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guidance on BGP community attribute.



I am looking at some guidance on implementing route preference using BGP communities in our WAN network. Our service provider provided MPLS network spans the entire globe. I spoke to our service provider on influencing default route selection based on regions. For example countries in AsiaPac should prefer Singapore and stuff like that.

Our service provider came back to me and said that this can be implemented as he has made the community attribute to be transparent throught the network. My question is even if he has done that how do I proceed. Does anyone have some links/docs on communities implementation?



5 Replies 5

John Blakley


Communities are somewhat arbitrary numbers, with the exception of 4 standard communities (no-export, no-advertise, internet, local-as), and they're used for routing decisions. If the provider has assigned you a community to send to them, let's use 555:100 for example, they are expecting you to send this community with your routes. You can do this with a route-map applied to the neighbor:

route-map Community permit 10

set community 555:100

router bgp 1

neighbor route-map Community out

neighbor send-community

The above would set 555:100 on all of your routes. If you needed to set a community for a subset of routes, you'd change the route-map to match on a prefix list and then set the community that way:

ip prefix-list Singapore permit

ip prefix-list Brazil permit

ip prefix-list US permit

route-map Community permit 10

match ip address prefix Singapore

set community 100:200

route-map Community permit 20

match ip address prefix Brazil

set community 200:200

route-map Community permit 30

match ip address prefix US

set community 300:200

and the BGP configuration is the same. The provider on the other side would be looking for any routes that have these communities attached and they'd make routing decisions based on them.

Here's a document on named communities:



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Hi, the service provider has not assigned any communities to me. He has just said he has set the communities to be transparent in his network.

Okay, their telling you that they're not going to strip them from your routes. That would mean that you would need to match on your communities on the other side and then make routing decisions based on whatever community you were matching on.


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I dealt with a similar situation several years ago.  Used community values to define a "virtual" AS topology, i.e. each site would tag its outbound routes with a community denoting it's virtual AS.  On received routes, examined received community and prepended original AS by virtual AS hops.


Perhaps there is some aspect of your situation and your question that I do not understand correctly. But it seems to me that you have a fairly simple issue and I do not see where communities are needed.

If I am understanding correctly you have multiple remote sites. Each remote site has a Customer Edge Router (CE) which connects to the Provider Edge Router (PE). The PE will advertise a default route to the CE. The CE will use the default route to send its outbound traffic to its connected PE. Once the traffic is in the provider clould you no longer are making choices about how it will be forwarded.

So where would communities enter into this? If I have misunderstood something then please clarify.




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