7 Replies Latest reply: May 5, 2012 7:44 AM by BIGEVIL RSS

    MPLS QoS

    Steven Williams

      If I am a customer using an MPLS network via ISP, and I am doing QoS marking on my local enterprise network, when that traffic is sent out to the MPLS cloud, how does the ISP know what QoS priority that traffic has? Maybe this is a dumb question, but I know that QoS is end to end, and the ISP is dealing with different customers, I dont have control of their edge router, so how are they getting my QoS configurations and priorities?

        • 1. Re: MPLS QoS
          Bradford Chatterjee (CCIEx2/CCDE)

          Your SP isn't psychic, so if you have QoS negotiated with your service provider, they'll usually provide you a set of tiers for your traffic, the bandwidth constraints of those tiers, and the markings you need to use for the tiers.


          For example; if your SP offers a QoS SLA, they'll tell you "we'll provide you with three levels of service; the priority tier supports 5Mbit and must be marked DSCP EF, the middle tier supports 20Mbit and must be marked CS3, the bottom tier supports 50Mbit and must be marked AF11, and the rest of your service is best effort." They'll also tell you how each tier will be treated in case of congestion on their backbone.


          Then they'll read the markings on your traffic that you set, they'll classify and maybe remark them to whatever markings have significance on the ISP, and then do the opposite on the other end when it goes back into your network.


          MPLS QOS marking is easy because they can use something called QOS groups to match and mark specific customer's traffic as it enters an LSP. They just declare traffic coming into the network that meets certain conditions, say incoming interface and DSCP EF, to belong to a QOS group. Then when it enters the LSP on the core side of the router they match on the QOS group and set the MPLS EXP bits for all the labeled traffic to something appropriate for the QOS rules to which you agreed.


          Let's say, though, that you don't have any agreement with your service provider. Since you have an MPLS service it's still easy for them. They just put your packets in an LSP and ship them across their backbone with the QoS markings untouched. The internal ToS bits are unchanged and the label is probably marked with EXP 0 instead of copying the first three ToS bits into the EXP field as is the default behavior. They don't pay attention to your markings but they don't erase them, either. That way your internal marking is preserved and you can do "end to end" QoS on the bits of your network that you control, but it's not really end-to-end because the SP wasn't doing QoS for you.


          For non-MPLS customers with no SLA, the SP will probably just remark all incoming traffic to a standard marking or no marking.

          • 2. Re: MPLS QoS
            Steven Williams

            Wow, you are very knowledgable and helpful. I figured something had to be done specific with the ISP, just wasn't sure how it all fit together. So without QoS on the MPLS network it just attempts to be best effort in a sense?

            • 3. Re: MPLS QoS
              Bradford Chatterjee (CCIEx2/CCDE)

              Yes, unless you have an explicit agreement with your ISP then they are probably treating all of your traffic as best effort. As long as there's no congestion along your LSPs, though, that's probably fine.

              • 4. Re: MPLS QoS
                Steven Williams

                Can you recommend any sites, blogs, or articles that provide good MPLS information? I think people recommend a book called traffic engineering with MPLS, which I will probably get, but I don't really have time to read a whole book, I need to sleep sometimes. 

                • 5. Re: MPLS QoS
                  Bradford Chatterjee (CCIEx2/CCDE)

                  Ivan Pepelnjak is the author of one of Cisco's MPLS books, and he maintains a blog at http://blog.ioshints.info/. He sometimes writes about MPLS on his blog and there's lots of other good info there besides. Traffic Engineering with MPLS might be a good book (I haven't read it), but the subject matter of MPLS-TE is pretty heavy if you don't already have a good foundation of MPLS knowledge. I'd recommend starting with the MPLS Fundamentals book.

                  • 6. Re: MPLS QoS
                    Steven Williams

                    Good call. Just the word fundamentals sounds like a good starting point for me since I know about .0001 percent about MPLS. Thanks for the help, appreciate it!

                    • 7. Re: MPLS QoS

                      This is also worth a read -




                      The ISP i worked for used short pipe