BGP: How to Apply Communities to Originated Routes

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in this section we are going to look at how you can scale BGP in the ISP backbone so inside iBGP. So using communities for iBGP the ISPs are going to tag prefixes learned from their BGP and their static customers with different communities to do different things so for example they would like to identify services that the customer may have purchased or you want to identify prefixes which are part of the highest peace provider assigned space PA space or to identify the customers PII addresses provider independent addresses that they might have got from the original internet registry or you want to control prefix distribution into ibgp or you want to control the prefix announcements to customers and up streams and several other reasons so for service identification what the ISP is going to provide is transit very abstruse connectivity to major ixps and connectivity to private peers or to customers now the customer can choose to buy all or any of the above access options and each option would be identified with a unique community then the ISP identifies where the address space from the customer comes from their provider assigned block or is their customers own provider independent space and you'll have one community for the provider signed block and one community for the provider independent space so this slide shows the typical county definitions and we assuming that we ain't as 100 so 1000 would be the aggregates out of s one hundred one thousand one would be the sub prefixes that come out of the a s 100 aggregate then one thousand five for example could be the static customer P I space providing independent space mm could be customers who get transit 2100 could be customers who get access to the ixp 2200 could be customers who get bgp customer access this is fruits which are announced by other bgp customers then all routes land from the IXP could be tagged with 3000 and this is how you could implement it in Cisco IOS you have a community list 10 for too much the aggregates which is 100 : 1000 11 for 1,000 112 for 1000 five which is a customer static PA space all the way down to 16 which is for the 3000 community which is routes land from the ixv inside s 100 you have sample just a little bit of a BGP session and you see you have Ruta BGP 100 on a slide and you're redistributing static routes using a particular route map static - BGP not that this is an advanced configuration and typically we would not redistribute statics into BGP however for this use case because you have lots of customers who have static routes that you would like to announce then with control redistribution it makes it ok so you first need a prefix block that is going to match which statics are permitted to reduce beauty into BGP and so the first thing that you do is have the aggregate and then say I'm going to have an IP prefix list s 100 - block permit 164 1000 it's a slash 19 all the way to a slash 32 so any prefix out of this block will be redistributed then for your s 100 prefixes you set the community to 1000 which remember is the aggregate community set and now for the static - BGP route map that we talked about just earlier you first have a match statement to match the prefix list is 100 - block for the very reason that when you're redistributing statics you need to be very careful about which prefixes you distribute and once you matched anything out of that block you set the community to 1001 which means these are static prefixes which come out of the aggregate block so for permit 20 which you don't have a match command you set the community to one thousand five which means these are prefixes from your customers out of their P I space so the prefixes s 100 - block would need to have all the address space that is 100 uses in this case it's just a single slash 19 but if s 100 had add address space you need to add it as well - this prefix list.

Exercises

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