from discussion I learn that MED is carried in MPLS world due to reason MED is carried in Extened Communities , which is not there in normal BGP.

Thx,

G

New Member

Re: MED not being accepted

Yes you are correct.

In a normal envirnonment it is carred from eBGP to eBGP, but not iBGP peers.

In MPLS it can be tricky, used on the extended communities. So it can be tricky to work with. If you end goal is to identify routes and make routing decisions on them, use SOO to identify the routes and then use local pref or prepend to influence routing decisions.

This has worked for us...

Hope that helps,

Karl Solie

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