Igmp Snooping; Igmp Snooping Disabled - Allied Telesis SwitchBlade x3100 Series Manual

Introduction

5.2 IGMP Snooping

IGMP snooping allows the SBx3112 to conserve the local area network bandwidth by not flooding (broadcast-
ing) the multicast frames but rather forwarding the multicast frames only to those ports that have expressed an
interest in receiving such frames. The product must examine (or snoop) some layer 3 information (join and
leave) in the IGMP host membership report message and the IGMP host leave group messages sent by the host
to a multicast router. The snooping of these messages is used to learn (or forget) which ports are interested
(or not interested) in receiving multicast packets.
In simple terms, upon the receipt of an IGMP host membership report message for a particular multicast group,
the IGMP learning process adds the port to the MAC address table against the multicast MAC address if it is
not already present. Upon the receipt of an IGMP host leave group message for a multicast group, the IGMP
learning process deletes the port from the MAC address table if it is present.
The forwarding process then utilizes the MAC address table populated by the learning process above to do
efficient forwarding of the received multicast frame.

5.2.1 IGMP Snooping Disabled

When IGMP snooping is disabled, the treatment of multicast frames by the SBx3112 is the same as any other
layer 2 switch.
Each time a frame is received, the learning process reads the source MAC address and updates the address
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tables if required. The forward process then uses these address tables to do an address lookup on the des-
tination MAC address to determine where to forward the frame.
Initially, the SBx3112 starts out by broadcasting/flooding (default forwarding) the received unicast frames on
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all its ports other than the port it was received on. This continues until the learning process learns and pop-
ulates the MAC address table (consisting of MAC address - port entries) after which the received unicast
frames are forwarded only to the intended destination. The exact port of the intended destination is
obtained by using the destination MAC address in the received frame as a key to locate the address - port
entry (inserted by the learning process earlier on) in the MAC address table. This is called the address
lookup phase as part of the forwarding process to exactly forward the unicast frames. Note that there is
one entry per unicast MAC address in the MAC address table since the unicast addresses are unique.
For any broadcast frames (with a destination MAC address of all 1's), the frame is forwarded on all the LAN
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switch ports (flooding) by the forwarding process obviously not including the port the broadcast frame was
received on.
For any multicast frames the lookup fails to determine the ports to send this frame on, and so floods them
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to all ports in the VLAN. There is no Source Address with the Multicast Address since it has not been
learned.
Creating a VLAN of type VLAN is actually the same, except the frame may be flooded on only
Note:
member ports of the VLAN.
5-2
Software Reference for SwitchBlade x3100 Series Switches (Internet Group Management
Protocol (IGMP))
IGMP Snooping Disabled