Operational BGP Communities

Operational 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

Please note that if the $PEER-AS is a four byte AS number you have to use the BGP Extended or Large Communities. 

Control of prefix redistribution 

Redistribution can be controlled with BGP Communities, BGP Extended Communities and BGP Large Communities

Evaluation Order

Group

Action

BGP Standard Community (RFC 1997)

BGP Extended Community (RFC 4360)

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

rt:0:$PEER-AS

56858:0:PEER-AS

3

Redistribute to $PEER-AS* 

56858:$PEER-AS

rt:56858:$PEER-AS

56858:1:$PEER-AS

4

General

Do not redistribute

0:56858

rt:0:56858

56858:0:0

5

Redistribute to all (default)

56858:56858

rt:56858:56858

56858:1:0

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 (56858:65281) or (56858:65282) respectively. This is also possible on a per-peer basis using BGP Large Communities:

  • (56858:901:$PEER-AS) for selective NO_EXPORT and
  • (56858:902:$PEER-AS) for selective NO-ADVERTISE

 

BGP Standard Community (RFC 1997)

BGP Large Community (RFC 8092)

add NO_EXPORT

65535:65281

/

add NO_ADVERTISE

65535:65282

/

 

 

 

add NO_EXPORT selective

/

56858:901:$PEER-AS

add NO_ADVERTISE selective

/

56858:902:$PEER-AS

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

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 SEECIX Looking Glass. If you type your ASN into the global search field, you will find all matching prefixes from all SEECIX route servers. 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.

How the different communities can be used 

Please note: In the following examples we only make use of Extended and 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):

  • (0:56858)
  • (56858:64501)
  • (56858:64502)
     

BGP announcements marked with the following communities are re-distributed to all peers/ASNs except AS64501 and AS64502:

  • (0:64501)
  • (0:64502)
  • (56858:56858)

BGP announcements tagged with the following communities are only re-distributed to AS65550 (4 Byte ASN) and AS64501 (2 Byte ASN):

  • (0:56858)
  • (rt:56858:65550) or (56858:1:65550)
  • (56858:64501)

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 Standard Community (RFC 1997)

BGP Extended Community (RFC 4360)

BGP Large Community (RFC 8092)

Prepend once

65001:0

/

56858:101:0

Prepend twice

65002:0

/

56858:102:0

Prepend three times

65003:0

/

56858:103:0

 

 

 

 

Prepend one selective

65001:$PEER-AS

rt:65001:§PEER-AS

56858:101:$PEER-AS

Prepend twice selective

65002:$PEER-AS

rt:65002:$PEER-AS

56858:102:$PEER-AS

Prepend three times selective

65003:$PEER-AS

rt:65003:§PEER-AS

56858:103:$PEER-AS

Graceful BGP Session Shutdown 

The SEECIX 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
Graceful BGP Shutdown

Graceful BGP Session Shutdown