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  1. Hi,

    Why do we need a router ? . If I have only a switch , how does it work

    Thank you

  2. Hello Ananth. I will attempt to answer all of your questions in this single reply.

    1. You wrote: H1 and H2 are still interested so they will respond with a membership report. The switch will intercept these two messages and forwards them to the CPU. One of the two membership reports is then forwarded to the router. In the above statement , only one the membership report is forwarded to the router . Is this a form of report suppression? Also explain why only one report is sent?

    Yes, only one membership report is forwarded to the router, however this one membership report includes all of the interfaces that want to take part in the multicast group. This is not a form of membership report suppression, because as mentioned in the lesson:

    "our hosts don’t hear each others membership reports. This overrules the membership report suppression mechanism which I described in the IGMP version 2 lesson. This is required since the switch has to hear the membership report from all interfaces that need to receive the multicast traffic."
    1. You wrote: Also one more query , if the router sends a general query , the switch forwards the query to all hosts? Is that correct? Will the switch able to generate query message on its own?

    Yes, that is correct. The switch will forward the query to all hosts. The switch does not generate its own queries, but transmits the queries received from the router.

    1. You wrote: It might be a dum question. How the PC has interface address as 0/1 . Normally the PC is connected to the switch right?

    Rene here is using routers to act as the PCs so that information about IGMP can be viewed. That is why the interface is GigabitEthernet 0/1 on the logging output in the lesson.

    1. You wrote: In the below message, Send v2 Report for on GigabitEthernet0/1. Which 0/1 interface it is referiing to ? Is it switch or the host interface

    Again, this is the interface of the router that is playing the role of the PCs. Router with hostname H1 plays the role of Host 1 and router with hostname H2 for Host 2.

    1. You wrote: In the below message , u had mentioned the switch will send a message if any-one else is still interested? where is this message seen?

    Yes, you are right. The first line should read
    IGMPSN: group: Group exist - Leave for group received on Vlan 1, port Gi0/2, group state (1)
    and not Gi0/3

    1. You wrote: Also generally only one host will be connected to one port of the switch right? Then why there is a need to send the group specific query to the port when one of the host decides to leave from that port?

    Actually, you may have more than one host connected to one port of a switch. What if you connect a hub to this port? (not that you would, but this must also be taken into account). What if you connect an unmanaged switch to this port? So you may have more than one host on a port…

    1. You wrote: My question here us why the switch sends the IGMP general query? [as a response to a leave group message] how different it is from a group specific query sent by a router?

    For this question, Andrew has written a very good answer. Take a look at this post: https://networklessons.com/topic/igmp-snooping/#post-23806

    I hope these answers have been helpful!


  3. Hello Say Hian

    I will attempt to answer your questions below:

    H1 and H2 are hosts. However, Rene has used routers there to simulate the hosts. This is why he has command line access to them.[quote=“sayhian16, post:28, topic:1321”]
    Does the following commands assign multicast ip address to ports in switch?
    ip igmp snooping vlan (vlan number) static (multicast address) interface (interface number)

    ip igmp snooping vlan 2 static int gi1/0/3

    This command will configure a layer 2 port as a member of the group defined by the static multicast address that you specify.

    The IGMP Snooping Querier feature can be enabled either globally or on a per-VLAN basis. If you only want to query only the specific multicasting ports, then you can limit it to the VLAN. For more information, take a look at this Cisco documentation.

    I hope this has been helpful!


  4. Hi Hussein,

    You can try:

    R1# show mac address-table multicast vlan 1
     vlan   mac address     type    qos             ports
       1  0100.5e02.0203  static   --  Gi0/1,Gi0/2,Router
       1  0100.5e00.0127  static   --  Gi0/1,Gi0/2,Router
       1  0100.5e00.0128  static   --  Gi0/1,Gi0/2,Router
       1  0100.5e00.0001  static   --  Gi0/1,Gi0/2,Router,Switch
  5. Hi Hussein,

    You can see the multicast groups with show ip igmp snooping groups but it won’t show the corresponding MAC addresses that are used per group. About your questions:

    You can use this if you expect packet loss on your subnet. It changes the interval for some IGMP messages. The downside of changing this is that you increase the leave latency:

    * Group member interval: this is the amount of time that the router waits before it determines there are no members left in the group. It is calculated as (robustness variable * query-interval) + (1x query response interval).
    * Other querier present interval: the amount of time that a router has to wait before determining there is no other multicast router that is the querier. Calculated as (robustness variable * query interval) + (0.5 x query response interval).
    * Last member query count: number of group specific querier that the router sends before it determines there are no members left in the multicast group. The number of queries is equal to the value of the robustness variable.

    The reason it can’t be zero is probably because of the last member query count. A setting of 0 would indicate it won’t send any last member query count messages.

    That is a good question…I didn’t see this before but you are right, the output is exactly the same. Probably a Cisco IOS quirk…

    Before Cisco IOS 12.1(11b)EW, the default behavior of the switch was to flood multicast traffic on all interfaces when it receives a TCN (which indicates a spanning-tree topology change). The idea behind this is that multicast traffic is not interrupted but it can get pretty ugly if you have a lot of multicast traffic which is why there is the no ip igmp snooping tcn flood command.

    This command disables the flooding of multicast when a TCN is received, and you can set it per interface.

    Hmm I think this depends on the platform and IOS version. I don’t believe there is a separate command to enable/disable IGMPv3 snooping.

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