CCIE SP MPLS FAQ: MPLS Migration and Configuration Case Study
Figure: TransitNet Backbone Network Topology
Q1. Carrying external customer routes within BGP has two advantages; list them.
Answer: Having numerous routes within the IGP can lead to stability and scalability issues that are not present within BGP. The internal routing infrastructure of the service provider is protected from outside route flapping of customer routes.
Q2. Why must Cisco Express Forwarding (CEF) be globally enabled on all routers within an MPLS network?
Answer: Because CEF is the only Cisco switching mechanism that builds an FIB, which is necessary for label imposition.
Q3.Why is it possible to remove BGP from internal routers when running MPLS?
Answer: Label switching to external destinations is performed toward the BGP next hop of the route, and therefore, internal LSRs do not need to hold routing information for the external routes; they need to hold only their next-hop addresses.
Q4. Is it possible to run MPLS on only a certain number of links within a service provider backbone?
Answer: Yes, although you must take care to ensure that routing information can forward any unlabeled packets.