Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Using Communities when multi-homed to two ISPs


I have a downstream customer who has requested that we implement communities. We've never had a request for them so we just never set them up... He is multi-homed. one connection to us and one connection to Provider X.

As I see it, I can provide him the typical communities, Local Pref and MED. Prepending is not going to be useful to him since he has his own public AS# and can just prepend his own AS#.

Since local pref is stripped to ebgp peers, and MED isn't propogated outside my upstreams AS#s, what real benefit will communities have on deciding whether incoming traffic chooses a route through me or through my customers other provider?

All I can think of is creating a community with a route-map that could block traffic out one of my upstreams if he had some sort of dislike to that particular ISP

Any super feature I'm overlooking here?




Re: Using Communities when multi-homed to two ISPs

Hi andy,

It is right time to ask customer what exactly is purpose of using communities.

there is different scenario when provider need some sort of community advertised from customer. look

1- Customer want that his prefix will not be advertised to specific that time u have to ask customer to attach specific community and

   you will deny those at upstream..

2- Customer want that for few prefix provider set specific local preference...just to ensure that written traffic will move via. specific provider only.

3- Customer want that he will advt. prefix with specific community and this should be advertised to every upstream.

so this list can be very longer..but here we need to know what customer want to achieve with this accordingly you will prepare your policy by considering

security and other process which suits to you as a provider.

hope this helps



New Member

Re: Using Communities when multi-homed to two ISPs

The only thing that immediately comes to mind is that he could have you set a lower local preference value for his prefixes to keep you from sending some (or any) traffic across the connection he has with you. Maybe to have you act as a backup connection only.

In that scenario, you'll presumably also learn his prefixes from your other peers and since these would have a higher local pref value you'd never use the direct connection between you. In fact, you wouldn't even advertise that you had a direct connection to him unless his other connection were down.

I'm sure someone will think of some other scenarios that probably make more sense.