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Re: Ask the Expert- MPLS Best Practices- Configuration & Troubleshooting

Hi David,

Quick question. I have MP-BGP core which consists from two Cat6800 running iBGP between them. OSPF is running as IGP. There is also CORP VRF where all corporate subnets are. Recently I added two prefixes to both core switches BGP and installing static route pointing to NULL0(AD 254). Strange thing is one core see these prefixes in local in VPNv4 routing table second see only route that is being advertised by first CORE but it's local route is not showing up which also pointing to NULL0.

 

My quess is that is because of AD iBGP is preferred because it has 200. Am I correct? Strange thing is both CORE switches see previously added prefixes local and iBGP advertised by neighbor. I'm confused a little bit.

Hope it will help. Best regards, Abzal

Re: Ask the Expert- MPLS Best Practices- Configuration & Troubleshooting

Hello Abzal
 
I will start by pointing out a general rule: if a protocol redistributes a route from the routing table, it never offers that very same route back to the routing table. This goes for any routing protocol, BGP or any other.
 
Now, focusing in your particular situation:
 
I think that what you are experiencing is a race condition. Your results would vary depending on the timing of your configurations.
 
Allow me to elaborate my statement: 
 
1.) On Core1, add the static route to Null0 with AD of 254 and redistribute it into BGP.
2.) Core2 will learn it from Core1 over iBGP and install it into RIB with AD of 200.
3.) On Core2, add the same static route with an AD of 254, but it wouldn't not make it into RIB because the iBGP-learned route with AD of 200 is better.
 
If you would add the static route to both Core routers first, and then manage to have it redistributed into BGP before they advertise it to each other, none of the routers will install the route into the RIB as the iBGP-learned route.
 
This is because in BGP RIB, there will be two alternatives:
- The locally injected (redistributed one)
- The one learned from the other Core
 
And BGP bestpath picks the locally injected one as the preferred one.
 
This would be true for non-Cisco implementations.
 
In case of Cisco IOS, since all locally-injected routes get the weight of 32768 by default while all learned routes have a weight of 0, the locally redistributed route will win thanks to its weight. Different means, the same result :)
 
You were right in your assumption.
 
Hope this helps
 
David 
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