Operational BGP Communities can be used to control various functions of the route server. With this communities, you can:
- control the redistribution of advertised prefixes
- prepend your own AS up to three times
- trigger the calculation of a new alternate path (if available) for your advertised prefixes before you start commencing a maintenance
Please note that if the $PEERAS is a four byte AS number you have to use the BGP Extended or Large Communities.
Control of prefix redistribution
You can control which BGP announcements you send to the route servers are redistributed to other peers. In fact, you can also control which peer / AS receives which BGP announcements you send to the route servers. For this, BGP Communities, BGP Extended Communities and BGP Large Communities can be used.
The following BGP Communities are supported:
The route servers remove the aforementioned BGP Communities and BGP Extended Communities from a BGP announcement before re-distributing it.
The well-known BGP Communities NO-EXPORT (65535:65281) and NO-ADVERTISE (65535:65282) are also honored meaning that a BGP announcement marked by one of these communities is not re-distributed to any peer. If you want the route server system to add a NO-EXPORT or NO-ADVERTISE community for a given BGP announcement before re-distributing, you have to add the community (6695:65281) or (6695:65282) respectively. This is also possible on a per-peer basis using BGP Large Communities: (6695:901:$PEERAS) for selective NO-EXPORT and (6695:902:$PEERAS) for selective NO-ADVERTISE.
More than one of the aforementioned BGP Communities and BGP Extended Communities can be added to a single BGP announcement. DE-CIX recommends not to add more than 50 of these communities as it makes handling complex and error-prone. If you need to do this, please contact Customer Service. The following table lists the evaluation order of the different BGP Communities and BGP Extended Communities which helps to build complex filter rules. In case two or more BGP (extended) Communities are contradicting the community with the lowest evaluation order wins.
All BGP Communities and BGP Extended Communities that are not listed above are not touched by the route servers and transparently re-distributed. For backwards compatibility, routes with no community at all are distributed to all peers as well.
You can check this in the DE-CIX Looking Glass. If you type your ASN into the global search field, you will find all matching prefixes from all DE-CIX route servers on all DE-CIX Internet Exchagnes. By clicking on a prefix, you get the prefix information dialogue with details (BGP Comunities etc.). You can apply further filters, e.g. filter for a specific route server or IX. More information how to use the DE-CIX Looking Glass can be found here.
How the different communities can be used
Examples for Frankfurt
BGP announcements marked with the following communities are only re-distributed to AS64501 and AS64502 (both 2 Byte ASNs):
BGP announcements marked with the following communities are re-distributed to all peers/ ASNs except AS64501 and AS64502:
BGP announcements tagged with the following communities are only re-distributed to AS65550 (4 Byte ASN) and AS64501 (2 Byte ASN):