I have two MX80 routers which are connected. When I enable the interface on one router I get the following packet from the other router which I can't explain:

17:23:46.004429  In IP6 (hlim 1, next-header: Options (0), length: 32) fe80::aad0:e501:335e:b368 > ff02::2: HBH (rtalert: 0x0000) [icmp6 sum ok] ICMP6, multicast listener report max resp delay: 0 addr: ff02::2

Also the local router sends a identical packet:

17:23:41.441762 Out IP6 (hlim 1, next-header: Options (0), length: 32) fe80::aad0:e501:3358:2a1b > ff02::2: HBH (rtalert: 0x0000) [icmp6 sum ok] ICMP6, multicast listener report max resp delay: 0 addr: ff02::2

MLD is off on the routers and I'm not sure what purpose this packet serves.

Any ideas would be welcome.

  • 1
    Sure looks like MLDv1, check RFC2710 and RFC3810. You might want to save it in PCAP and review in wireshark. – ytti Aug 7 '13 at 16:57
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 8 '17 at 14:07
  • @RonMaupin No, the answer didn't really help me and currently I don't have an answer or I would've posted+accepted it. It's not really important so I don't know if this will ever get a conclusion... – Sebastian Wiesinger Aug 9 '17 at 8:10
  • OK. If you don't think you will get an acceptable answer, or the question is no longer relevant, you could delete the question. Otherwise, you can leave it open, and it will periodically pop to the top looking for an answer. If it no longer matters, that probably serves no purpose, but if you are still interested, then that is fine. – Ron Maupin Aug 9 '17 at 13:48
  • @RonMaupin I would have deleted it but the very stern warning when pressing the button made me hesitate. ;) – Sebastian Wiesinger Aug 9 '17 at 13:49

This is not so much related to MLD so much as it is to listening on the built-in IPv6 multicast groups.

ff02::1 is the all-nodes multicast - I'm actually surprised you don't see that too (are you sure that you don't?)

ff02::2 is the all-routers multicast, any device configured to forward IPv6 packets will listen on it.

  • I checked and I only see ff02::2. Sending a ff02::1 message seems redundant to me, as everybody listens on it anyway. With ff02::2, I know that routers listen on it I'm just not so sure what the MLD packet will achieve. It actually never reaches the other router anyway as our RE firewall filter catches and discards it. – Sebastian Wiesinger Aug 7 '13 at 16:06
  • 1
    Such packets are useful for switches that do MLD snooping so they know which systems are interested in such multicast traffic. It can then filter multicast traffic sent to ff02::1 to ports that don't do IPv6 at all for example. – Sander Steffann Aug 7 '13 at 16:57
  • Yes but the router only announces ff02::2 not ff02::1. bit strange. – Sebastian Wiesinger Aug 7 '13 at 17:07
  • But wouldn't all need ff02::1? As IPv6 does not have proper broadcast, if you could 'opt-out' for ff02::1, then things would be rather broken for RA+ND. – ytti Aug 7 '13 at 17:34
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    rfc4541 'The only exception is the address FF02::1 which is the all hosts link-scope address for which MLD messages are never sent.'. So it's expected not to see MLD 'registration/announce' for desire to listen to ff02::1, there is no 'opt-out'. – ytti Aug 7 '13 at 17:47

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