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The IGMP snooping problem
IGMP snooping is a valuable technique to limit and shape (multicast) network traffic. But as
IGMP snooping is a technique based upon eavesdropping on other protocols, it is necessary
to ensure that the entire IGMP infrastructure is there. Also, when the topology changes (for
example, changes in the spanning tree due to switch maintenance), the network administrator
must make sure that IGMP information remains consistent. These issues are fully described
in RFC4541.
The DAPs act as multicast hosts and the network must provide for a node that performs the
Querier function; otherwise, no IGMP traffic is generated except on host startup. In a full
multicast-routed network, the Querier function is performed by the multicast-aware router (see
Multicast configuration
router as there is usually no need for it. The result is that the Querier function is not
performed.
The absence of the Querier function, and therefore the absence of regular IGMP traffic, makes
it difficult for the IGMP-snooping Layer 3 switches to determine where the multicast hosts are
located. Even though the Layer 3 may know locally which switch ports are connected to
multicast hosts, this is usually not known for the uplink connections.
Unless IGMP snooping is disabled, multicast problems frequently occur due to incorrect
configuration of the network.
The IGMP snooping solution
The simplest solution, and most frequently implemented, is to switch off IGMP snooping in the
Layer 3 switches for the VoIP VLAN. Even though this causes multicast traffic to reach ports
which are not connected to multicast group members (for example, other parties on the VoIP
VLAN), this is usually not an issue.
When IGMP snooping cannot be switched off, the network administrator must ensure that the
Querier function is implemented at the correct location in the network. This must be done in
such a way that all involved IGMP-snooping Layer 3 switches converge their multicast groups
so that multicast group members in all locations of the network can communicate through
multicast. This must be ensured when there are changes in the topology of the network.
The way in which this is done completely depends on the capabilities of the IGMP-snooping
Layer 3 switch and the network topology. Some switches can act as Querier; other switches
must be statically configured.
SIP DECT Fundamentals
on page 260), but many networks do not contain a multicast-aware
The IGMP snooping problem
October 2012
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