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Foundry Configuration Guide for the FESX, FSX, and FWSX
18 - 2
© Foundry Networks, Inc.
December 2005
receives another Group Membership report for the group, the device removes the entry from the table. The
default is 140 seconds. You can change the interval to a value from 10 – 1220 seconds.
•
Forwarding policy – The device forwards all IP multicast traffic by default but you can enable the device to
forward IP multicast traffic only for groups for which the device has received a Group Membership report, and
drop traffic for all other groups.
Support for IGMP V2 Snooping in Layer 3 Software Images
IGMP V2 snooping has been supported on FESX devices running
switch
software since release 02.0.00. Release
02.2.00 supports IGMP V2 snooping on both
router
and
switch
software images. On both images, IGMP V2
snooping is MAC-based. This differs from IGMP V2 snooping on the BigIron/FastIron router images, which match
on both IP source and group (S,G) entries programmed in the Layer 4 CAM. In contrast, the FESX router images
match on Layer 2 destination MAC address entries.
When Layer 2 CAM is used, traffic is switched solely based on the destination MAC address. Consequently, traffic
of the same group coming to the same port, regardless of its source, is switched in the same way. In addition, the
lowest 23 bits of the group address are mapped to a MAC address. In this way, multiple groups (for example,
224.1.1.1 and 225.1.1.1) have the same MAC address. Groups having the same MAC address are switched to
the same destination ports, which are the superset of individual group output ports. Thus, the use of Layer 2 CAM
might cause unwanted packets to be sent to some ports. However, the switch generally needs far less Layer 2
CAM than it does Layer 4 CAM, which is required for each stream with a different source and group.
NOTE:
Layer 2 IGMP snooping is automatically enabled with Layer 3 multicast routing. If Layer 3 multicast
routing is enabled on your system, do not attempt to enable Layer 2 IGMP snooping.
Configuring IP Multicast Traffic Reduction
By default, Foundry devices forward all IP multicast traffic out all ports except the port on which the traffic was
received. To reduce multicast traffic through the device, you can enable IP Multicast Traffic Reduction. This
feature configures the device to forward multicast traffic only on the ports attached to multicast group members,
instead of forwarding all multicast traffic to all ports. The device determines the ports that are attached to
multicast group members based on entries in the IGMP table. Each entry in the table consists of MAC addresses
and the Foundry device ports from which the device has received Group Membership reports for that group.
By default, the device broadcasts traffic addressed to an IP multicast group that doesn’t have any entries in the
IGMP table. When you enable IP Multicast Traffic Reduction, the device determines the ports that are attached to
multicast group members based on entries in the IGMP table. The IGMP table entries are created when the VLAN
receives a group membership report for a group. Each entry in the table consists of an IP multicast group address
and the Foundry device ports from which the device has received Group Membership reports.
When the device receives traffic for an IP multicast group, the device looks in the IGMP table for an entry
corresponding to that group. If the device finds an entry, the device forwards the group traffic out the ports listed in
the corresponding entries, as long as the ports are members of the same VLAN. If the table does not contain an
entry corresponding to the group or if the port is a member of the default VLAN, the device broadcasts the traffic.
NOTE:
When one or more Foundry devices are running Layer 2 IP Multicast Traffic reduction, configure one of
the devices for active IGMP and leave the other devices configured for passive IGMP. However, if the IP multicast
domain contains a multicast-capable router, configure all the Foundry devices for passive IGMP and allow the
router to actively send the IGMP queries.
Enabling IP Multicast Traffic Reduction
To enable IP Multicast Traffic Reduction, enter the following command:
FESX424 Router(config)# ip multicast
Summary of Contents for FastIron Edge Switch X424
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