and here is a tcpdump
root@OpenWrt:~# tcpdump -i br-wan igmp
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on br-wan, link-type EN10MB (Ethernet), snapshot length 262144 bytes
13:42:14.472896 IP 192.168.20.1 > 233.136.0.202: igmp v2 report 233.136.0.202
13:42:15.812889 IP 192.168.20.1 > 233.32.36.161: igmp v2 report 233.32.36.161
13:42:18.109196 IP one.one.one.one > all-systems.mcast.net: igmp query v2
13:42:18.432900 IP 192.168.20.1 > 233.136.0.202: igmp v2 report 233.136.0.202
13:42:24.572968 IP 192.168.20.1 > 239.255.255.250: igmp v2 report 239.255.255.250
13:42:25.212889 IP 192.168.20.1 > 233.32.36.161: igmp v2 report 233.32.36.161
13:42:28.216225 IP 192.168.20.1 > all-routers.mcast.net: igmp leave 233.32.36.161
13:42:28.310807 IP one.one.one.one > 233.32.36.161: igmp query v2 [max resp time 4] [gaddr 233.32.36.161]
13:42:28.710590 IP one.one.one.one > 233.32.36.161: igmp query v2 [max resp time 4] [gaddr 233.32.36.161]
13:42:29.110694 IP one.one.one.one > 233.32.36.161: igmp query v2 [max resp time 4] [gaddr 233.32.36.161]
13:42:48.108512 IP one.one.one.one > all-systems.mcast.net: igmp query v2
13:42:57.212946 IP 192.168.20.1 > 239.255.255.250: igmp v2 report 239.255.255.250
13:42:57.212973 IP 192.168.20.1 > 233.136.0.202: igmp v2 report 233.136.0.202
13:43:18.104342 IP one.one.one.one > all-systems.mcast.net: igmp query v2
13:43:20.492911 IP 192.168.20.1 > 233.136.0.202: igmp v2 report 233.136.0.202
13:43:27.933058 IP 192.168.20.1 > 239.255.255.250: igmp v2 report 239.255.255.250