Solved: BGP Community AA:NN puzzled

p.mckay · ‎04-04-2006

I am a bit puzzled by the following statements

I understand where 30540 is the AS as well as 6539 but where does the :1 or :100 come from.

ip community-list 10 permit 30550:1

ip community-list 20 permit 6540:100

vettrock · ‎04-05-2006

Your provider is taging various routes with various tag for some purpose that is sometimes listed on their web page or in the whois database. Normally if you are tagging routes for your own purpose you should use your own AS or a private AS.

Lots of providers will let you tag routes with various communities in order for them to manipulate the local prefrence, prepend it, or supress advertisement to various peers.

If you do a whois -h radb.net AS702 you get the following at the end of the entry:

remarks: --------------------------------------------------------------

MCI filters out inbound prefixes longer than /24.

We also filter any networks within AS702:RS-INBOUND-FILTER.

--------------------------------------------------------------

MCI uses the following communities with its customers:

702:80 Set Local Pref 80 within AS702

702:120 Set Local Pref 120 within AS702

702:20 Announce only to MCI AS'es and MCI customers

702:30 Keep within Europe, don't announce to other MCI AS's

702:1 Prepend AS702 once at edges of MCI to Peers

702:2 Prepend AS702 twice at edges of MCI to Peers

702:3 Prepend AS702 thrice at edges of MCI to Peers

--------------------------------------------------------------

Advanced communities for customers

702:7020 Do not announce to AS702 peers with a scope of

National but advertise to Global Peers, European

Peers and MCI customers.

702:7001 Prepend AS702 once at edges of MCI to AS702

peers with a scope of National.

702:7002 Prepend AS702 twice at edges of MCI to AS702

peers with a scope of National.

702:7003 Prepend AS702 thrice at edges of MCI to AS702

peers with a scope of National.

702:8020 Do not announce to AS702 peers with a scope of

European but advertise to Global Peers, National

Peers and MCI customers.

702:8001 Prepend AS702 once at edges of MCI to AS702

peers with a scope of European.

702:8002 Prepend AS702 twice at edges of MCI to AS702

peers with a scope of European.

702:8003 Prepend AS702 thrice at edges of MCI to AS702

peers with a scope of European.

--------------------------------------------------------------

Additional details of the MCI communities are located at:

http://global.mci.com/uk/customer/bgp/

--------------------------------------------------------------

Details of MCI's peering policy and how to get in touch with

MCI regarding peering policy matters can be found at:

http://global.mci.com/peering/

--------------------------------------------------------------

View solution in original post

pkhatri · ‎04-05-2006

That is absolutely correct. You are free to allocate that number to be anything you want as long as you match on it accordingly within your route-maps.

The reason you see multiple communities from the provider is probably because the provider has their own mechanism and reasons for tagging the routes with the various communities. The actual values are irrelevant as far as you are concerned.

Pls do remember to rate posts.

Paresh

View solution in original post

pkhatri · ‎04-04-2006

Hi,

A community is a 32-bit community which is typically structured as 16-bits of the AS number and 16-bits of a number locally significant to that AS. The use of suc a convention allows some degree of uniqueness of community values.

Therefore, a community of 6540:100 simply indicates an AS# of 6540 and a locally significant community identifier of 100.

Pls do remember to rate posts.

Paresh

p.mckay · ‎04-05-2006

Thanks so the :100 is something that you can make up to tag routes coming from the provider. Therefore if I had two providers plus my own AS I could change the :100 to virtually anything as long as the route maps refeenced are also changed so I could change the :100 to :10

ip community-list 10 permit 30550:1

ip community-list 20 permit 6540:10

ip community-list 30 permit 577:20

The thing that confuses me since is that when I do look at some routes that come from the provider 6540 as in sh ip bgp 65.19.196.0/23 I see

Community: 6540:10 and on other routes I can see :20 :30 so I must assume that the upstream provider is sending the :10 :20 :30

Once again thanks

vettrock · ‎04-05-2006

Your provider is taging various routes with various tag for some purpose that is sometimes listed on their web page or in the whois database. Normally if you are tagging routes for your own purpose you should use your own AS or a private AS.

Lots of providers will let you tag routes with various communities in order for them to manipulate the local prefrence, prepend it, or supress advertisement to various peers.

If you do a whois -h radb.net AS702 you get the following at the end of the entry:

remarks: --------------------------------------------------------------

MCI filters out inbound prefixes longer than /24.

We also filter any networks within AS702:RS-INBOUND-FILTER.

--------------------------------------------------------------

MCI uses the following communities with its customers:

702:80 Set Local Pref 80 within AS702

702:120 Set Local Pref 120 within AS702

702:20 Announce only to MCI AS'es and MCI customers

702:30 Keep within Europe, don't announce to other MCI AS's

702:1 Prepend AS702 once at edges of MCI to Peers

702:2 Prepend AS702 twice at edges of MCI to Peers

702:3 Prepend AS702 thrice at edges of MCI to Peers

--------------------------------------------------------------

Advanced communities for customers

702:7020 Do not announce to AS702 peers with a scope of

National but advertise to Global Peers, European

Peers and MCI customers.

702:7001 Prepend AS702 once at edges of MCI to AS702

peers with a scope of National.

702:7002 Prepend AS702 twice at edges of MCI to AS702

peers with a scope of National.

702:7003 Prepend AS702 thrice at edges of MCI to AS702

peers with a scope of National.

702:8020 Do not announce to AS702 peers with a scope of

European but advertise to Global Peers, National

Peers and MCI customers.

702:8001 Prepend AS702 once at edges of MCI to AS702

peers with a scope of European.

702:8002 Prepend AS702 twice at edges of MCI to AS702

peers with a scope of European.

702:8003 Prepend AS702 thrice at edges of MCI to AS702

peers with a scope of European.

--------------------------------------------------------------

Additional details of the MCI communities are located at:

http://global.mci.com/uk/customer/bgp/

--------------------------------------------------------------

Details of MCI's peering policy and how to get in touch with

MCI regarding peering policy matters can be found at:

http://global.mci.com/peering/

--------------------------------------------------------------

p.mckay · ‎04-06-2006

What are you using as a whois tool. I live in a Windows world and all I have been able to find are tools that show domain names none seem to accept commnad lines other then IP address or Hostname.

pkhatri · ‎04-05-2006

That is absolutely correct. You are free to allocate that number to be anything you want as long as you match on it accordingly within your route-maps.

The reason you see multiple communities from the provider is probably because the provider has their own mechanism and reasons for tagging the routes with the various communities. The actual values are irrelevant as far as you are concerned.

Pls do remember to rate posts.

Paresh