Web Management Guide

Chapter 13 | Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
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◆ TCN Query Solicit – Sends out an IGMP general query solicitation when a
spanning tree topology change notification (TCN) occurs. (Default: Disabled)
When the root bridge in a spanning tree receives a TCN for a VLAN where IGMP
snooping is enabled, it issues a global IGMP leave message (or query
solicitation). When a switch receives this solicitation, it floods it to all ports in
the VLAN where the spanning tree change occurred. When an upstream
multicast router receives this solicitation, it immediately issues an IGMP general
query.
A query solicitation can be sent whenever the switch notices a topology
change, even if it is not the root bridge in spanning tree.
◆ Router Alert Option – Discards any IGMPv2/v3 packets that do not include the
Router Alert option. (Default: Disabled)
As described in Section 9.1 of RFC 3376 for IGMP Version 3, the Router Alert
Option can be used to protect against DOS attacks. One common method of
attack is launched by an intruder who takes over the role of querier, and starts
overloading multicast hosts by sending a large number of group-and-source-
specific queries, each with a large source list and the Maximum Response Time
set to a large value.
To protect against this kind of attack, (1) routers should not forward queries.
This is easier to accomplish if the query carries the Router Alert option. (2) Also,
when the switch is acting in the role of a multicast host (such as when using
proxy routing), it should ignore version 2 or 3 queries that do not contain the
Router Alert option.
◆ Unregistered Data Flooding – Floods unregistered multicast traffic into the
attached VLAN. (Default: Disabled)
Once the table used to store multicast entries for IGMP snooping and multicast
routing is filled, no new entries are learned. If no router port is configured in the
attached VLAN, and unregistered-flooding is disabled, any subsequent
multicast traffic not found in the table is dropped, otherwise it is flooded
throughout the VLAN.
◆ Forwarding Priority – Assigns a CoS priority to all multicast traffic. (Range: 0-7,
where 7 is the highest priority; Default: Disabled)
This parameter can be used to set a high priority for low-latency multicast
traffic such as a video-conference, or to set a low priority for normal multicast
traffic not sensitive to latency.
◆ Version Exclusive – Discards any received IGMP messages which use a version
different to that currently configured by the IGMP Version attribute.
(Default: Disabled)
◆ IGMP Unsolicited Report Interval – Specifies how often the upstream
interface should transmit unsolicited IGMP reports when proxy reporting is
enabled. (Range: 1-65535 seconds, Default: 400 seconds)