OSPF in MPLS VRF with no Area 0

Unanswered Question
Aug 9th, 2010

I tried to link two client sites in the same area (for example area 5 on both sides) with no area 0 in the VRF at all. The client has a link to area 0 in another part of their network. Our task is simply to connect their regular area together across the WAN. Is this possible or must I have an area 0 somewhere in the VRF. I've had trouble with this before, It seems whenever I have a VRF connection to area 0 it works, and whenever there is no link to area 0 from the VRF I never get the routes through successfully.

Normal area 5, redistributing OSPF/BGP with internal external 1 and external 2 specified in the redistribution command, I manually specify are OSPF domain ID on both sides, just two VRF's each in OSPF area5 with no area 0

Thanks

I have this problem too.
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Chetan Kumar Ress Tue, 08/10/2010 - 07:21

Hi

Accourding to me your senario will not work. Without Area 0 you can't have communication over WAN.

But still i will check & come back with exact result.

The concept behind MPLS VPN with ospf protocol for CE-PE is to have the non-backbone areas, Area 1 and Area 2, are directly connected to the MPLS VPN superbackbone that functions as an OSPF Area 0. Therefore, an actual Area 0 is not required as in the traditional OSPF. Area 0 is a requirement only when the PE router is connected to two different non-backbone areas belonging to the same OSPF domain on a PE router.

Whenever you will configure the MPLS VPN and use OSPF for CE-PE , you will see that if you are using only Area 0 then also you will get route as        "O IA".

You will never get "O" route type until an unless you use OSPF sham-link. Because it always treat as Interarea i.e MPLS Domain is Super backbone & Connected area is non-backbone area.

Using OSPF without MPLS Domain need an Backbone know as Area 0 in same way when using OSPF with MPLS Domain you need Supper Backbone known as Area 0. The same rule apply in both senario that without having connectivity with Area 0 your different area will not communicate.

Regards

Chetan kumar

http://chetanress.blogspot.com

Chetan Kumar Ress Wed, 08/11/2010 - 07:46

Hi

Today i tried that same to configure  It works.

Topology was :

CE --- Area2 ------ Area 2 --PE -----------P-------- PE--Area 2 -------- Area 2 ---CE

The above topology works.

The basic behaviour of OSPF is to have an Backbone area and in traditional OSPF it is known as Area 0.

But in MPLS Domain the SP Domain is known as MPLS VPN Super Backbone. So it is not identified by any Area ID when ever you connect OSPF with MPLS Domain then SP domain itself say that "I AM MPLS VPN SUPER BACKBONE" i don't need any Area ID to become Backbone link traditional OSPF doamin. Becasue in MPLS PE router area identified as a ABR router that is Super Backbone , So requriment of OSPF is fulfilled .

4.2.3.  OSPF Areas

   A PE-CE link may be in any area, including area 0; this is a matter
   of the OSPF configuration.

   If a PE has a link that belongs to a non-zero area, the PE functions
   as an Area Border Router (ABR) for that area.

   PEs do not pass along the link state topology from one site to
   another (except in the case where a sham link is used; see Section
   4.2.7).

   Per [OSPFv2, Section 3.1], "the OSPF backbone always contains all
   area border routers".  The PE routers are therefore considered area 0
   routers.  Section 3.1 of [OSPFv2] also requires that area 0 be
   contiguous.  It follows that if the OSPF domain has any area 0
   routers other than the PE routers, at least one of those MUST be a CE
   router, and it MUST have an area 0 link (possibly a virtual link) to
   at least one PE router.

http://tools.ietf.org/html/rfc4577

Please refer the RFC for same.

So without Area 0 in MPLS Domain you can have communication with same area or different.

Regards

Chetan Kumar

http://chetanress.blogspot.com

Atif Awan Tue, 08/17/2010 - 23:17

In case each site is using a single area (can be same or different) there is no requirement to have Area 0 within the VRF as the SP backbone will serve this purpose. However, if any site has Area 0 in it's topology then that Area 0 needs to be extended to the local PE either via a virtual-link or through a direct interconnect.

Atif Awan Mon, 08/23/2010 - 17:08

Shamlink is between PE devices and not between a CE device and a PE device and is used for a completely different application (avoiding backdoor intra area routing in certain scenarios) as opposed to extending Area 0 for contiguity purposes. Virtual-link is required between CE and PE in case Area 0 exists in the CE domain. I believe you should modify the wiki so that it provides others with the correct information.

Atif

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