SOO Site of origin!!... Clarification please!!

Question

Hello to all, I'm having problems in understanding how bgp choose one route and discard other based in its SOO, for example
suppose this example
look at the network diagram in the link:
Figure 1 Network Diagram for SoO Example
(BGP per Neighbor SoO Configuration)
http://www.cisco.com/en/US/partner/products/ps6441/products_feature_guide09186a0080783eac.html
For my example I will tag the ingress router on PE1 from CPE1 with soo 65000:1, and I will tag again the ingress on PE2 from CPE2 with the same soo 65000:1, obvious that I'm using as-overide in this example....
My question is, suposse we are analising PE2, and from the left receive the routes of CPE2 tagged with soo through IGP and from EGP is receiving the routes directly from CPE2 tagged with the same soo... so the question is, because of the same SOO the PE2 have to choose a network path and discard other, how is the selection process, the PE2 look first at soo and after that makes a decision on external internal protocol or how is the algorithm in the decision for discarting, sorry if it seems banal, but I undertand the soo but I don't understand if we have two same soo network how to choose one and discard other?....
Best Regards!

Answer 1

Marcelo, neither PE1 or PE2 discards any route. The BGP decision making is also not altered, so when in the same example PE2 receives routes from PE1 with SOO tag, it runs the best path selection on the routes as usual, ignoring the SOO, when it receives these updates, now after receiving when it has to send these updates out that is when the SOO is helpful. So PE2 observes that his neighbor also has the same SOO tag, so he does not send those routes out to that neighbor. But PE2 very much has those routes in the BGP table.
One simple example why these routes are kept in the BGP table and not discarded is, on the same PE2 these may be CE3 also connected. But CE3 may not have a backdoor to CE1 or CE2, in that case the PE2 needs the routes so it can give it to CE3 which is not configured with the same SOO.
So its very much neighbor/site dependent, even if the neighbor(CE2) doesnt have a route still the PE2 will not send anything received from CE1/PE1.
So if implementing SOO, its as good as mandatory to have a IBGP mesh between your sites where you configure SOO.
HTH-Cheers,
Swaroop