There is a problem with IGMP snooping across switches since the IGMP message on one switch doesn't get send to the other switch, so the switch with the source will not know that the port to the other switch should receive the multicast frames. Cisco has a document which describes the problem (and Cisco-specific solutions). This is not a vendor-specific problem, but a solution would be vendor-specific. From what I can tell, for the HP 2810, you will need to manually set the switch-to-switch ports to
Forward: Causes the switch to forward all IGMP and IP multicast transmissions through the port.
Cisco's explanation of the problem:
Understand the Problem and Its Solutions
By default, the Catalyst switches have IGMP snooping enabled. With
IGMP snooping, the switch snoops (or listens) for IGMP messages on all
the ports. The switch builds an IGMP snooping table that basically
maps a multicast group to all the switch ports that have requested it.
Assume that, without any prior configuration, Receiver 1 and Receiver
2 have signaled their intentions to receive a multicast stream for
18.104.22.168 that maps to the L2 multicast MAC address of 01.00.5e.6f.ef.ef. Both Switch 1 and Switch 2 create an entry in their snooping tables for these receivers in response to the IGMP reports
that the receivers generate. Switch 1 enters port Gigabit Ethernet
2/48 in its table, and Switch 2 enters port Fast Ethernet 1/0/47 in
Note: At this point, the multicast source has not started its traffic, and none of the switches knows about the switch mrouter port.
When the source on Switch 1 starts to stream multicast traffic, Switch
1 has "seen" the IGMP report from Receiver 1. As a result, Switch 1
delivers the multicast out port Gigabit Ethernet 2/48. But, since
Switch 2 "absorbed" the IGMP report from Receiver 2 as part of the
IGMP snooping process, Switch 1 does not see an IGMP report (multicast
request) on port Gigabit Ethernet 2/46. As a result, Switch 1 does not
send any multicast traffic out to Switch 2. Therefore, Receiver 2
never gets any multicast traffic, even though Receiver 2 is in the
same VLAN but merely on a different switch than the multicast source.
The reason for this issue is that IGMP snooping is not really
supported on any Catalyst platform without an mrouter. The mechanism
"breaks down" in the absence of an mrouter port. If you want a fix for
this solution, you must have the switches somehow learn or know of an
mrouter port. The Solutions section of this document explains the
procedure. But how does the presence of an mrouter port on the
switches remedy the situation?
Basically, when the switches learn or statically know about an mrouter
port, two critical things occur:
- The switch "relays" the IGMP reports from the receivers to the mrouter port, which means that the IGMP reports go toward the
multicast router. The switch does not relay all the IGMP reports.
Instead, the switch sends only a few of the reports to the mrouter.
For the purpose of this discussion, the number of reports is not
important. The multicast router only needs to know if there is at
least one receiver that is still interested in the multicast
downstream. In order to make the determination, the multicast router
receives the periodic IGMP reports in response to its IGMP queries.
- In a source-only multicast scenario, in which no receivers have yet "joined" in, the switch only sends the multicast stream out its
When the switches know their mrouter port, Switch 2 relays out the
IGMP report that the switch received from Receiver 2 to its mrouter
port. This port is Fast Ethernet 1/0/33. Switch 1 gets this IGMP
report on the switch port Gigabit Ethernet 2/46. From the perspective
of Switch 1, the switch has received merely another IGMP report. The
switch adds that port into its IGMP snooping table and begins to send
out the multicast traffic on that port as well. At this point, both
the receivers receive the requested multicast traffic, and the
application works as expected.
But how do the switches identify their mrouter port so that IGMP
snooping works as it is expected to work in a simple environment like
this? The Solutions section provides some answers.
HP's explanation of the problem:
When the switch receives an IGMP Join, it accepts the host request and
begins forwarding the IGMP traffic. This means ports that have not
joined the group and are not connected to routers or the IGMP Querier
will not receive the group's multicast traffic.
It appears that you could set the switches up as IGMP Queriers:
Using the Switch as Querier
The function of the IGMP Querier is to poll other IGMP-enabled devices
in an IGMP-enabled VLAN to elicit group membership information. The
switch performs this function if there is no other device in the VLAN,
such as a multicast router, to act as Querier. Although the switch
automatically ceases Querier operation in an IGMP-enabled VLAN if it
detects another Querier on the VLAN, you can also use the Command
Prompt to disable the Querier capability for that VLAN.
Note A Querier is required for proper IGMP operation. For this reason, if you disable the Querier function on a switch, ensure that
there is an IGMP Querier (and, preferably, a backup Querier) available
on the same VLAN.
If the switch becomes the Querier for a particular VLAN (for example,
the DEFAULT_VLAN), then subsequently detects queries transmitted from
another device on the same VLAN, the switch ceases to operate as the
Querier for that VLAN. If this occurs, the switch Event Log lists a
pair of messages similar to these:
I 01/15/01 09:01:13 igmp: DEFAULT_VLAN: Other Querier detected
I 01/15/01 09:01:13 igmp: DEFAULT_VLAN: This switch is no longer Querier
In the above scenario, if the other device ceases to operate as a
Querier on the default VLAN, then the switch detects this change and
can become the Querier as long as it is not pre-empted by some other
IGMP Querier on the VLAN. In this case, the switch Event Log lists
messages similar to the following to indicate that the switch has
become the Querier on the VLAN:
I 01/15/01 09:21:55 igmp: DEFAULT_VLAN: Querier Election in process
I 01/15/01 09:22:00 igmp: DEFAULT_VLAN: This switch has been elected as Querier