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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 Rene,

    Thanks for your reply.
    As you mentioned, information inside the RIB is use to build the LIB.
    So i would imagine to have a LABEL for all the available routes.

    e.g. I have 3 best routes to the same destination with rip, eigrp, ospf in the RIB.  via  - rip  via  - eigrp  via  - ospf

    q1) Can i check for FIB, does it contain the same amount of available routes as in the RIB ?

    q2) if the above is yes, i would believe FLIB and FIB would have the same amount of labels ?

    q3) it is said that in MPLS, the LABELs are use to do switching/routing from Source to Destination and it is faster because there is no need to do best route selection using the IP headers. – is this true ?
    But how do we ensure that the LABELs being used (added) and the next hop to send to represent the best routing path available ?

    or to put it in another way

    • in normal routing, best route selection is done via (ad, metric, prefix length)…
    • in mpls, since all available routes will have a tag, how does the router know which tag to use (without looking at the routing table for the best route) ?


  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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