Inter-AS MPLS VPN - The whole story (1)

The IETF released the first RFC for MPLS L3VPNs; RFC 2547, “BGP/MPLS VPNs” in 1999 as a standardized form for the emerging Cisco’s tag switching at that time, succeeded with revised drafts in 2003, 2004 and 2005; draft-ietf-2547bis, mainly adding Inter-Domain MPLS L3VPNs (Multi-AS backbone (Inter-AS) and Carriers’ Carriers), and was finally approved and RFC 2547 was obsolete in 2006 with RFC 4364, “BGP/MPLS IP Virtual Private Networks (VPNs)”.

NOTE RFC 4364 was updated with a couple of RFCs; RFC 4577 "OSPF as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)" and RFC 4684 "Constrained Route Distribution for Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs)", both RFCs just addressed certain points as illustrated in their subjects without addressing the whole contents of RFC 4364.

To maintain the continuity of VPN services across multiple service providers, mainly for customers who span world wide on different service providers, IETF described 3 types of options (A, B and C) for Inter-AS or Inter-Provider MPLS VPN solutions, while Cisco implemented three options (1, 2 and 3) with Cisco IOS (these options are also known in Cisco documents as 10A, 10B and 10C).

Cisco's IOS options 1 (AKA 10A), 2 (AKA 10B), and 3 (AKA 10C) correspond to options A, B and C mentioned in draft-ietf-2547bis section 10 "Multi-AS Backbones" (which was obsolete by RFC 4364).

The available Cisco solutions are:

Option 1: Back-to-back VRF
Option 2: ASBR-to-ASBR Multiprotocol eBGP (MP-eBGP) for VPNv4
- Option 2a: Using the next-hop-self approach
- Option 2b: Using the redistribute connected approach
- Option 2c: Multi-hop MP-eBGP approach
Option 3: Multi-hop MP-eBGP between route-reflectors (RRs)
- Option 3a: Directly connected approach
- Option 3b: Multi-hop eBGP approach

Moreover in late 2007, Cisco introduced a new Inter-AS option; Option AB. This feature combines the best aspects of options 1 (10A) and 2 (10B).

I'll try to discuss all the available options in the upcoming posts in details while providing configuration examples.

I hope that I've been informative.

Mohammed Mahmoud.

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