2 Replies Latest reply: Feb 23, 2011 5:38 PM by jneiberger RSS

    Why does PIM enable IGMP?


      IGMP is enabled automatically when you enable PIM on an interface and

      I can't figure out why. I've talked to a few other engineers and no

      one can thing of why IGMP would need to be enabled on a

      router-to-router connection running PIM. My only thought was that it

      was a leftover from PIMv1, which used IGMP type 14 packets, but that's

      a total guess.


      Do any of you happen to know the answer to this piece of trivia?




        • 1. Re: Why does PIM enable IGMP?

          Hello jneiberger,

          In addition to the main role of IGMP, managing multicast hosts membership in LAN, it is also used by mtrace command for path checking :



          Type      Name                                    Reference

          ----      ----------------------                  ---------

          0x11      IGMP Membership Query                   [RFC1112]

          0x12      IGMPv1 Membership Report                [RFC1112]

          0x13      DVMRP                                   [RFCDVMRP]

          0x14      PIM version 1                           [PIMv1]

          0x15      Cisco Trace Messages

          0x16      IGMPv2 Membership Report                [RFC2236]

          0x17      IGMPv2 Leave Group                      [RFC2236]

          0x1e      Multicast Traceroute Response           [Fenner]

          0x1f      Multicast Traceroute                    [Fenner]

          0x22      IGMPv3 Membership Report                [RFCIGMPv3]

          0xf0-0xff Reserved for experimentation            [RFCIGMPIANA]

          From the previous post about mtrace :



          • 2. Re: Why does PIM enable IGMP?

            Oh, we were just talking about that in the other thread!  lol   I should have realized that. Now I feel silly that we had a whole thread about mtrace and it didn't even enter my mind with regard to this question. Thanks!