BGP Community attribute for loop preventing

[#1]

Can I use community attribute for loop preventing? If yes, please, explain how?

[Reza Sharifi]

In BGP, As-path attribute is used for loop prevention

Have a look at this post:

https://supportforums.cisco.com/discussion/10864336/bgp-loop-prevention-aspath

HTH

[#3]

Ofcource I knew about AS_Path, but my question about "how I can make loop preventing using comminity attribute".

[Francesco Molino]

Hi 

The default mechanism had been well described. 

Now if you want to ensure that a learned route isn't advertised back by using community, you'll need to set a community as inbound using route-map with your neighbor bgp (for all learned routes) and another route-map outbound with this same neighbor bgp that will deny announcement of all subnets tagged with a specific community. 

In terms of logic, bgp had its default mechanism for loop prevention (ebgp and ibgp are different). But maybe you can explain a bit more what is your final goal? 

If you want some help to build the config let us know. 

Thanks 

PS: Please don't forget to rate and mark as correct answer if this answered your question

[abache]

I'm sure you're aware that BGP's inbuilt loop prevention technique using AS Path is solid and works reliably and well. Where it tends to break down however is perhaps when you have 2 more of entry points and there is two-way redistribution configured between BGP and some IGP. In such cases, often tried and trusted loop prevention techniques like tagging, AD manipulation etc work well.

However there is a community called SoO (site of origin) that prevents prefixes originating from an AS being advertised back to that AS. This feature is normally configured on your upstream eBGP neighbours (i.e. PE routers). Once configured the PE's will no longer advertise your prefixes back to you.This is a slight deviation from standard eBGP behaviour whereby an eBGP neighbour will typically advertise all prefixes to you (including ones you orignated) and its up to your device to check the AS path attribute for each prefix it receives.

The following link goes into more detail about the SoO community:

http://www.cisco.com/c/en/us/td/docs/ios/12_4t/12_4t11/htbgpsoo.html#wp1054840