4-2 - Inter-AS for Layer 3 MPLS VPNs

Question 1

Describe the following Inter-AS L3VPN Design: Option A - Back-to-Back VRF

Answer 1

• Simplest method
• Not scalable
• Packets are forwarded as IP packets between the ASBRs

Question 2

Describe the following Inter-AS L3VPN Design: Option B - Single-hop MP-EBGP

Answer 2

• Allows for medium scalability
• Produces some routing overhead on ASBRs
• VPNv4 prefixes are exchanges between ASBRs

Question 3

Describe the following Inter-AS L3VPN Design: Option C - Multihop MP-EBGP

Answer 3

• Most scalable
• ASBRs hold no VPNv4 prefix or label info
• VPNv4 routes are exchanged between route reflectors

Question 4

Describe the following Inter-AS L3VPN Design: Option D - Combination of Options A and B

Answer 4

• Consolidates the best functionality of A and B
• The EBGP peers on the ASBRs are called “option AB peers”

Question 5

IOS/IOS-XE Command:
mpls bgp forwarding

Answer 5

• in interface configuration mode, this enables the ASBR to send/accept labeled packets over a configured interface
• used in Option B

Question 6

IOS/IOS-XE Command:
neighbor <ip> inter-as-hybrid</ip>

Answer 6

• Configured in BGP subconfig mode
• If any prefixes import into the option AB VRFs, then the imported paths advertise to this peer.
• If any prefixes are received from this peer and import into the option AB VRFs, then the imported paths advertise to the IBGP peers.

Question 7

IOS/IOS-XE Command:
inter-as-hybrid next-hop global

Answer 7

• In vrf/address-family subconfig mode
• Specifies that the next-hop address for BGP updates for paths that import to the VRF and that are received from an option AB peer are placed in the global routing table.
• The address that is used is the address of the interface that is at the remote end of the EBGP global shared link. The next-hop context is retained as global and not modified to that of the importing VRF.

Question 8

IOS/IOS-XE and IOS-XR Command:
neighbor <ip> next-hop-unchanged</ip>

Answer 8

• in BGP/address-family subconfig mode
• when RRs receive an announcement and reflect it to the other AS’s route-reflector (i.e. EBGP), it overcomes the default behavior involving the next-hop
• the next-hop would remain unchanged. original next-hop (the originating PE) is preserved.

Question 9

Match the inter-AS option with the correct description

Description:
back-to-back VRF
multihop MP-EBGP
combination of A and B
single-hop MP-EBGP

Options:
Option D
Option C
Option B
Option A

Answer 9

back-to-back VRF = Option A
multihop MP-EBGP = Option C
combination of A and B = Option D
single-hop MP-EBGP = Option B