Action BGP Communities can be used to control various functions of the route server. With these communities, you can:
- control the redistribution of advertised prefixes, based on an ASN
- prepend your own ASN up to three times
- trigger the calculation of a new alternate path (if available) for your advertised prefixes before you start commencing a maintenance
- trigger blackholing
Please note that if the <PEER-AS> is a four byte AS number you have to use BGP Large Communities.
Control of prefix redistribution
Redistribution can be controlled with BGP Communities and BGP Large Communities.
In case two or more BGP (large) Communities are contradicting the community with the lowest evaluation order is processed.
| Priority | Group | Action | BGP Community (RFC 1997) | BGP Large Community (RFC 8092) |
|---|---|---|---|---|
| 1 | Well-known | Well-known NO_EXPORT | 65535:65281 | / |
| Well-known NO_ADVERTISE | 65535:65282 | / | ||
| 2 | ASN based | Do not Redistribute to <PEER-AS> | 0:<PEER-AS> | 42476:0:<PEER-AS> |
| 3 | Redistribute to <PEER-AS> | 42476:<PEER-AS> | 42476:1:<PEER-AS> | |
| 4 | General | Do not redistribute | 0:42476 | 42476:0:0 |
| 5 | Redistribute to all (default) | 42476:42476 | 42476:1:0 |
The route servers remove the aforementioned BGP Communities and BGP Large 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 (42476:65281) or (42476:65282) respectively. This is also possible on a per-peer basis using BGP Large Communities:
- (42476:901:<PEER-AS>) for selective NO_EXPORT and
- (42476:902:<PEER-AS>) for selective NO-ADVERTISE
| BGP Community (RFC 1997) | BGP Large Communities (RFC 8092) | |
| add NO_EXPORT | 42476:65281 | / |
| add NO_ADVERTISE | 42476:65282 | / |
| add NO_EXPORT selective | / | 42476:901:<PEER-AS> |
| add NO_ADVERTISE selective | / | 42476:902:<PEER-AS> |
More than one of the aforementioned BGP Communities and BGP Large Communities can be added to a single BGP announcement. SwissIX 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.
All BGP Communities and BGP Large 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 SwissIX Looking Glass. If you type your ASN into the global search field, you will find all matching prefixes from all SwissIX route servers. By clicking on a prefix, you get the prefix information dialogue with details (BGP Communities etc.). You can apply further filters, e.g. filter for a specific route server or IX. More information how to use the SwissIX Looking Glass can be found here.
How the different communities can be used
Please note: In the following examples we only make use of BGP Large Communities when it becomes necessary.
BGP announcements marked with the following communities are only re-distributed to AS64501 and AS64502 (both 2 Byte ASNs):
- (0:42476)
- (42476:64501)
- (42476:64502)
BGP announcements marked with the following communities are re-distributed to all peers/ASNs except AS64501 and AS64502:
- (0:64501)
- (0:64502)
- (42476:42476)
BGP announcements tagged with the following communities are only re-distributed to AS65550 (4 Byte ASN) and AS64501 (2 Byte ASN):
- (0:42476)
- (42476:1:65550)
- (42476:64501)
AS Path Prepending
You can use BGP communities to prepend your own ASN up to three times. This can be done to all other peers or selective to only certain peers.
| BGP Community (RFC 1997) | BGP Large Communities (RFC 8092) | |
| Prepend once | 65001:0 | 42476:101:0 |
| Prepend twice | 65002:0 | 42476:102:0 |
| Prepend three times | 65003:0 | 42476:103:0 |
| Prepend once selective | 65001:<PEER-AS> | 42476:101:<PEER-AS> |
| Prepend twice selective | 65002:<PEER-AS> | 42476:102:<PEER-AS> |
| Prepend three times selective | 65003:<PEER-AS> | 42476:103:<PEER-AS> |
Blackholing
| BLACKHOLE (RFC 7999) |
| 65535:666 |
Please also have a look at our Blackholing Guide.
Graceful BGP Session Shutdown
The SwissIX route servers support RFC 8326 (Graceful BGP Session Shutdown). With this well-known BGP Community, you can instruct the route servers to calculate and redistribute an alternate path (if available) for your advertised prefixes before you start commencing your maintenance. This makes sure that routers of other customers have fully converged before you interrupt L2 connectivity and thereby so called micro blackholing is prevented.
Details:
- Setting BGP Community GRACEFUL_SHUTDOWN (65535:0) on all you advertised prefixes. The route server will set BGP local preference to 0 for these prefixes.
- The route server will calculate alternative paths for your advertised prefixes (if available) and redistribute these to other peers. Prefixes with no alternative path will get redistributed with BGP Community GRACEFUL_SHUTDOWN
- You should also apply GRACEFUL_SHUTDOWN on the inbound policy of you eBGP session
- After convergence has completed, you can safely shut down the BGP session. At this point, routers of other customers have learned alternative paths for your prefixes (if available) from the route server and forward traffic on the new path
