Saturday, January 3, 2009

Inter AS MPLS VPN Options

Option A (Back to Back VRF)

In this Option, logical interfaces will be configured between ASBRs and each logical interface will be associated with one VRF. Each ASBR will treat the other ASBR as CE and will support any PE-CE routing protocol between ASBRs. In this option, label will be assigned at three routers for a prefix.

Traffic between ASBR will be forwarded as IP packet.

Option B

With Option B, VPNv4-eBGP neighborship will be established between ASBRs without LDP between ASBRs.

Now when ASBR receives an VPNv4 prefix from other ASBR, as per iBGP rule, it will advertise the same to

other iBGP neighbors (other PEs) without changing the Next hop. As normally, the link between ASBRs will not

be advertised into our AS, we have 3 sub options to get it work. The 3 sub options as below,

Option B -2a – On ASBRs; configure “next-hop-self” to all iBGP neighbors (PEs). Label allocation happens in

PE1, ASBR1 and ASBR2.

Option B -2b – On ASBRs, when next-hop is not modified, a host entry (/32) will be installed as connected for

other ASBR. We need to redistribute this route into IGP using “redistribute connected” command. (If we have

more connected interface and which we don’t need to be redistributed, we can use a route-map that matches

only the host entry of ASBR). Label allocation happens in PE1 and ASBR1.

Option B – 2c – With above options (Option 2a, Option 2b), we run the risk of having single link failure. As part

of redundancy, we can have multiple links between ASBRs and establish eBGP neighborship between loopback

interfaces. Static routes need to be configured at ASBRs to reach the other ASBR’s loopback address and

should be redistributed using “redistribute static” command.

Option C – This option required ASBR only to advertise IPv4 prefix about PE and RR between AS and not

VPNv4. RR will be configured with Multihop eBGP to propogate VPNv4 prefixes with labels. Normaaly this is recommended when two AS merge happens. Else this may not be a right choice as not all the Service porviders will

agree to advertise the IPv4 prefixes about PEs and RRs to other AS.

Option AB – To be Updated

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