CCIE SP MPLS FAQ: MPLS VPN Troubleshooting
Figure: Sample MPLS VPN Network
Q1. What are the common errors that occur in an MPLS VPN network that are not a result of MPLS VPN design or configuration error?
Answer: The common errors, apart from the users reporting a network error when something else is broken, are MPLS MTU issues in the core of the MPLS backbone and broken end-to-end LSP.
Q2. When would the RIP routing process between the CE and PE router be broken?
Answer: Most commonly, this occurs as a result of RIP Version 1 being used on PE or CE router.
Q3. When would an RIP subnet get inserted as a major network into MP-BGP?
Answer: When you enable auto-summarization in the BGP routing process.
Q4. Why would a route received from a CE router not be propagated to other PE routers?
Answer: Because you forgot to redistribute routes from PE-to-CE routing protocol into BGP.
Q5. When would the export route target specified in the VRF definition not be attached to the MP-BGP route?
Answer: If you use an export map and the route-map used in the export process sets the route target with the set extcommunity command with no additive keyword, all route targets previously attached to the route are lost.
Q6. Why would a receiving PE router ignore an MP-BGP route?
Answer: The receiving PE router that is not a BGP route reflector would ignore a MP-BGP route if none of the route targets attached to the route match any import route target configured in the VRFs on the PE router.
Q7. Why would a receiving PE router decide to ignore an MP-BGP route even though there is a match in the route targets attached to the MP-BGP route and route targets configured in the VRF?
Answer: If you use a misconfigured import route-map, the routes passing the route target check are still ignored because they are rejected by the import route-map.
8: Why would an MP-BGP route received from another PE router not be propagated to the CE router?
Answer: If you forget to configure redistribution from MP-BGP to PE-to-CE routing protocol, the remote routes never propagate to the CE routers. Similarly, if the routes are redistributed into the PE-to-CE routing protocol but with an incorrect metric, they are not advertised.