12 Replies Latest reply: Nov 20, 2011 7:59 PM by Keith Barker - CCIE RS/Security, CISSP RSS

    Does Frame Relay use Broadcast ?

    Arul

      1.Does Frame Relay use Broadcast to forward the information from one router to another router ?

      2.I know and i have used in GNS3, the Cisco router as a frame relay switch. Does Cisco have a dedicated Frame relay switch, that just come out of the box and do just frame relay switching ?

         I mean what ISP is doing to transport the frame relay packet from one are to another area ?

       

      Please explain....

        • 1. Re: Does Frame Relay use Broadcast ?
          Paul Stewart  -  CCIE Security

          Frame relay always uses a DLCI.  If you have a broadcast or multicast upper layer protocol you can tell frame-relay to transmit those layer 3 broadcast using a dlci.  This is typically traffic that is sourced from the router (since routers don't forward broadcast that are received on other interfaces).  Examples are RIP, OSPF, and EIGRP.  To enable this, you use the "broadcast" keyword on the end of frame-relay map statements.  If you are using inverse arp, it is enabled by default.

           

          I'll defer question two to those who have worked on the telecom side of things.

          • 2. Re: Does Frame Relay use Broadcast ?
            Brian

            For Question#2, yes Cisco use to have dedicated FR & ATM switches when they first bought StrataCom back in 1996.  Not sure when they stopped the dedicated FR switches, but the ATM (Lightstream 1010) switch went EOL I believe in 2005.

             

            Hope this helps.

             

            Brian

             

            • 3. Re: Does Frame Relay use Broadcast ?
              Keith Barker - CCIE RS/Security, CISSP

              Here is a video on how to configure an IOS router to provide the "frame relay switch" function.

               

              Best wishes,

               

              Keith

               

              • 4. Re: Does Frame Relay use Broadcast ?
                Arul

                You mean Cisco router will start sending broadcast after specifying the broadcast keyword in the frame relay map command ?

                • 5. Re: Does Frame Relay use Broadcast ?
                  Keith Barker - CCIE RS/Security, CISSP

                  Arul wrote:

                   

                  You mean Cisco router will start sending broadcast after specifying the broadcast keyword in the frame relay map command ?

                   

                   

                  Hello-

                   

                  Frame relay sends data (at L2) down a PVC based on DLCI numbers.   At L3, the destination may be unicast or multicast or broadcast (regarding the IP address being use).   The broadcast keyword, associated with a frame mapping, tells the router:  "If on ethernet, you would be forwarding this routing update out this interface (such as a routing update), go ahead and send it down this PVC on DLCI XYZ".   That is what the broadcast keyword does for the mapping.

                   

                  The L3 address destination address may be multicast or broadcast, but the L2 frame is sent on a specific DLCI as there is no special L2 "broadcast" address in frame relay.  

                   

                  Keith

                  • 6. Re: Does Frame Relay use Broadcast ?
                    Brian

                    Frame-Relay does not use Layer 2 or Layer 3 broadcasts.  Frame Relay operates at the physical and data link layers of the OSI reference model.  With Frame-Realy, an interface is by default a "multipoint" interface and non broadcast in nature.  Frame−relay inverse−arp is on by default (for point−to−point, there is no Inverse ARP) and IP split horizon checking is disabled by default for Frame Relay encapsulation.  This means routing updates come in and out the same interface. The routers learn the data−link connection identifiers (DLCIs) they need to use from the Frame Relay switch via LMI updates. The routers then use Inverse ARP for the remote IP address and create a mapping of local DLCIs and their associated remote IP addresses.  Since frame−relay inverse−arp is enabled, broadcast IP traffic will go out over the connection by default.

                     

                    You can verify this with RIP and EIGRP.  The exception being OSPF, which has many different network types.  You will have to change the default behavior of the interface for OSPF to propagate the routing updates.

                     

                    Hope this helps.

                     

                    Brian

                     

                     

                     

                    P.S. - Attached is a Cisco PT file showing an example of this using RIPv2.  Feel free to change the routing protocol to EIGRP or OSPF.  Enjoy!

                    • 7. Re: Does Frame Relay use Broadcast ?
                      Keith Barker - CCIE RS/Security, CISSP

                      Brian wrote:

                       

                      Frame-Relay does not use Layer 2 or Layer 3 broadcasts. 

                      <snip>

                       

                       

                      Hi Brian-

                       

                      A friendly question for you.

                       

                      If we are running RIPv1 (and I know RIPv1 is a bad idea in general but just as a "what if") , and we are using frame relay, what will be the L3 destination address of the updates that are sent over the individual PVCs?

                      • 8. Re: Does Frame Relay use Broadcast ?
                        Brian

                        RIPv1 uses the IP broadcast address (255.255.255.255) to send its updates.  This has nothing to do with Frame-Relay.  Frame-Relay is a layer 2 WAN protocol  and IP is layer 3 protocol.  As stated in the last sentence of the first paragraph, "Since frame−relay inverse−arp is enabled, broadcast IP traffic will go out over the connection by default."

                         

                        Below are some debug outputs for frame-relay and RIPv2 and RIP v1.

                         

                        R1#debug frame-relay lmi
                        Frame Relay LMI debugging is on
                        Displaying all Frame Relay LMI data
                        R1#
                        Serial0/0/0(out): StEnq, myseq 1, yourseen 0, DTE up

                        datagramstart = 0xE7829994, datagramsize = 13

                        FR encap = 0x00010308

                        00 75 51 01 00 53 02 01 00

                        Serial0/0/0(in): Status, myseq 1, pak size 21
                        nRT IE 1, length 1, type 0
                        nKA IE 3, length 2, yourseq 1 , myseq 1
                        nPVC IE 0x7 , length 0x6 , dlci 102, status 0x0 , bw 0

                        R1#
                        Serial0/0/0(out): StEnq, myseq 1, yourseen 0, DTE up

                        datagramstart = 0xE7829994, datagramsize = 13

                        FR encap = 0x00010308

                        00 75 51 01 00 53 02 01 00

                        Serial0/0/0(in): Status, myseq 1, pak size 21
                        nRT IE 1, length 1, type 0
                        nKA IE 3, length 2, yourseq 1 , myseq 1
                        nPVC IE 0x7 , length 0x6 , dlci 102, status 0x0 , bw 0

                        R1#

                         

                         

                        R1#debug ip rip
                        RIP protocol debugging is on
                        R1#RIP: received v2 update from 192.168.123.2 on Serial0/0/0
                              20.20.20.0/24 via 0.0.0.0 in 1 hops

                        R1#RIP: received v2 update from 192.168.123.3 on Serial0/0/0
                              30.30.30.0/24 via 0.0.0.0 in 1 hops

                        R1#RIP: sending  v2 update to 224.0.0.9 via Loopback10 (10.10.10.1)   <--- note the multicast address for RIPv2
                        RIP: build update entries
                              20.20.20.0/24 via 0.0.0.0, metric 2, tag 0
                              30.30.30.0/24 via 0.0.0.0, metric 2, tag 0
                              192.168.123.0/29 via 0.0.0.0, metric 1, tag 0
                        RIP: sending  v2 update to 224.0.0.9 via Serial0/0/0 (192.168.123.1)
                        RIP: build update entries
                              10.10.10.0/24 via 0.0.0.0, metric 1, tag 0

                        R1#RIP: received v2 update from 192.168.123.3 on Serial0/0/0
                              30.30.30.0/24 via 0.0.0.0 in 1 hops

                        R1#RIP: received v2 update from 192.168.123.2 on Serial0/0/0
                              20.20.20.0/24 via 0.0.0.0 in 1 hops

                        R1#

                         

                         

                        R1#debug ip rip
                        RIP protocol debugging is on
                        R1#RIP: sending  v1 update to 255.255.255.255 via Loopback10 (10.10.10.1)   <--- note the broadcast address for RIPv1
                        RIP: build update entries
                              network 20.0.0.0 metric 2
                              network 30.0.0.0 metric 2
                              network 192.168.123.0 metric 1
                        RIP: sending  v1 update to 255.255.255.255 via Serial0/0/0 (192.168.123.1)
                        RIP: build update entries
                              network 10.0.0.0 metric 1

                        R1#RIP: received v1 update from 192.168.123.2 on Serial0/0/0
                              20.0.0.0 in 1 hops

                        R1#RIP: received v1 update from 192.168.123.3 on Serial0/0/0
                              30.0.0.0 in 1 hops

                        R1#

                         

                        So, the question asked does Frame-Relay use broadcast?  To which the answer is no.  Frame-Relay does not use either Layer 2 or Layer 3 broadcasts.  However, When you enable frame-relay emcapsulation, inverse ARP is on by default and allows IP broadcasts to go out the interface.  No different then how HDLC allows Layer 3 broadcasts to go out the interface.

                         

                        Hope this helps.

                         

                        Brian

                         

                        • 9. Re: Does Frame Relay use Broadcast ?
                          Keith Barker - CCIE RS/Security, CISSP

                          How about the concept of a pseudo-broadcast being used with frame relay?

                           

                          Keith

                          • 10. Re: Does Frame Relay use Broadcast ?
                            Arul

                            I think the pseudo broadcast is false broadcast messages.

                            It is more like muticast that is send from one DLCI to another DLCI.

                            Am i right ?

                            • 11. Re: Does Frame Relay use Broadcast ?
                              Brian

                              Sure you can call it a pseudo-broadcast if you like, but the fact remains there is no layer 2 broadcast field in the frame-relay header.   Frame Relay has no inherent broadcast or multicast capabilities whatsoever.

                               

                              What happens when the layer 3 address is the broadcast address?  Frame-relay will send a copy of the packet to all DLCLs on the link.  Take the simple networkl I used in my earlier post (forgot ot add the file - I will fix that).  I have three routers in a full-mesh scenario.  So when router R1 needs to send a layer 3 broadcast message, R1 sends a copy to each of the DLCIs on the link that is one to R2 and one to R3.  Now imagine there are 10 routers or neighbors, R1 would need to send 10 copies, one to each remote router.

                               

                              The problem with the Frame-Relay pseudo-broadcast is it does not scale well because the pseudo-broadscasts are process-switched.  In addition, pseudo-broadcasts are treated as broadcast traffic and sent to all neighbors on the WAN link, regardless of their need to receive them, resulting in packets being dropped from the broadcast queue due to oversubscription.

                               

                              Hope this helps.

                               

                              Brian

                               

                              • 12. Re: Does Frame Relay use Broadcast ?
                                Keith Barker - CCIE RS/Security, CISSP

                                Fantastic!

                                 

                                Thanks for the explanations.

                                 

                                Cheers,

                                 

                                Keith