9 Replies Latest reply: Feb 26, 2016 4:50 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

    What is Overlay Network?

    Praveen Rai

      Hi team,


      today while  reading MPLS i got to know this term m confused about whats all about it after reading i found overlay network is somewhat like Frame relay in which we create VC to connect customer L3 IP to L2 services and isolate it from other so pls correct me m right or it is something else pls make this clear to me


      whats the advantage of it..............................if somebody clear it with visual it would give morwe sense....................

        • 1. Re: What is Overlay Network?

          Don't allow the term to confuse you, Overlay Networks are nothing new and have evolved over time, its just a matter of understanding the operation, today MPLS is the most prominent and the advantages are clear!  

          • 2. Re: What is Overlay Network?
            Anthony Sequeira, CCIE,VCP

            There have been two primary VPN models that have been used by Service Providers:


            • The overlay model - where the service provider provides emulated leased lines to the customer. The overlay model typically uses the virtual circuits of a Frame Relay or ATM service. The overlay model has advantages such as permitting the duplication of addresses and the isolation of the control and security planes.
            • The peer-to-peer model - where the service provider and the customer exchange Layer 3 routing information and the provider relays the data between the customer sites on the optimum path between the sites. Layer 3 MPLS VPNs permit the creation of a peer-to-peer model with many of the advantages of the overlay model, such as the duplication of addresses.


            Anthony Sequeira


            Twitter: @compsolv

            Facebook: http://www.facebook.com/compsolv

            • 3. Re: What is Overlay Network?
              Scott Morris - CCDE/4xCCIE/2xJNCIE

              When one type of network runs on top of another network, that's an overlay.  As noted, MPLS VPNs are a primary example of that, because what a customer views as "the network" is a very limited scope of the REAL network making all that possible.


              So one is overlaid on top of the other.





              • 4. Re: What is Overlay Network?

                Hi Scott, really make sense your explanation for the term overlay. I was reading your response on the other thread regarding SONET. 


                And you commented on that link that MPLS is overlay,  then i found another post of yours that clears the dark clouds of my understanding for the term overlay.


                Thank you so much.


                I just hope one day, when CISCO wants you to write a book that you will do the same, stick to the point and clear as a crystal water the explanation.


                Because some of the author for Cisco book, they will drive you around in circles before you get the idea. when in fact it is just a very small matter.


                anyway i think that's how marketing strategy works.


                to the original poster, don't really  mean to hijack this post.


                But can't find a way to thank Scott except to do this. Thanks again Sir Scott.

                • 5. Re: What is Overlay Network?
                  Scott Morris - CCDE/4xCCIE/2xJNCIE

                  heheheh...  I'm discovering the difficulty in that is it takes a lot of time and energy to write a book!  But yes, what I'm working on, I'm trying to make crystal clear as well as fun and simple to read! 


                  Glad to help!



                  • 6. Re: What is Overlay Network?

                    Just curious who will win the battle, peer to peer or overlay networks interns of usage?

                    • 7. Re: What is Overlay Network?

                      well...as much as I respect Scott and everything he had achieved and contributed to so many CCIEs, I have to respectfully disagree with MPLS being an ‘overlay network’ and agree with Anthony Sequeria.  It is all about who you peer routing with.  Frame-Relay is an overlay network because the provider does not participate in customer routing.  MPLS is a ‘peer-to-peer’ model network because the provider Does participate in customer routing. 

                      • 8. Re: What is Overlay Network?

                        MPLS L3 VPN is peer to peer  because the provider network is aware about the routing table of  the customer sites

                        and MPLS L2 VPN is overlay because the provider network is only a transport.




                        • 9. Re: What is Overlay Network?
                          Scott Morris - CCDE/4xCCIE/2xJNCIE

                          It's ok to disagree with me.     However, that just means that I have to do a better job of explaining things.  Anthony's explanation of SP models was how they used to do things from an architectural point of view but not necessarily having to do with logical function.  Although even he mentions the L3VPN part of using the overlay to create the peer-to-peer!


                          At a base level...  Forget about details...  The term "overlay" simply means to function on top of something else.  In other words, take away the base level and your overlaid stuff can't work.  MPLS is an example of that.


                          Your service provider has an IP network that is only usable/reachable insider their own devices.  On TOP of that, MPLS is running.  Take away the base IP network, and MPLS/LDP/BGP/etc cannot function.  Hence MPLS is an overlay.


                          You have some examples within MPLS of Layer3 VPNs and Layer2 VPNs.  Each of those is an overlay as well.  Again, take away IP base or even MPLS and those cannot function by themselves.  Yet they serve to create connections between companies and sites that will emulate a peer-to-peer network.  And yet they are still an overlay!


                          It's complicated.  No doubt about it.  But you have to think of the LAYERS upon which you build a network in order to achieve your end result of "virtually private peer-to-peer-like" functionality yet over a common (shared) core network.


                          In your last post arguing that L3 VPNs are peer to peer, I think you may need to read a bit more about how BGP works within the IPv4 VRF address families as well as the VPNv4 address families on the SP side in order to make that work.  Again, without the base, the transport will not exist.