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New Member

IGMP Snooping - all mcast traffic forwarded to mrouter ports?

I've been looking into IGMP snooping and have read that a L2 switch will forward multicast traffic to all ports connected to an interested receiver AND all mrouter ports. In a L2 'V' topology this results in all multicast traffic routed onto a VLAN being forwarded to the 2nd distribution switch. My question is how should a 6500 Sup720 deal with this unwanted multicast traffic? Both a Local SPAN of the RP and a Netdr capture suggest that this traffic is punted to the RP and ultimately dropped. Is this expected behavior or should the traffic be dropped in H/W?



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Cisco Employee

IGMP Snooping - all mcast traffic forwarded to mrouter ports?

Hello Tim,

With IGMP snooping enabled the multicast traffic is not flooded (which would include the cpu) however there are still instances where the traffic may need to be punted.

Can you paste the netdr capture for at least one of those packets?



New Member

IGMP Snooping - all mcast traffic forwarded to mrouter ports?

Fabienne, thanks for the reply. Further reading suggests that PIM snooping would prevent the traffic from being forwarded on all mrouter ports. Unfortunately due to a bug that isn't an option. Below is an exmaple of a punted packet from the netdr capture taken on the distribution switch that is NOT the PIM DR for the segment:

------- dump of incoming inband packet -------
interface Vl703, routine mistral_process_rx_packet_inlin, timestamp 1w5d
dbus info: src_vlan 0x2BF(703), src_indx 0x182(386), len 0x346(838)
  bpdu 0, index_dir 0, flood 0, dont_lrn 0, dest_indx 0xB08(2824)
  68020400 02BF0400 01820003 46080000 00110520 0E000040 00000000 0B080000
mistral hdr: req_token 0x0(0), src_index 0x182(386), rx_offset 0x76(118)
  requeue 0, obl_pkt 0, vlan 0x2BF(703)
destmac 01.00.5E.40.01.62, srcmac 00.1C.F9.09.94.00, protocol 0800
protocol ip: version 0x04, hlen 0x05, tos 0x00, totlen 820, identifier 11384
  df 0, mf 0, fo 0, ttl 13, src, dst
    udp src 1168, dst 8798 len 800 checksum 0x51B7