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OSPF Domain-ID || Domain-Tag

OSPF Domain ID

When OSPF is used as the routing protocol on a provider edge to customer edge (PE-CE) link in a multiprotocol label switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute derived from the OSPF process number to indicate whether the route originated within the same OSPF domain or from outside it.

Importance of Domain-ID

In MPLS-VPN network ISP cloud is treated as a super backbone area, and PEs are considered as ABRs or ASBRs depending on the domain-id value, then routes redistributed to CEs would be OSPF inter- area routes or would be OSPF external routes.

Why would we care to have OIA or E or O routes in our OSPF database?

The answer is simply that OSPF prefers intra-area routes then inter-area routes and finally external routes to be installed in routing table. And we don’t want to come in to the case that routes are leaked from a back door in then being redistributed to the ISP cloud creating a loop in the network and disrupting traffic.

So if we had a different processes on our PEs then so we have to explicitly configure the domain-id value under the OSPF process.

OSPF loop prevention in PE-CE routing

  • DN bit

o    When a type 3 LSA is sent from a PE router to a CE router, the DN bit [OSPF-DN] in the LSA Options field MUST be set. This is used to ensure that if any CE router sends this type 3 LSA to a PE router, the PE router will not redistribute it further.

  • Domain-Tag

o   PE routers originate Type 5 LSAs reporting the extra-domain routes as AS-external routes. Each such Type 5 LSA MUST contain an OSPF route tag. This tag identifies the route as having come from a PE router. The VPN Route Tag MUST be used to ensure that a Type 5 LSA originated by a PE router is not redistributed through the OSPF area to another PE router.

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Impossible Is Nothing.


I created this blog to be my companion in the long journey towards my Oscars and Golden Globe awards; My CCIE and JNCIE. As those are the most valuable trophies for any Network Engineer and just like any artist these trophies shall be your biggest achievements in your career.

Getting the CCIE means instant credibility, suddenly, you become an expert in your group. Your understanding of different technologies gives you a large arsenal of tools with which to design networks and troubleshoot problems in a faster and more reliable way. Now people will not hesitate to bring their network problems to you, nor hesitate to solicit your advice or look to your guidance.

In short you’ll be instantly transformed from a trained doctor to a neurosurgeon, when you talk other will listen.

This is the kind of recognition and appreciation these certificates gives and that is what I am after. So even if this journey will be full of obstacles or hold back, I would still go through it and finish it as I should know better not to give up and never surrender and not to let despair get into me as I already have a belief that Impossible Is Nothing.

“Impossible is just a small word that is thrown around by small men who find it easier to live in a world they’ve been given to explore and not use the power they have to change it. Impossible is not a fact. It is an opinion. Impossible is not a declaration. It is a dare. Impossible is potential. Impossible is temporary. Impossible is nothing!” –Muhammad Ali