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?
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...
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..
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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