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Forum Replies

  1. Hi René,
    Under the loopback. what exact command? Is i pim proxy-server or service?


  2. Hi Rene,

    In the picture, R2 has two interfaces Gi0/2, and I think interface towards R3 should be Gi0/1



  3. Hi Ulrich,

    Here’s the loopback of R2:

    interface Loopback0
    ip address
    ip pim sparse-mode
    ip igmp helper-address udl GigabitEthernet0/2
    ip igmp proxy-service

    You can find the complete configurations of all routers at the end of the lesson :smiley:


  4. R2(config-if)#ip igmp helper-address udl GigabitEthernet 0/2

    ip igmp helper-address udl: this command tells the router to send IGMP membership reports to an upstream router that is connected on our UDL interface.
    Why are we forwarding traffic to R2 Gig 0/2 towards R1 Gig 0/2 when R1 is blocking all income traffic?

    R2(config-if)#ip igmp unidirectional-link

    If it is unidirectional how would we know if traffic is inbound or outbound only?


  5. Hello Mark.

    The lesson here is simulating a situation where there is a unidirectional link such as is often the case when using satellite connections. All traffic on the satellite link goes from the internet towards the user, whereas all upload data goes via an alternate physical link. So, in order to simulate this situation, all incoming data on R2 G0/2 interface is blocked.

    Having said that, let’s get to your questions:

    What the R2(config-if)#ip igmp helper-address udl GigabitEthernet 0/2 command is doing is providing a helper address (similar to a DHCP helper address) that according to Cisco

    “helpers [yes helpers] the report to the IGMP querier associated with the UDL interface identified in the ip igmp helper-address interface configuration command. This command should be used on every downstream router on every interface to a potential multicast receiver.”

    You can find more info about this at this Cisco Documentation link.

    So it is used as a destination address for responses from R1 to multicast queries from H1 that reached R1 via the back-channel. If this was not configured, then any IGMP packets destined for H1 would reach R2 and would be dropped as these did not originate from the Gi0/2 interface. Rene’s explanation of the command may be a little bit confusing, I will let him know to revise.

    You do not need to specify whether the direction is sending or receiving. IGMP will learn which direction it is based on the nature of the connection. In other words, the direction is determined based on which direction the packets are actually flowing.

    I hope this has been helpful!


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