9 Replies Latest reply: May 14, 2011 11:04 AM by Julian RSS

    Frame relay- non broadcast

    Subramaniya Karthic

      I have a really simple doubt

       

      In FR networks, while using OSPF we configure static neighbors in OSPF. This is because the FR doesnot allow multicasting.

       

      My doubt is when we use "frame-relay map ip <address> <dlci> broadcast" in a interface it means we are sending broadcast packets for the network

      and when "broadcast" command is not used we are statically configuring neighbors in OSPF.

       

      Does this means the capability of a FR nw to broadcast depends on the "frame-relay map" command? I dont think so?

      If this is not the case why would a neighborship be formed in a network only when "broadcast" keyword is used ?

       

      Can there be any configuration made in the frame relay switch that can stop broad casts?

       

      Thank you all


      R.S.Karthic

        • 1. Re: Frame relay- non broadcast
          tnewshott

          Technically, frame relay still doesn't support "broadcast" or "multicast" traffic, even with the broadcast command on the frame relay map, or when using Inverse ARP. 

           

          What happens, is that it sees that broadcast/multicast traffic needs to be sent to the far-end destination across a specific DLCI, it then wraps the broadcast/multicast in a unicast header, which allows it to be sent across the frame relay network just like any other traffic. 

          • 2. Re: Frame relay- non broadcast
            Subramaniya Karthic

            Travis wrote:

             

            Technically, frame relay still doesn't support "broadcast" or "multicast" traffic, even with the broadcast command on the frame relay map, or when using Inverse ARP. 

             

            What happens, is that it sees that broadcast/multicast traffic needs to be sent to the far-end destination across a specific DLCI, it then wraps the broadcast/multicast in a unicast header, which allows it to be sent across the frame relay network just like any other traffic. 

             

             

            Yes.Got the point.

             

            1)So what is the "broadcast" command really doing

            2) If we can send "multicast like" packets over the DLCI's(i.e by wrapping and unicasting) them, then

              why is there a non-broadcast mode where we statically configure neighbors.

              If our routers can form neighborship by using the "broadcast" keyword then why are we not using it..why is ther a nonbroadcast mode?

            • 3. Re: Frame relay- non broadcast
              tnewshott

              There is a difference between OSPF network types, and frame-relay topologies.

               

              If your frame relay supports broadcasts, you can allow OSPF to communicate dynamically.

               

              If your frame relay does not support broadcasts, you need to tell OSPF this by setting the corresponding network type under the interface, and if necessary, configure static neighbors.

               

               

              Frame relay topologies are the key factor here, it sounds like you are still in the process of learning frame relay. I am no frame relay expert, but I will certainly help try to help until I reach the limit of my own personal comprehension of the topic!

              • 4. Re: Frame relay- non broadcast
                Subramaniya Karthic

                Travis wrote,

                 

                If your frame relay does not support broadcasts, you need to tell OSPF this by setting the corresponding network type under the interface, and if necessary, configure static neighbors.

                 

                 

                Thats my question... In our case we are not sending framer relay broadcasts..we are just unicasting(you explained me that broadcasts are being wrapped in unicast packets)..

                 

                Thanks for your help

                • 5. Re: Frame relay- non broadcast
                  Scott Morris - CCDE/4xCCIE/2xJNCIE

                  yes, the capability of FR to do broadcasts is dependent on the "broadcast" keyword (or point to point subinterfaces).  If you don't want broadcasts, don't use that keyword.  And it won't occur!

                   

                  Remember though, that routing protocols are just part of that.  CDP is similar.  IP Multicast traffic (if you run it) is similar.

                   

                  But by default, Frame Relay is an NBMA network (NON Broadcast Multiple Access).

                   

                  HTH,

                   

                  Scott

                  • 6. Re: Frame relay- non broadcast
                    Subramaniya Karthic

                    Gotcha..So we control if we need to send multicasts(though they are sent as unicasts)..

                     

                    Thank you for all help to get me to this level of understanding..

                     

                    Thank you Travis and Scott

                    • 7. Re: Frame relay- non broadcast
                      Scott Morris - CCDE/4xCCIE/2xJNCIE

                      The part of "though they are send as unicasts" you need to be careful with so that you don't confuse yourself.

                       

                      The format of the packet really doesn't change.  The IP header will still say 224.0.0.5 (OPSF) or 255.255.255.255 (if broadcast), or whatever particular group address you have...

                       

                      There's no MAC in a frame-relay header, so the "all-F's" is not really applicable.  It's just that the broadcast/multicast message will get COPIED to each of the DLCIs that has the "broadcast" parameter set.  That doesn't technically MAKE it a unicast message, it just makes it a replicated message.

                       

                      HTH,

                       

                      Scott

                      • 8. Re: Frame relay- non broadcast
                        tnewshott

                        Scott, that is good to know.  I honestly haven't gotten to that kind of packet level detail on FR - nor have I seen it used outside of the lab in several years.

                        • 9. Re: Frame relay- non broadcast
                          Julian

                          Hi all,

                           

                          Only three short questions about this topic:

                           

                          1 - By the way, it is worthwhile to built a non-broadcast FR network, as it is by default?

                           

                          2 - I mean, what are the inconvenients of allowing broadcasts in such a network?

                           

                          3 - So, if we used for example RIP or OSPF in a FR network, we should allow broadcast to make it works well, shouldn't we?

                           

                          Regards!