How to validate community if its workin?

Unanswered Question
Aug 14th, 2008

Hi Guru's,

I implemented some community on my bgp , when i do show ip bgp <network>, i cant see any community info, how will i validate this ?

In addition, sending this community to my provider, how will they interpret this ?


I have this problem too.
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Giuseppe Larosa Fri, 08/15/2008 - 05:59

Hello Jefferson,

have you configured

neighbor x.x.x.x send-community

where x.x.x.x is your provider ?

a very useful command in global config is:

ip bgp-community new-format

without this the community is shown as a number and not as ASN:

the sh ip bgp accepts options to show the advertised prefixes and their attributes.


show ip bgp community

I would check with that version of the show because if you are setting the community on received advertisements may be you still see the original advertisement and the change is sent to the neighbor.

About what the provider will do with the BGP community you need to work with ISP stuff on this.

They can increase the local preference based on the BGP community, many other actions are possible.

Hope to help


jeffersoncbriones Fri, 08/15/2008 - 17:25

Hi Giuseppe,

Here's what i did.

router bgp MYAS#

bgp log-neighbor-changes

neighbor Neighbor-IP remote-as RemoteAS#

neighbor Neighbor-IP version 4


address-family ipv4

neighbor Neighbor-IP activate

neighbor Neighbor-IP send-community

neighbor Neighbor-IP soft-reconfiguration inbound

neighbor Neighbor-IP# route-map toRemoteAS# out

ip as-path access-list 1 permit ^$

ip as-path access-list 1 deny .*

route-map toRemoteAS# permit 10

match as-path 1

set community RemoteAS#:120

When i do show ip bgp community no output is shown.



Giuseppe Larosa Sat, 08/16/2008 - 00:21

Hello Jeff,

everything looks like fine but the BGP community new format is not set.

try to do a sh ip bgp

where your.prefix matches the empty AS path.

Do you see at the end of the show something like Community: there is here some big number?

In this case you need ip bgp-community new-format in global config to see Community as you typed it.

Verify in the output that it tells advertised to and the list is not empty.

If you dont' see a Community field this can derive from the fact that you are setting the community towards a neighbor.

To be sure to see the community attribute:

you can try to get the prefix advertised into your router via some form of redistribution.

suppose you want to advertise x.x.x.0/24

instead of using network x.x.x.0

we use

redistribute eigrp 20 route-map set-com

route-map set-com permit 10

match ip address 15

set community RemoteAS#:120

in this case the BGP advertisement gets the community when enters the BGP table.

sh ip bgp community RemoteAS#:120 should show the prefix x.x.x.0/24

BGP has actually different tables and one of them is for the updates to be sent.

Some IOS releases allows to show what you send to a neighbor that is to show the updates' table.

Hope to help


jeffersoncbriones Sat, 08/16/2008 - 00:52

Hi Giuseppe ,

Thanks for the clarification, i have already set the new format config (just forgot to paste it on the config). So basically i cannot see the effect of my config on my side ? how bout checking on route servers?


rajivrajan1 Sat, 08/16/2008 - 01:28

Q.On the far end router ( to which communities are sent) is there any output for command -

sho ip bgp community


If it is there

Sh ip bgp < prefix-from the output of above command>

will result in showing communities.

if it's not there try using command

neighbor send-community both

and don't forget to refresh routing updates.


use an access-list and find any matches are there in local router for sending communities.

Giuseppe Larosa Sat, 08/16/2008 - 06:42

Hello Jeff,

if you send the advertisement to your own reflector server you can check on it.

Or you can use the looking glass of your provider, if it has a public looking glass.

Hope to help


arunsing Sat, 08/16/2008 - 05:05

Hi Jeff,

Can you pls paste the output of the show ip bgp command. I hope you are not using any private AS on the internal routers.

arunsing Sat, 08/16/2008 - 05:11

You can try the following command to check if there is any NLRI to match the regex expression being used by the as path list.

sh ip bgp regexp ^$

jeffersoncbriones Mon, 08/18/2008 - 17:25

Hi guys,

Im not using private ASN. when i do show ip bgp community not output is shown. Based on my route map config, the local preference should be 120. And this is going out to my provider. When i do show ip bgp regexp ^$, the local preference for my networks are still 100. Should i see 120 local pref on these ?



micahcox Mon, 08/18/2008 - 22:35

If I understand the scenario correctly, you send a community to your ISP and they set local preference toward you within their AS based on the community you set. If thats the scenario then you will NOT see local preference change on your routers. You also will not see it anywhere outside of your ISP's AS. Local preference is non-transitive, it will not be propagated to the route servers.

Think of local preference is used to make an outbound routing decision from an AS. When you look at local preference in your own AS, you are viewing your outbound preference toward that network.

You will have to call your provider or hope they have a public route server available within their AS.


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