multicast on frame relay

johnifanx98johnifanx98 Member Posts: 329
in CCNA & CCENT
When a FR router runs OSPF/EIGRP, it would multicast messages to its peer. My question is how it knows which PVC listens on a specific multicast address, or, it assumes all PVCs listen on all multicast addresses, and then multicast to all PVCs by default?

In a real broadcast network, a multicast IP is translated into a layer 2 multicast address. Another question is if there is such a counterpart in DLCI.
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  • Dieg0MDieg0M Member Posts: 861
    In an NBMA network, you will use the broadcast command after the DLCI mapping so that broadcast and multicast can be pseudo-broadcasted through the PVC. This means that the multicast or broadcast will be encapsulated in a single unicast frame destined for each remote router that you have a frame-relay map command configured. This is my understanding of it anyways.
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  • johnifanx98johnifanx98 Member Posts: 329
    Dieg0M wrote: »
    In an NBMA network, you will use the broadcast command after the DLCI mapping so that broadcast and multicast can be pseudo-broadcasted through the PVC. This means that the multicast or broadcast will be encapsulated in a single unicast frame destined for each remote router that you have a frame-relay map command configured. This is my understanding of it anyways.

    What is DLCI in each frame? Is it DLCI of each PVC, or, there is a multicast DLCI across all PVCs?
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    The DLCI is mapped to the IP address.
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  • johnifanx98johnifanx98 Member Posts: 329
    networker050184 wrote: »
    The DLCI is mapped to the IP address.

    I am talking about layer 2, or, frame address.
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    Yes I know. Do you know what a frame map is?
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  • theodoxatheodoxa Member Posts: 1,340 ■■■■□□□□□□
    A DLCI is simply Frame Relay's addressing method. A DLCI is assigned to each end of a PVC. A Frame Relay Map is used to map these DLCIs to their corresponding IP Addresses.
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  • johnifanx98johnifanx98 Member Posts: 329
    When routing a packet, the router finds the DLCI from the next hop IP address in the frame map. Clear. Now my question is when a router multicasts, how does it find the DLCI? I bet there is not a multicast IP mapping in the frame map.
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    When you configure OSPF/EIGRP you tell it which network to run on right? So the frame map tells it which DLCI is associated with that network.
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  • theodoxatheodoxa Member Posts: 1,340 ■■■■□□□□□□
    johnifanx98 wrote: »
    When routing a packet, the router finds the DLCI from the next hop IP address in the frame map. Clear. Now my question is when a router multicasts, how does it find the DLCI? I bet there is not a multicast IP mapping in the frame map.

    My understanding is that it simply sends it to the DLCIs on the other end of all PVCs configured on that port. Its a Psuedo (False) Broadcast because Frame Relay simply creates a Unicast copy of the packet for each DLCI. For example, let's say R1 had 3 PVCs, each point-to-point, R1 would create 3 copies of the packet and Unicast them over each of those PVCs to the other end.
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  • johnifanx98johnifanx98 Member Posts: 329
    networker050184 wrote: »
    When you configure OSPF/EIGRP you tell it which network to run on right? So the frame map tells it which DLCI is associated with that network.

    OSPF might, but not for EIGRP or RIPv2.
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    So when you configure EIGRP or RIP you don't use a network statement? How does the router know which interfaces to include in the process?
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  • johnifanx98johnifanx98 Member Posts: 329
    networker050184 wrote: »
    So when you configure EIGRP or RIP you don't use a network statement? How does the router know which interfaces to include in the process?

    what I meant is EIGRP/RIP does not differentiate NBMA type network.
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  • johnifanx98johnifanx98 Member Posts: 329
    theodoxa wrote: »
    My understanding is that it simply sends it to the DLCIs on the other end of all PVCs configured on that port. Its a Psuedo (False) Broadcast because Frame Relay simply creates a Unicast copy of the packet for each DLCI. For example, let's say R1 had 3 PVCs, each point-to-point, R1 would create 3 copies of the packet and Unicast them over each of those PVCs to the other end.

    Here is the output showing which layer 2 interface (ethernet )subscribes to which multicast addresses. I assume NBMA FR has similar features, like which DLCI subscribes to which multicast addresses. Then when a FR router multicasts, it knows which DCLI to unicast to (pseudo multicast)


    # netstat -g

    IPv6/IPv4 Group Memberships
    Interface RefCnt Group
    lo 1 all-systems.mcast.net
    eth0 1 224.0.0.251
    eth0 1 all-systems.mcast.net
    lo 1 ff02::1
    eth0 1 ff02::202
    eth0 1 ff02::1:ffb4:da21
    eth0 1 ff02::1
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    johnifanx98 wrote: »
    what I meant is EIGRP/RIP does not differentiate NBMA type network.

    Why would it need to? You specify the address in the network command then the address is mapped to a dlci. Not sure what else you are not getting besides that.
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  • johnifanx98johnifanx98 Member Posts: 329
    networker050184 wrote: »
    Why would it need to? You specify the address in the network command then the address is mapped to a dlci. Not sure what else you are not getting besides that.

    Assuming router A has two PVCs, and runs EIGRP. One PVC connects to router B which does not run EIGRP. If it happens we run network commands to add both PVCs, then EIGRP multicast traffic will go to router B whatsover, though it will be filtered out at router B at layer 3, right?

    This conflicts with my understanding of multicast. I thought multicast traffic will be handled at layer 2, like when it'll be accepted or rejected.
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  • Dieg0MDieg0M Member Posts: 861
    Multicast can be L2 or L3, it is just a method for data delivery to a group of destinations.
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  • EdTheLadEdTheLad Member Posts: 2,111 ■■■■□□□□□□
    johnifanx98 wrote: »
    Assuming router A has two PVCs, and runs EIGRP. One PVC connects to router B which does not run EIGRP. If it happens we run network commands to add both PVCs, then EIGRP multicast traffic will go to router B whatsover, though it will be filtered out at router B at layer 3, right?
    This conflicts with my understanding of multicast. I thought multicast traffic will be handled at layer 2, like when it'll be accepted or rejected.

    So you have 2 PVCs, one that needs eigrp and one that doesn't? This could be configured on a FR point of view in two different ways as you haven't given details.

    1) Multipoint PVC's
    In this case both PVCs share the same subnet and will participate in eigrp via the same network command. Eigrp uses multicast for hello's, dest 224.0.0.10, a frame-relay pvc will only send broadcast/multicast ip packets if broadcast is enabled on the pvc, so you could just enable broadcast on one of the two pvc's. If broadcast was on both, the hello's would be sent on both, since eigrp isn't enabled on R2, R2 is not listening for multicast traffic destined for multicast group 224.0.0.10 so therefore any multicast traffic arriving on R2 will be dropped after FR encapsulation is removed.


    2) Point-to-Point PVCs
    In the case of ptp pvc's these will be on different subnets and can be controlled using the network statement under eigrp.
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  • theodoxatheodoxa Member Posts: 1,340 ■■■■□□□□□□
    johnifanx98 wrote: »
    Assuming router A has two PVCs, and runs EIGRP. One PVC connects to router B which does not run EIGRP.

    I would imagine that it would not send it over the PVC to the router that doesn't run EIGRP, but I'm not absolutely sure.
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