CCIE SP MPLS FAQ: Provider Edge (PE) to Customer Edge (CE) Connectivity Options

CCIE SP MPLS FAQ: Provider Edge (PE) to Customer Edge (CE) Connectivity Options


Figure: PE-to-CE Connectivity Options—Sample Topology

Q1. What are the four ways in which routing information can currently be learned from a CE router?

Answer: Static, BGP, OSPF, and RIP version 2.

Q2. When running a dynamic routing protocol between PE and CE, how can the PE identify which routing update belongs to which VRF?

Answer: This is identified by the routing context that is configured on the PE router as an address-family.

Q3. Is it possible for multiple routing contexts of different routing processes to be associated with the same VRF?

Answer: Yes. This is possible by configuring the address-family into the relevant VRF.

Q4. When running OSPF PE CE, how many layers of routing hierarchy are necessary?

Answer: Three: area 0 (backbone area), other OSPF areas, and MPLS VPNbackbone.

Q5. Is it necessary for a VPN site OSPF area 0 to be directly attached to the MPLS VPN backbone?

Answer: Yes. Because every PE router acts as an ABR and, therefore, must to be attached to the backbone, either directly or through a virtual link.

Q6. When running OSPF as the routing protocol between the PE routers and the C routers, are routing adjacencies formed across the MPLS VPN backbone?

Answer: No. OSPF routing information is distributed through the use of MP-BGP.

Q7. How are routing loops prevented between sites when running OSPF PE CE?

Answer: Through use of the down bit, which prevents a PE router from distributing a summary-LSA from an attached site if the bit is set.

Q8. Which feature is necessary to allow VPN sites to run the same autonomous system number?

Answer: The AS-override feature, which enables the PE router to rewrite the AS_PATH to contain only the service provider AS number rather than the originating site.

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