BGP community attributes

Unanswered Question
Dec 15th, 2008

Hello all. I read that you can use BGP community attributes to influence how an EBGP neighbor routes to you. If you (the customer) have 2 EBGP neighbors (2 separate ISPs), after tagging routes with a community attribute and configuring them outbound to the EBGP neighbors. what filter can you (the customer) use to influence how they route to you? how does this work from the ISP perspective? Do the ISPs have to configure anything? do you have to call the ISP and tell them what to configure for the community that you sent to them? please be specific. thanks

in other words, I am trying to find out if BGP community attributes on their own can influence how ISPs get to you and how. please be specific. thanks

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Overall Rating: 5 (2 ratings)
Richard Burts Mon, 12/15/2008 - 20:34


Other than tagging/inserting the community value into what you advertise to the ISP there is not much that you do (you do not configure any filters or anything like that). The rest is up to the ISP. The ISP will configure filters to recognize the community value and to take appropriate action based on the community value that you have set.

And you need to negotiate with the ISP to agree on what community values they will accept and process. Many ISPs have some standard community values which they offer for customers to use, and you can negotiate with your ISPs for unique community values if the ones they have as standard do not fill your requirements.



marikakis Tue, 12/16/2008 - 00:11


Rick is right. Just to make it a little more specific, an action that is commonly taken when an ISP detectes a particular community in a received route is to set the local preference for the route in the ISP AS. Other possible actions for ISP include prepending of the ISP AS in customer routes or blackhole traffic (as suggested/triggered remotely by customer). So, this type of customer initiated setting of community is a specific hint from customer to ISP about which action the ISP should take to influence path selection and that's pretty much the power of those communities.

For a sample policy, please have a look at the following link:

You can also browse other sample policies here:

Note also, that a standard policy which is available for all customers is normally preconfigured by ISP (e.g. they have filters in the BGP session with you and those filters look for the communities in the routes you are sending to them and take the appropriate action such as setting the local preference in their own AS). Communicating with ISP is always wise, just to confirm you have a common understanding of the specific policy.

Kind Regards,


Richard Burts Tue, 12/16/2008 - 10:25


This is a nice answer and a good complement to my response. It deserves the rating that I gave it.



marikakis Wed, 12/17/2008 - 06:00

Thank you Rick. I thought the same about your post yesterday. This means that I rated it yesterday though I did not say it (it is a habit of mine not to necessarily talk about rating). Sometimes I write a lot and sometimes nothing (trying to keep average number of words low :-).


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