User manual

C HAPTER 17 | Multicast Filtering
Layer 2 IGMP (Snooping and Query)
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ES-2000 Series
◆ TCN Flood – Enables flooding of multicast traffic if a spanning tree
topology change notification (TCN) occurs. (Default: Disabled)
When a spanning tree topology change occurs, the multicast
membership information learned by switch may be out of date. For
example, a host linked to one port before the topology change (TC)
may be moved to another port after the change. To ensure that
multicast data is delivered to all receivers, by default, an switch in a
VLAN (with IGMP snooping enabled) that receives a Bridge Protocol
Data Unit (BPDU) with TC bit set (by the root bridge) will enter into
“multicast flooding mode” for a period of time until the topology has
stabilized and the new locations of all multicast receivers are learned.
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 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