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Forum Replies

  1. Hi Niranjan,

    The routing decisions remain the same, we use our routing table for this. Other routers can advertise all the labels they want to us but our local router will decide the path we use, just like with normal routing without labels.


  2. Hi MD Arif T,

    The main difference is that:

    • In MPLS we are switching based on labels.
    • In IP we are routing based on destinations.

    Theoretically doing lookups for labels is faster than lookups in the routing table for destinations. I doubt there is much of a performance different nowadays though...keep in mind that MPLS is > 15 years old, back then it might have made more of a difference.

    The main advantage of MPLS is that we are able to transport non-IP traffic and we have VPNs.


  3. Hi Alan,

    If OSPF, EIGRP and RIP have learned the route then only one will be installed in the routing table. EIGRP will be selected since it has the lowest administrative distance.

    By default, LDP will generate a label for each route in the routing table. This is something that you can change by using access-lists if you want.

    The LFIB has pretty much the same information as the FIB, however some entries in the FIB (like directly connected interfaces) won't show up in the LFIB.

    Theoretically, switching based on labels is faster than routing where we do a lookup for the IP destination. Nowadays this doesn't matter much. Keep in mind MPLS is already quite old.

    The RIB only has the best routes in it, the FIB is built with information from the RIB. That's why you know that we also use the best path with label switching. Labels are advertised between routers, that's how they know which label to use.

    If you want to see this in action, boot a couple of routers and configure OSPF on EIGRP on them...advertise some loopbacks. Then enable MPLS on the interfaces and take a look at the different tables:

    • show ip route = RIB

    • show ip cef = FIB

    • show mpls ldp bindings = LIB

    • show mpls forwarding-table = LFIB


  4. Hi Rene,
    Could you expand on this a bit please. It seems to me that it is possible to receive a label for a destination that we do not have in the routing table.
    The first example where we have BGP between PE and CE but no BGP on the core. Here the PE's were advertising a route for the Customer.
    If the core is more complex there could be multiple ways for PE1 to get to PE2 so it could learn the label from more than one neighbour.

    How would a router chose which label route to use where it has a label for a destination but no route ?


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