1-6 - VRF and Routing in the MPLS VPN Control Plane

Question 1

What routing requirements did the designers of MPLS VPN technology face?

Answer 1

• customer edge routers should not be MPLS VPN-aware
• PE routers must support MPLS VPN services and traditional Internet services
• To make the MPLS VPN solution scalable, P routers must not participate in customer VPN routing

Question 2

What are the two (broad) categories of VRFs?

Answer 2

• default VRF: core routes that core IGP installs and Internet routes that BGP installs
• VPN VRF: sets of sites with matching routing requirements; information from customer edge routers; MP-BGP info from other PE routers

Question 3

T/F: MP-BGP exchanges routes between PE routers by using the neighbor definitions, VPNv4 info, and any BGP extended communities (e.g. route targets).

Answer 3

True

Question 4

Multi-VRF support

Answer 4

a feature that allows you to configure and maintain more than one instance of a routing and forwarding table within the same PE device

Question 5

Outer Transport MPLS Label

Answer 5

MPLS label used to transport the traffic across the backbone network from PE to PE

Question 6

Inner VPN MPLS label

Answer 6

• uniquely identifies which VRF the packet belongs to
• the egress PE sets this (based on info it received from the far-end PE via VPNv4 announcements)
• the ingress PE reads this and thats how it knows what VRF the traffic belongs to

Question 7

What are the L3VPN Label Modes?

Answer 7

per-customer edge mode: a unique VPN label is allocated for each CE
per-VPN mode: the same label generates all prefixes in the VRF
per-prefix mode: each prefix is assigned a unique VPN label

Question 8

What is Flexible L3VPN Label Allocation Mode?

Answer 8

basing the label mode configuration on arbitrary match criteria, like prefix value and communities. Route-policies are leveraged to do this.

Question 9

What elements are contained in MP-BGP updates that are exchanged between PE routers?

Answer 9

• VPNv4 address
• extended BGP communities (at least one route target is necessary for MPLS VPNs)
• label for VPN packet forwarding
• Mandatory BGP attributes such as AS-path (optionally, the MP-BGP update can contain any other BGP attribute)

Question 10

PE routers can perform the following exchanges:

Answer 10

• PE routers exchange IPv4 VPN routes with CE routers via various routing protocols that run in the virtual routing tables
• PE routers exchange VPNv4 routes via MP-BGP sessions with other PE routers
• PE routers exchange core routes with provider routers and other PE routers via core IGP

Question 11

Which of the following is true about VPN labels?

A. A unique VPN label is allocated for each PE router
B. The egress provider edge advertises the VPNv4 prefix to the CE router
C. Each prefix is assigned a duplicate VPN label before advertisement
D. In the per-customer edge mode, a unique VPN lable is allocated for each CE router

Answer 11

D