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bgp communities and numbers?

All,

I understand most of the bgp communities like no-advertise, no-export, etc., but our provider had us add a community full of numbers to workaround another provider advertising our address space. What do these numbers do exactly?

It's something like:

route-map set-community permit 10

match ip address 5

set community 528739922

I can only assume that they're putting a priority on this community in a route-map on their side and then making the other providers advertisement and lower priority. This was a cutover from a legacy system to a newer system with the legacy system let unmodified. It screwed up our routing last night, and it seems like this is what they did to fix it. I wasn't involved in this one, but I'm curious as to what the above route-map does.

Thanks,

John

HTH, John *** Please rate all useful posts ***
3 ACCEPTED SOLUTIONS

Accepted Solutions

Re: bgp communities and numbers?

Communities can be set both inbound or outbound. when done outbound (towards another AS) its purpose needs to be coordinated and agreed.

I think your neighboring AS knows about this community and is taking action...else what is the point of setting it outbound.

Sam

Hall of Fame Super Bronze

Re: bgp communities and numbers?

John,

Communities provide a tagging mechanism where you can group customers and treat their traffic accordingly.

Based on your community, the ISP is making traffic decision within their network and perhaps with other ISP carriers as well.

What type of routing decisions are made? Only the ISP can answer that.

HTH,

__

Edison.

Re: bgp communities and numbers?

John

If u add this line "ip bgp-community new-format" to ur box, u will see that the long number is actually community 8067:61010 so ur AS + tag for action.

HTH

Sam

7 REPLIES

Re: bgp communities and numbers?

route map is merely setting a community for ACL 5 prefixes.

there must be an action taking place further away in the network with a statement matching this community Number.

some ISPs use community values to take specific action in their own AS. so Customer set the community to get ISP to take an automatic action based on the community agreed.

You need to find out more about what action is taking place when this community is matched.

HTH

Sam

Re: bgp communities and numbers?

That's what I had thought at first. Normally, from what configs I've seen, it would set the community in-bound (we're setting it out), and then a route-map would match the community and advertise to it's other peers, correct? Well, if that's the case, the only other thing that I could think of is maybe the provider is matching on our community and block, and they're sending a community to the other provider with a no-advertise? I can't see any other way that they could keep the other provider/system from advertising our block.

John

HTH, John *** Please rate all useful posts ***

Re: bgp communities and numbers?

Communities can be set both inbound or outbound. when done outbound (towards another AS) its purpose needs to be coordinated and agreed.

I think your neighboring AS knows about this community and is taking action...else what is the point of setting it outbound.

Sam

Hall of Fame Super Bronze

Re: bgp communities and numbers?

John,

Communities provide a tagging mechanism where you can group customers and treat their traffic accordingly.

Based on your community, the ISP is making traffic decision within their network and perhaps with other ISP carriers as well.

What type of routing decisions are made? Only the ISP can answer that.

HTH,

__

Edison.

Cisco Employee

Re: bgp communities and numbers?

John,

This technique is described in RFC1998. It doesn't mean that this is exactly what your SP is doing with the community attribute you are sending them but this is by far the most commonly used approach.

http://www.ietf.org/rfc/rfc1998.txt?number=1998

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Re: bgp communities and numbers?

John

If u add this line "ip bgp-community new-format" to ur box, u will see that the long number is actually community 8067:61010 so ur AS + tag for action.

HTH

Sam

Cisco Employee

Re: bgp communities and numbers?

Sam,

Good point. It certainly makes it easier to read ;-)

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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