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New Member

Carring BGP MED through an AS

Hello,

I wish to carry MED (BGP) from an AS1 to an AS3 via an AS2.

By default, AS2 reset MED before sending BGP announcement to AS3.

Is there a command a method to solve this ?

Thanks

3 REPLIES
New Member

Re: Carring BGP MED through an AS

Sounds like you are not using the right attribute for the job. MED advertised from AS1 is intended to indicate preferred path from AS2 to AS1 and is not relevant to AS3. Can you use As-path prepend or communities instead?

Tim

New Member

Re: Carring BGP MED through an AS

In fact, My site 1 & 3 are interconnected to BGP Backbone (AS2)

with redundant architecture (on each site: 2 routers, 2 links).

In the Backbone, I have also 2 symetric links between site 1 & site 3.

Site 1 & site 3 run OSPF and are connected to backbone in BGP.

I wish that the path used inside the backbone be driven by remote

OSPF cost.

So, I can choice the link and force go/back traffic on the same backbone link

(no asymetric session) by "playing" with OPSF cost.

It seems MED should be the solution, But BGP backbone reset It before

announcing to remote site.

Any Idea to carry Med end to end or other method.

Thanks.

VIP Purple

Re: Carring BGP MED through an AS

Hello,

I am not clear on what you want to accomplish. When BGP sends the MED update to another AS, the MED is reset to 0. No way around it. If you want to use MED on the backbone to chose a preferred path to a specific AS, you could define different MED values and use the bgp always-compare-med command (because by default the backbone router will only compare MED values from neihjbors in the same AS, with this command it will comapre MED values from different ASs). So with the following config, the backbone will always chose Router1 as the preferred path (lower MED value is preferred):

Backbone

router bgp 2

neighbor 1.1.1.1 remote-as 1

neighbor 3.3.3.3 remote-as 3

bgp always-compare-med

Router1

router bgp 1

neighbor 2.2.2.2 remote-as 2

neighbor 2.2.2.2 route-map SETMEDOUT out

!

route-map SETMEDOUT permit 10

set metric 50

Router 3

router bgp 3

neighbor 2.2.2.2 remote-as 2

neighbor 2.2.2.2 route-map SETMEDOUT out

route-map SETMEDOUT permit 10

set metric 120

Is this what you want to accomplish/know ?

Regards,

Georg

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