Chapter 24
| Multicast Filtering Commands
IGMP Snooping
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ip igmp snooping
router-alert-option-
check
This command discards any IGMPv2/v3 packets that do not include the Router
Alert option. Use the no form to ignore the Router Alert Option when receiving
IGMP messages.
Syntax
[no] ip igmp snooping router-alert-option-check
Default Setting
Disabled
Command Mode
Global Configuration
Command Usage
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.
Example
Console(config)#ip igmp snooping router-alert-option-check
Console(config)#
ip igmp snooping
router-port-expire-
time
This command configures the querier timeout. Use the no form to restore the
default.
Syntax
ip igmp snooping router-port-expire-time seconds
no ip igmp snooping router-port-expire-time
seconds - The time the switch waits after the previous querier stops before
it considers it to have expired. (Range: 1-65535;
Recommended Range: 300-500)
Default Setting
300 seconds
Command Mode
Global Configuration
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