Brocade Communications Systems Brocate Ethernet Access Switch 6910 - Manual (Page 1111)

Brocade 6910 Ethernet Access Switch Configuration Guide 1061
53-1002581-01
Layer 2 IGMP (Snooping and Query)
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If a topology change notification (TCN) is received, and all the uplink ports are subsequently
deleted, a time out mechanism is used to delete all of the currently learned multicast
channels.
When a new uplink port starts up, the switch sends unsolicited reports for all currently learned
channels out the new uplink port.
By default, the switch immediately enters into “multicast flooding mode” when a spanning tree
topology change occurs. In this mode, multicast traffic will be flooded to all VLAN ports. If many
ports have subscribed to different multicast groups, flooding may cause excessive packet loss
on the link between the switch and the end host. Flooding may be disabled to avoid this,
causing multicast traffic to be delivered only to those ports on which multicast group members
have been learned. Otherwise, the time spent in flooding mode can be manually configured to
reduce excessive loading.
When the spanning tree topology changes, the root bridge sends a proxy query to quickly
re-learn the host membership/port relations for multicast channels. The root bridge also sends
an unsolicited Multicast Router Discover (MRD) request to quickly locate the multicast routers
in this VLAN.
The proxy query and unsolicited MRD request are flooded to all VLAN ports except for the
receiving port when the switch receives such packets.
• 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.