All things related to multicast

[Gordon Rogier <>]

I am having a little problem with the 3Com SuperStack II 3000 ethernet
switch and IGMP Multicast Learning.  I am interested to see if anyone else
has experience and/or reported this issue to 3Com.

This issue was identified up to and including the latest SSII 3000 code
available of v2.43.

When in IGMP Multicast Learning mode, it appears to me that the 3Com 3000
does not forward the IGMPv2 Membership Report (8bit 'type' field of 0x16)
to which is sent to the 224.0.0.2 group in response to an IGMP Group
Specific Query (it does not send it anywhere, esp. back towards the
querier router). Because this membership report does not get back to the
querier and in the case that this is _not _ the only receiver on the
segment, the mcast router prunes down the group until a General Query
rolls around.  This effectively can cut off the mcast traffic
inappropriately and cause mcast traffic 'ghosting' type problems for most
applications.  Effectively bad systemic jitter at best.

Turning off IGMP Multicast Learning clearly does "flood" all mcast traffic
as expected.

It basically looks like the 3Com 3300 actually "eats" all IGMP packets and
makes specific forwarding decisions for IGMP packets.

Has anyone else seen this problem???

I am quite concerned about this.  It is quite debatable to me if this
feature should be turned on or off.  If high bandwidth applications are
present, it almost has to be on for reasonable traffic control..... but,
it you need to support lower bandwidth applications like SLP services,
then this could cause some noticable ghosting issues... and for video type
services where side by side receivers are 'channel-surfing' they can very
easily cause noticable IGMP induced "jitter" in the traffic stream on the
local segment.

If I can figure out a valid support email address for 3Com I will forward
this them also. 

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Gordon Rogier                        

            
Network Engineer                     785-864-0381wk 785-550-4468 cell
Great Plains Network                 785-864-9330 FAX