From: Paresh Khatri (Paresh.Khatri@aapt.com.au)
Date: Sun Feb 20 2005 - 20:00:14 GMT-3
Hi Andre,
Let's call the PE setting the LP to 100 - PE1
Let's call the PE setting the LP to 150 - PE2
The third PE is, of course, PE3.
I assume there is a full MP-BGP mesh between the 3 PEs.
When PE1 receives the BGP route from PE2, it will select that route as the best path. Therefore, if it has previously advertised the route that it learnt from the CE (with a LP of 100), it will withdraw that route. Since this is IBGP, it will not be advertising any route for that network at all now, to the other PEs. This behaviour stems from the fact that a BGP peer only advertises what it considers to be the "best" path. For PE1, the best route is through PE2, so it does not advertise it's locally learnt route. The only PE on which you should see both routes is PE1.
HTH,
Paresh.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Andre Scalco
Sent: Saturday, 19 February 2005 02:00 PM
To: ccielab@groupstudy.com
Subject: MPLS MP-BGP router behavior
I'm testing some features of MPLS in a LAB and I got a scenario that I would like to discuss with you.
I have two CEs configured injecting the same network (10.0.0.0/24) to their respective PEs, one PE is setting the Local Preference as 100 and the other one as 150.
On a Third PE I do see the network 10.0.0.0/24 being correctly learned from the PE that is setting the LP as 150, but I don't actually see on my bgp table for that VRF the other possible path from the PE that's setting as 100.
My assumption is that when the Third PE do the "scan" to place that 10.0.0.0/24 under the VRF it does select only one possible path regardless the network is coming from more than one place. However if I do a "Show ip bgp vpn rd all" then I should see this network from the two PEs that are advertising this network.
So in other words, in MP-BGP I should consider this rules with extended community RT before any standard best path selection algorithm "weight,lp,med,etc"
Let me know what you guys think,
Andre
Best Regards,
Andre Scalco
"Think Outside the Bun"
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