MP-BGP SOO Community

[mounir.mohamed]

HI,

I wanna ask about what is the actual use from appliy the Soo community on the incoming routes also how its working i mean how its attached and how its travel the path.

[Giuseppe Larosa]

Hello Mukarram,

I'm not Martin, however I will try to answer from my experience on this subject

>> its a clear fact the PE will not accept a route from iBGP which it already received from CE through IGP because of ADs......than y do we need SoO ?????

This is the theory, in practice when you deal with a multihomed VRF site with two PE and two CE nodes you can see one PE picking as best path the MP BGP that comes from colocated PE instead of eBGP session with connected CE node.

I saw this some years ago during testing of 12.0ST code on 7500 and 12000 GSR nodes.

So the message is that you cannot consider the sessions between PEs regarding VRFs as treated as true iBGP sessions.

One of the countermeasures we took was the use of neigh .. weight command to give preference to true eBGP session with connected CE over routing feedback provided by colocated PE node.

Notice that this helps in correctly restoring BGP best paths after a failure between PE and CE node that leads to installation of routes coming from colocated PE node serving the same VRF site

Site of origin can be used in a scenario like this to avoid this routing feedback in case of single PE-CE link failure and to provide correct behaviour on restore of failed link.

Hope to help

Giuseppe

[mounir.mohamed]

The SOO community is used as loop prevention future because AS-override and allowas-in can overwrite the AS loop prevention mechanisms which may lead to routing loops but are u talking about customer that running BGP as CE-PE and used the same AS number in all branches?

[inderdeeps]

As per my knowledge the SOO is used at the following situation:

The SOO is used in the case where 2 or more CE routers at the same site have the backdoor link between them. The problem arise when PE recieve route from CE1 and then send out to CE2 which is also updated back to CE1 again via backdoor link. This may cause the suboptimal route or routing loop in some of IGP. That is where SOO came in. SOO is an ext. community of BGP - when PE receive route from CE1 and redistribute to BGP it will be impose with SOO string. The PE will not advertise these route if it see that the another CE such as CE2 has same SOO value in the vrf they are connected to because same SOO imply that CE1 and CE2 is in the same site hence the name Site of Origin.

Cheers!!!!

Inderdeep