OSPF Superbackbone

[zikalamc]

Hi,

When the PE-CE link is in are 0, is the backbone seen as area 0 as well or just another area or is it transaparent to other routers in other in other areas in the c-network?

Is this different when the PE-CE link is in a normal area?

[mheusinger]

Hi,

To answer your question let me start with a short explanation of the term "OSPF superbackbone". Actually what is meant by this are two things:

1) every PE will announce itself as ABR and ASBR to the attached CE routers i.e. into the attached OSPF area independantly of having area 0 or a virtual link

2) for a PE ABR/ASBR to produce the LSA3, LSA4, LSA5 when required, all nessessary information to do so is transported in MBGP extended communities

The result of 1) is, that actually you do not need area 0 at all in a multi-area MPLS VPN environment. Area 0 in OSPF is mainly needed to let a router know it is an ABR. This is accomplished differently in the case of an OSPF PE.

Because of 2) a PE gets all the information to decide, which LSA type to create when inserting a route into the OSPF area towards the CE. (Sidenote: the difference to "normal" OSPF is that you get inter-area routes even within an "all area 0" MPLS VPN)

As usually in MPLS L3VPNs a CE does not know anything about all this. So it simply gets LSAs from an ABR/ASBR (the PE). If the CE is in area 0 routes from other locations learned through MPLS VPN look like routes injected from an area outside area 0 (LSA3 or LSA4+LSA5 for external routes). If the CE is in a normal area the PE acts like a normal ABR/ASBR to him.

Hope this helps.

Martin

[zikalamc]

Very well explained!!!

Thanks Martin.

[sshang2]

Hi, Martin:

I have two further questions regarding your post.

1. Do you mean PE router will unconditionally announce itself as ABR to its attached router?

2. If the OSPF area between PE and CE router is not Area 0.0.0.0, is PE able to summarize those OSPF routes learned from the local site to Type 3 route before advertising them to the remote site? My intention is to minimze the OSPF update to the remote site so the change in the local site will stop at PE router. I believe route summarization will help me for this purpose. But I am not sure whether and how PE can do it.

Thanks a lot!

SShang

[mheusinger]

Hi again!

1) Yes.

2) You can achieve that with "area 12 range ..." and a route-map utilizing a prefix-list during OSPF->BGP redistribution on the PE. The trick is, that the "area range"-command will insert a OSPF route to the Null interface into VRF ip routing table. BGP will pick that up during redistribution and the route-map/prefix-list makes sure you get rid of the other unwanted stuff.

regards

Martin

[sshang2]

Hi, Martin:

Thanks a lot!

SShang