MPLS – Why Does MPLS Need an IGP Like OSPF?

igpmplsospf

I saw a network using MPLS, BGP and OSPF as main network protocols. I understand that BGP was used to exchange traffic with another AS, and OSPF was used as Internal Gateway Protocol. So, why to use also MPLS if we already had OSPF?

Best Answer

Why does MPLS need an IGP (like OSPF) in order to work?

In an MPLS network, OSPF is not used to route customer traffic. It is only used to provide routing for the internal provider network so that labels can be generated. LDP is used to advertise these labels to neighboring label-switched routers.

So, why to use also MPLS if we already had OSPF?

You're really asking how MPLS works. The answer to that is too long for this forum.

But briefly, OSPF is a poor choice for routing customer networks because

  1. Customer addresses can (and often do) overlap. If customer A and customer B both use the 10.0.0.0 network, it is necessary to keep them separate. OSPF can't do that.
  2. Customer networks need to be isolated. You don't want Customer A to see routes from Customer B. OSPF can't do that either.
  3. You can have hundreds of customers, each with thousands of routes. OSPF doesn't scale well to that size.
  4. You may want to route your customer traffic over a particular path. OSPF has a very simple routing metric that doesn't allow that kind of path manipulation.
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