4 Replies Latest reply: Jan 25, 2010 6:01 PM by TheOtherTomJones, pACE, CIPT RSS

    How MPLS work?

    Parveen Rao

      Hi all,



      Is there any body tell me briefly about MPLS. How it works on labeling. I learned alot, but I am not able to unerstand the real motto. pls explain.

        • 1. Re: How MPLS work?
          Scott Morris - CCDE/4xCCIE/2xJNCIE

          Just so you know, there are entire books written on this subject. 


          Magic.  Pure magic.  Otherwise known as labels.


          One interesting thing about your network as you grow it...  Take the internet as an example.  If you have a full-feed in BGP, you will notice a whopping 310,000 routes these days plus whatever internal ones you have.  But the interesting thing about this, from any router's perspective is that while you may have 310,000+ unique ROUTE entries, you have a VERY limited number of NEXT HOP addresses.  So why not simplify things down to next-hop paths?


          In a sense that is what MPLS allows you to do.  You can have two edge routers running full BGP feeds to the Internet.  They may exchange routes with each other as well.  Some packets you want to exit Router-A and others Router-B.  Well, what about the rest of your network?  Do you really want to propogate that many routes through your entire network?


          What happens if you don't?  Well, if you simply give your internal routers a default route, you'll most likely induce a routing loop.  Router-B wants to go towards Router-A, but the first internal router wants to hand it back to Router-B.  And the fight ensues until TTL is exceeded!   Or, if there's no route entry, the traffic is simply dropped.  Not as dramatic, but equally effective!


          So instead, your internal network needs to have reachability out to your own edges.  makes sense, as its your network!  They exchange labels based on those routes.  As as long as there is a label switched path from Router-A to Router-B (and vice-versa) then those two routers can send ANY traffic to one another, because the underlying IP packet is never viewed.  All packet movement (switching) is done via the label lookup instead of a route lookup.






          PS.  The Cisco Press book MPLS Fundamentals is a good place to get started!

          • 2. Re: How MPLS work?

            I second the MPLS Fundamentals book, written by Luc de Ghein.  I've been pushing through the book in both preparing for the lab, but also to really develop a strong basic knowledge of MPLS operations.  The writer does a great job of getting you involved and interested, and lays out the info rather well.

            • 3. Re: How MPLS work?

              Hi Parveen


              Very simply each router has a table which says when a mpls packet arrives look at the label. Search in the table and when found replace the label and send it out of the router on the interface associated with this new label.


              Table = {old_label, new_label, new_interface}. Since the labels are fixed length they can be looked up quickly.


              If you go from router A to Z then the labels you use ab bc cd de...    yz are called the label path. Not strictly true.


              So you need to establish a label path using the traditional static routes or a dynamic routing protocol  before you start. After which you only need to worry about a failure in the label path.


              If you look under my name or in the Advanced Routing and Switching section you will see example of simple MPLS.


              Regards Conwyn

              • 4. Re: How MPLS work?
                TheOtherTomJones, pACE, CIPT



                I still need to make some small changes Scott recommended, but Ive just been lazy about it