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20. IGMP Snooping
Introduction
The BiNOS switch can use IGMP (Internet Group Management Protocol) snooping to
constrain the flooding of multicast traffic. This is done by dynamically configuring physical
interfaces to forward multicast traffic only to interfaces that are associated with IP multicast
devices. IGMP snooping requires the LAN switch to snoop on the IGMP transmissions
between the host and the router and to keep track of multicast groups and member ports.
When the switch receives an IGMP report from a host for a particular multicast group, the
switch adds the host port number to the forwarding table entry. When it receives an
IGMP
Leave Group
message from a host, it removes the host port from the table entry. It also
deletes entries periodically if it does not receive IGMP membership reports from the multicast
clients.
The multicast router sends out periodic general queries to all VLANs. All hosts interested in
this multicast traffic send
join
requests and are added to the forwarding table entry. The
switch forwards only one
join
request per IP multicast group to the multicast router. It creates
one entry per VLAN in the forwarding table for each MAC group from which it receives an
IGMP
join
request.
Multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure MAC multicast groups by using the
ip igmp snooping vlan static
configuration
command. If you specify group membership for a multicast group address statically, your
setting supersedes any automatic manipulation by IGMP snooping. Multicast group
membership lists can consist of both user-defined and IGMP snooping-learned settings.
Joining a Multicast Group
When a host connected to the switch wants to join an IP multicast group, it sends an
unsolicited
IGMP
join
message, specifying the IP multicast group to join. Alternatively, when
the switch receives a general query from the router, it forwards the query to all ports in the
VLAN. Hosts wanting to join the multicast group respond by sending a
join
message to the
switch. The switch CPU creates a multicast forwarding-table entry for the group if it is not
already present. The CPU also adds the interface where the
join
message was received to the
forwarding-table entry. The host associated with that interface receives multicast traffic for
that multicast group.
Leaving a Multicast Group
The router sends periodic multicast general queries and the switch forwards these queries
through all ports in the VLAN. Interested hosts respond to the queries. If at least one host in
the VLAN wishes to receive multicast traffic, the router continues forwarding the multicast
