Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
Operation With or Without IP Addressing
You can configure IGMP on VLANs that do not have IP addressing. The benefit
of IGMP without IP addressing is a reduction in the number of IP addresses
you have to use and configure. This can be significant in a network with a large
number of VLANs. The limitation on IGMP without IP addressing is that the
switch cannot become Querier on any VLANs for which it has no IP address—
so the network administrator must ensure that another IGMP device will act
as Querier. It is also advisable to have an additional IGMP device available as
a backup Querier. See the following table.
Table 4-1.Comparison of IGMP Operation With and Without IP Addressing
IGMP Function Available With IP Addressing
Configured on the VLAN
Available
Without IP
Addressing?
Operating Differences Without an IP Address
Forward multicast group traffic to any port on Yes None
the VLAN that has received a join request for
that multicast group.
Forward join requests (reports) to the Querier. Yes None
Configure individual ports in the VLAN to Auto Yes None
(the default)/Blocked, or Forward.
Configure IGMP traffic forwarding to normal or Yes None
high-priority forwarding.
Age-Out IGMP group addresses when the last
IGMP client on a port in the VLAN leaves the
group.
Support Fast-Leave IGMP and Forced Fast-
Leave IGMP (below).
Yes
Yes
Requires that another IGMP device in the VLAN has an IP
address and can operate as Querier. This can be a multi-
cast router or another switch configured for IGMP oper-
ation. (HP recommends that the VLAN also include a
device operating as a backup Querier in case the device
operating as the primary Querier fails for any reason.
Support automatic Querier election. No Querier operation not available.
Operate as the Querier. No Querier operation not available.
Available as a backup Querier. No Querier operation not available.
Automatic Fast-Leave IGMP
IGMP Operation Presents a “Delayed Leave” Problem. Where multiple
IGMP clients are connected to the same port on an IGMP device (switch or
router), if only one IGMP client joins a given multicast group, then later sends
a Leave Group message and ceases to belong to that group, the IGMP device
retains that IGMP client in its IGMP table and continues forwarding IGMP
traffic to the IGMP client until the Querier triggers confirmation that no other
4-13
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