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Chapter 16
| Multicast Filtering
Layer 2 IGMP (Snooping and Query)
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 or 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.
◆
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)
When a new upstream interface (that is, uplink port) starts up, the switch sends
unsolicited reports for all currently learned multicast channels via the new
upstream interface.
This command only applies when proxy reporting is enabled.
◆
Router Port Expire Time – The time the switch waits after the previous querier
stops before it considers it to have expired. (Range: 1-65535, Recommended
Range: 300-500 seconds, Default: 300)
◆
IGMP Snooping Version – Sets the protocol version for compatibility with
other devices on the network. This is the IGMP Version the switch uses to send
snooping reports. (Range: 1-3; Default: 2)
– 472 –
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