Volume 1 Part 1 of 6 Section 2.9

Why is it on routers 3 and 4 that broadcasts still work? I don't have a frame-relay map ip <ip> <dlci> broadcast command configured. Actually there is nothing configured on router 3 and 4's serial interfaces. So they are using LMI to find the PVC to use. I can see using the sh frame-relay map command that broadcasts are allowed i just don't understand why. I always through broadcasts were only allowed if you configured it for such. Router 3 and 4 use physical interfaces for the config and router 5 (hub) uses multipoint sub interfaces.




    Inverse-ARP automatically supports the sending of broadcast and multicast frames.




  • In addition,

    You can issue "show frame-relay map" and you will see the Dynamic mappings indicating "broadcast" support.


    Hi Carlos,

    He did that already ;), but he is wondering why broadcast keyword is displayed, indicating the support.

  • hahaha sorry, my bad, i havent read the whole post. I just read the first two of the sentences. ; )

  • Justin0104,

    Yes, this lab is tricky.

    You're requested to "TURN-OFF" inverse-arp on R1 and R2 BUT you're
    requested to assigned unused DLCI to s0/0.999 on R3 and R4.

    R3 and R4 will still have the inverse-arp ON but you just "mislead"
    them to an unused interface while still keeping main DLCI (eg. on R3
    the main DLCI is 305") to the hub.

  • Right which I did. The unused PVCs were placed into the s0/0.999 inteface. So are you saying that if the interface is able to use inverse arp to resolve the DLCI that it can actually use the broadcast service? I have always been under the impression that broadcasts only traversed a link if the broadcast keyword was used on the frame-relay map ip command.

  • Anyone have any ideas about this. Router 5 is the hub and is configured with multipoint subinterfaces. Under the subinterfaces on R5 i have configured frame-relay interface-dlci. On R1 and R2, they only use the physical interface, no subinterfaces. R1 and R2 also only use Inverse-arp to resolve the L3 to L2 mapping. So there is no frame-relay map ip or frame-relay interface-dlci commands configured on the interfaces. Yet on R1 and R2 I can send broadcast traffic. So if i poing router 5 will respond to R1 (broadcast). Any reason why this behavior works without the need for a frame-relay map ip with the broadcast keyword tacked on?

  • Justin0104,

    The Fram inverse-arp will resolve IP to DLCI and include the broadcast
    packets too.


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