It’s been a while so now MPLS is included in CCIE R&S v4.0, so high time I added some new nuggets.

I’ll start off with something I’ve recently used but otherwise rarely touched as it’s mostly used inter ISP. In an earlier article I explain an issue encountered when using one of a type-B MPLS interconnect, my MPLS interconnect type summary there was:

  1. PE physical link(s), ties one VRF to another using one subnet for each VRF. Commonly deployed using vlans across a DOT1q trunk (anyone still using ISL? ok, ok a tagged trunk but don’t tell me I’m not allowed to say “trunk”). Possible to use routing protocols between each individual VRF, most commonly staticly routed as it’s the safest model, the other side isn’t trusted…
  2. PE MP-eBGP, ties the two MPLS clouds together exchanging VPN labels. BGP can be used to exchange the vpn labels, no need for tag-switching of IP traffic. Requires the inter- link to be known in both IGPs as the next-hop changes, only the vpn label is preserved end-to-end.
  3. P tag-switching, joins the two domains together at the lowest level. The inter-link can be between P devices all labels are preserved end-to-end. Even the IGPs must have ‘full’ connectivity, of course I mean PE-to-PE…

Note: P, PE & CE/CPE are common ISP acronyms for Provider, Provider Edge & Customer (Premise) Edge.

Time to explain all three of them in detail. RFC4364 section 10 mentions multi-AS backbones specifically.

What if two sites of a VPN are connected to different Autonomous
Systems (e.g., because the sites are connected to different SPs)?
The PE routers attached to that VPN will then not be able to maintain
IBGP connections with each other, or with a common route reflector.
Rather, there needs to be some way to use EBGP to distribute VPN-IPv4

RFC4364 section 10 goes on to detail three options often referred to as type-A/B/C or type-1/2/3 MPLS (inter-AS) interconnects. the name/numbering confusion comes from the RFC just listing three options rather than naming them explicitly. Of course you can leave it to us engineers to adopt simple bullet points as law…

Section 10.A:

VRF-to-VRF connections at the AS (Autonomous System) border routers.

Section 10.B:

EBGP redistribution of labeled VPN-IPv4 routes from AS to neighboring AS.

Section 10.C:

Multi-hop EBGP redistribution of labeled VPN-IPv4 routes between source and
destination ASes, with EBGP redistribution of labeled IPv4 routes from AS
to neighboring AS.

Type-A is what I personally see most often. This is because it provides a simple L2/3 demarcation point, management wise there might be a cost due to static routes but it’s very predictable, easy to configure and easy to agree upon when dealing with 3rd parties.

Type-B is a logical choice when there is a level of trust and eBGP is already used.

As more and more sub-enterprise companies adopt MPLS internally I think we’ll see more of these type of interconnects.

[EDIT 2014-01-21] Not sure why but it seems this page was a draft for a long time (about 4 years *sigh*). Oh well, now you have it…

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