Creating BGP Communities

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Nov 1st, 2006
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I understand the concept of BGP communities and how they are used to flag routing updates. What I haven't been able to find is how the values of the communities are set. With a community value of 3275:100, the 3275 part is the ASN of the sending router, but what is the meaning of the 100 piece? Is that a well-known value or a value that is agreed upon by admins in each AS? What I'm asking here is whether the community values are unique in a relationship between a provider and customer. Can provider A agree that the 100 value represents a LOCAL_PREF of 220 with one customer and that it denotes a METRIC of 2 with a different customer?


Thanks.


Clay Hawks

Correct Answer by pkhatri about 10 years 9 months ago

Well, providers don't usually seek agreement from customers. They simply decide on a set of community values and publish them to make customers aware of them.


Paresh


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pkhatri Wed, 11/01/2006 - 16:29
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Hi Clay,


There is no universally accepted convention for the second part of the community. That is part of the reason that the first part of the community is usually the ASN. That makes the second part of the community a value locally significant to that AS.


In general, a provide will use the same community values for each customer, in order to carry out a specific action on BGP updates. However, there is nothing stopping a provider from using different communities for different customers (as long as the community is being acted upon at the interface to the customer). It is a management nightmare, though...


Paresh

clayhawks Wed, 11/01/2006 - 17:33
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Thanks. That does make it a little clearer. So the values in the second part of the community are agreed upon by the provider and any set of customers that would like to use them?

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pkhatri Wed, 11/01/2006 - 17:58
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Well, providers don't usually seek agreement from customers. They simply decide on a set of community values and publish them to make customers aware of them.


Paresh


PS. Pls do rate posts that help

clayhawks Wed, 11/01/2006 - 18:00
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Ahh! That makes a great deal of sense. I understand completely.


Thanks for your insight.


Clay

amarsjoshi Fri, 11/03/2006 - 12:40
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Hi Clay,


You can try this too - lets say you have BGP neighbor with AS# 1000, then create a routemap and set community 3257:1000 to all the routes that you get from them.. This will help you to see only these routes from any BGP router in your network (provided you are doing send bgp-community on your IBGP) by running the command : sh ip bgp community 3275:1000


Again, you can use any numbers in place of the 1000.


The service providers take this concept and add more stuff to it as to which is advertised out and other things..


HTH,

Amar

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