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  • Multicast static join group.

    Hi,
    I need a help in using a static-join for a below scenario,
    There is a multicast streamer/source streaming the multicast traffic out from a interface having IP address 192.168.1.1/32 with no gateway. The multicast streamer is pumping traffic for multicast group 239.150.151.4 & 239.150.151.9 on port 5500.This streamer is connected to a Cisco 4849 Switch (configured as a L2 switch) on access port G1/1.


    This L2 switch is further connected to another Cisco 4849 switch (configured a L3 switch, IP routing & multicast routing is enabled only) on access port G1/2.The L3 switch is having the same access VLAN 200 is extended to his another post G1/2 and I can join & see this multicast stream on my LapTop (running a VLC player) connected on same access VLAN 200 port. Attached is the diagram.

    My question is, when I am sending any IGMP join request from my laptop, is this join request is going to the L2 switch or L3 switch is only responding the IGMP join and forwarding & serving the multicast?

    If L3 is not doing this job, and IGMP joins are going to L2 switch each time, then how to restrict all these join request to L3 switch only, so that these request should not be forwarded to L2 switch and L3 should serve it and forwards the multicast…Is there any way of configuring multicast static-joins on L3 to restrict it? And what are the additional configurations to be done on L3 to achieve this objective and how?

    My Observations:-
    1)I can see the equal amount of traffic bit rate (10Mbps)as seen on G1/1 of L2 switch and G1/1 of L3 switch(without even joining any multicast group from laptop).So i guess all the traffic from streamer is coming till L3 switch.


    L2#sh int GigabitEthernet1/1 | inc rate
    5 minute input rate 10000000 bits/sec, 0 packets/sec
    5 minute output rate 2000 bits/sec, 314365 packets/sec
    L3#sh int GigabitEthernet1/1 | inc rate
    30 second input rate 9200000 bits/sec, 51879 packets/sec
    30 second output rate 1000 bits/sec, 7591 packets/sec
    L3#sh int GigabitEthernet1/2 | inc rate
    30 second input rate 0 bits/sec, 51879 packets/sec
    30 second output rate 0 bits/sec, 7591 packets/sec


    2)When I join any one multicast group I can see the traffic still to be same on the both G1/1 ports of L2 & L3 switch but some 4Mbps traffic on G1/2 of L3 switch due to the group I joined and is the desired & expected behavior.
    L3#sh int GigabitEthernet1/2 | inc rate
    30 second input rate 0 bits/sec, 51879 packets/sec
    30 second output rate 3800000 bits/sec, 7591 packets/sec
    Attached Files

  • #2
    Re: Multicast static join group.

    IGMP joins from the client are sent to the L3 device on that segment. Remember that igmp joins are between clients and either routers or L3 switches with multicast enabled. PIM joins are sent between routers only. If your L2 switch is in the transit path then it will forward the igmp join to that multicast enabled device. Once at the multicast enabled router it really depends on what version of PIM you are using as far as next steps go.

    You can use a switch/router to simulate a client with the

    igmp join-group command


    If your worried your multicast routers arent receiving the joins you can check by issuing the ip mroute command on them. You should see a *,G entry in there for that group.

    Also remember if your running pim-dense mode that mode is an implicit join meaning it wil flood that multicast traffic out to everyone and then start pruning it back, so it would be normal to start seeing traffic everywhere with dense-mode even if you havent requested it. Sparse-mode is an explicit join, meaning the client has to request the multicast feed. So think of it like this,

    Dense-mode - Push technology

    Spares-mode- Pull technology



    CCNA, CCNA-Security, CCNP
    CCIE Security (In Progress)

    Comment


    • #3
      Re: Multicast static join group.

      Also remember that PIM uses the underlying IGP for transport. Also be aware of the RPF check and how it works.
      CCNA, CCNA-Security, CCNP
      CCIE Security (In Progress)

      Comment


      • #4
        Re: Multicast static join group.

        Also from looking at your diagram you have no L3 interfaces in the multicast path so the switches will treat the multicast traffic as broadcast traffic and foward out all ports except the one it was received on. You have ip multicast-routing enabled on one switch but from you diagram you have no L3 interface nor do you have pim enabled anywhere. You can enable igmp snooping to help the switch learn what ports the multicast traffic should be forwarded too. Right now its treating it as broadcast and just flooding it out everywhere.
        CCNA, CCNA-Security, CCNP
        CCIE Security (In Progress)

        Comment


        • #5
          Re: Multicast static join group.

          Thanks for the elaborating reply & Yes you are right, apart from only these two commands(ip routing & ip multicast-routing) there is no other commands for L3 forwarding is used,not even PIM is enabled on G1/1 of L3 switch.Also there is no L3 interface on even L3 switch.

          So my questions are :-
          1)As per your advice for enabling snooping,I think igmp-snooping is enabled by default in 4849 switch. ?
          2)Consider If both the switches are in pure L2 domain, then any igmp-join request from the client will be forwarded to Multicast Streamer(Source) Or, if not ,then who will responde to this query the L2 or the L3 switch, OR there would not be any igmp-join/query/reply generated from client or to the source at all ?
          3)If I connect more clients of the L3 switch,will it increase the over all traffic on the source Interface G0/0..OR L3 switch will replicate the packets and will forwards on all clients connected on all its ports under same vlan 200?
          4)In any case I dont want any igmp-join query are fwded to L2 switch..how to restrict this (If this happening) and only L3 switch should respond to all igmp-join queries and replicate the packets on all clients connected on this switch and how to achieve it ?

          Comment


          • #6
            Re: Multicast static join group.

            Correct L2 switches dont reply to igmp joins as in order for an interface to be igmp enabled it must be running pim. With igmp snooping the switch is able to intercept the igmp joins and forward it to the port where the router is attached. The layer 3 interface then processes the join. In your case you have no L3 interfaces configured. igmp snooping will help with what ports the multicast should be replicated too in order to stop it replicating out to all ports. I know igmp snooping is enable on the 3560's and 3750's by default but not sure with your model. I would check the configuration guide for that particular switch.

            Also nothing is ever forwarded back to the multicast source. The only job of the multicast source is to put the traffic on the wire. The source doesnt care once that multicast traffic starts flowing. Its up to the routers and switches to get that traffic to the clients. The only devices that process igmp-joins are routers/L3 switches configured for multicast.
            CCNA, CCNA-Security, CCNP
            CCIE Security (In Progress)

            Comment

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