RFC 4541, "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", May 2006Source of RFC: magma (int)
Errata ID: 4032
Publication Format(s) : TEXT
Reported By: Josef Felber
Date Reported: 2014-06-30
Rejected by: Brian Haberman
Date Rejected: 2014-07-01
Section 2.1.2 says:
1) Packets with a destination IP address outside 224.0.0.X which are not IGMP should be forwarded according to group-based port membership tables and must also be forwarded on router ports.
It should say:
1) Packets with a destination IP address outside 224.0.0.X which are not IGMP should be forwarded according to group-based port membership tables.
IMHO it makes no sense to forward non-IGMP datagrams to router ports if these are not members of the refering groups.
Consider the following example:
A IGMP snooping switch is connected to two mcast servers and some hosts. Server A is the querier, server B is the non-querier.
Now with the prevailing version of RFC4541 any mcast data stream (outside 224.0.0.x, non IGMP) would be routed to the hosts which are members of the refering group (correct), but also to server A, because the switch recognizes this port as a mcast server port due to the querier function. But server A is not interested in the data streams of server B unless it joins the group. This behaviour uneccessarily loads the port of server A with bandwidth.
Obviously some (all?) switch models (like HP2920) show this erroneous behavior.
Kind Regards, Josef Felber
The text in the RFC is correct. Router ports need to see the multicast traffic in order to correctly forward it to interested members not connected to the switch.