CCNP SP MPLS FAQ: MPLS VPN

Q1. What is a route distinguisher?

Answer: A route distinguisher is a 64-bit value—chosen by the service provider—that is appended to the IPv4 routes of the customer and makes the routes unique.

Q2. How is a packet that is coming from the CE router identified as to which VRF it belongs?

Answer: The packet is identified as belonging to a VRF by the VRF configuration on the interface.

Q3. What is the purpose of RTs?

Answer: You use RTs to import vpnv4 routes into the VRF routing table and to export the vpnv4 routes to VRF sites that accept the RT.

Q4. What is an RR group?

Answer: An RR group can be configured on route reflectors to filter vpnv4 routes.

Q5. What is the BGP neighbor command with as-override used for?

Answer: The BGP speaker checks the as-path of the BGP route and replaces all occurrences of the autonomous system number of the BGP peer in the as-path with his own autonomous system number.

Q6. When would you use different route distinguishers for routes of the same VPN?

Answer: You would use different route distinguishers for hub-and-spoke topologies and when you need to make different vpnv4 routes from the same IPv4 route when sites are dual homed to PE routers and the MPLS VPN network has route reflectors.

Q7. What command should you configure on a Multi-VRF CE router that is running OSPF?

Answer: You should configure capability vrf-lite on a Multi-VRF CE router that is running OSPF.

Q8. What three characteristics does an OSPF sham link have?

Answer: An OSPF sham link has the following characteristics:

Unnumbered
Point-to-point
Demand-circuit

Q9. Why do MPLS VPN packets have two MPLS labels?

Answer: P routers use the IGP label to forward the packet to the correct egress PE router. The egress PE router uses the VPN label to forward the IP packet to the correct CE router.

Q10. Which BGP extended community can prevent routing loops from occurring in MPLS VPN networks?

Answer: Site-of-Origin (SOO) can prevent routing loops from occurring in MPLS VPN networks.

Q11. How can overlapping IP addresses be supported between customer sites when MPLS VPN is deployed?

Answer: Each VPN has access to its own routing and forwarding tables

Q12. What routing information is contained within the global routing table on

Answer: Routes internal to the P network (internal service provider

Q13. The combination of the per VPN routing table and per VPN forwarding

Answer: Virtual Routing and Forwarding Instance (VRF).

Q14. What is a route target?

Answer: A route target is a 64 bit value attached to a BGP route as an

Q15. What is the purpose of the route target?

Answer: To determine which routes must be imported into which VRFs so that VPN membership can be achieved.

Q16. How are client VPN routes distributed across the service provider P network?

ccnp-sp-mpls-faq-mpls-vpn

Answer: Through use of Multiprotocol BGP (MP-BGP), which provides extensions to BGP so that non-IP prefixes can be transported.

Q17. What is a route distinguisher (RD)?

Answer:The RD is a 64-bit prefix prepended to a VPN client IPv4 address to make it globally unique across the P network.

Q18. What is a routing context?

Answer: It is a routing instance within a routing process that distinguishes between different VPN routing information.

Q19. Using the basic MPLS VPN mechanisms is it possible to have overlapping

Answer: No. During the import process, one or more updates are lost because BGP selects one as the best path to the given

Q20. How can the issue of overlapping IP addresses between different sites in an extranet be resolved?

Answer: Through the use of Network Address Translation (NAT).

Q21.What are the two different formats that can be used for the route distinguisher (RD)?

Answer: ASN:nn and IP Address:nn.

Q22. Does the RD have any special meaning for BGP?

Answer: No. BGP interprets the RD as a sequence of bits that, together with the IP address, make up the VPNv4 prefix.

Q23: How are loops prevented between VPN client sites when these sites are multihomed to the backbone?

Answer: Through use of the Site of Origin (SOO) attribute.

Q24. What type-code is used for the route target within the Extended Community attribute?

Answer: Type-code 0x0002 or 0x0102.

Q25. What must you configure to enable static routing, or routing information learned through RIP version 2 or OSPF to be advertised between PE routers?

Answer: Redistribution between the various routing processes and BGP must be enabled.

Q26. Which specific BGP capability must be supported between PE routers to enable the successful exchange of VPNv4 prefix information?

Answer: Multiprotocol Extensions capability.

Q27. What information is carried within the MP_REACH_NLRI attribute?

Answer: Address family information, next-hop information, and NLRI.

Q28. During periods of convergence why does a cell mode MPLS

Answer: Cell-mode MPLS uses downstream-on-demand label distribution with conservative retention mode This means that an upstream

Q29. How often does the import scanner run on a PE router?

Answer: Frequency is based on a timer, which is 15 seconds, by default.

Q30. Why is it necessary to be careful when adjusting the scanner interval from its default of 60 seconds?

Answer: If there are many routes and a low scanner interval, the router’s CPU can be adversely affected.

Q31. Can a single BGP session carry routes for multiple address families?

Answer: Yes. In the case of MPLS VPN, this might be the IPv4 address family for Internet and the VPNv4 address family for VPN.

Q32. If only VPNv4 routes must be carried across a BGP session, which command prevents the advertisement of IPv4 routes?

Answer: The no bgp default ipv4-unicast command.

Q33. Describe the functionality of a route reflector.

Answer: The route reflector is a BGP-speaking router that propagates internal BGP routes to other internal BGP peers.

Q34. Describe the main benefit of route-reflector partitioning.

Answer: It allows the network to be split so that a certain set of PE routers can peer with a certain route reflector, or set of route reflectors. This setup has the advantage of helping to scale the BGP topology.

Q35. Describe the ORF capability.

Answer: ORF enables a router to push its inbound route filtering to a peer so that it can set its outbound route filtering to prevent the propagation of unwanted routing information.

Q36. In an IP routed backbone why is it necessary for P routers to hold full

Answer: P routers forward packets based on their destination IP address. If this address is not within the forwarding table the packets are

Q37. How does the full routing requirement change with the introduction of MPLS into the core?

Answer: Because MPLS forwards based on labels, the IP destination address information is no longer required on routers performing pure label switching, except when Multicast routing is deployed.

Q38. What are the two advantages to carrying external routing in BGP rather than in the service provider IGP?

Answer: Stability so that flapping within a customer site does not affect the backbone. The size of the internal routing structure within the service provider can be kept to a minimum.

Q39. When running OSPF, why is it necessary for the PE loopback addresses to be a /32 host address?

Answer: OSPF always sets a loopback to a /32 regardless of its configured mask, which causes a loss of connectivity because TDP/LDP advertises the address with the configured mask, thus causing a mismatch.

Q40. Why can’t PE loopback addresses be summarized within the backbone?

Answer: They cannot be summarized because if they were, the end-to-end LSP between PE routers would be broken, causing a loss of connectivity for the VPN client.

Q41. During the PE import process, what can be used to filter out certain routes?

Answer: An import-map can be used to filter on IP prefix, standard community, or extended community.

Q42. Is it possible to advertise routes from the same VRF with different RT values?

Answer: Yes. Through the use of export-maps.

CCNP SP MPLS FAQ: MPLS VPN

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