Netis ST3326(ST-3302) [94/118] Chapter10 multicast

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Chapter10:Multicast
Understanding IGMP Snooping
Layer 2 switches can use IGMP snooping to constrain the flooding of multicast traffic by dynamically
configuring Layer 2 interfaces so that multicast traffic is forwarded to only those interfaces associated
with IP multicast devices. As the name implies, IGMP snooping requires the LAN switch to snoop on
the IGMP transmissions between the host and the router and to keep track of multicast groups and
member ports. When the switch receives an IGMP report from a host for a particular multicast group,
the switch adds the host port number to the forwarding table entry; when it receives an IGMP Leave
Group message from a host, it removes the host port from the table entry. It also periodically deletes
entries if it does not receive IGMP membership reports from the multicast clients.
The multicast router sends out periodic IGMP general queries to all VLANs. When IGMP snooping is
enabled, the switch responds to the router queries with only one join request per MAC multicast
group, and the switch creates one entry per VLAN in the Layer 2 forwarding table for each MAC group
from which it receives an IGMP join request. All hosts interested in this multicast traffic send join
requests and are added to the forwarding table entry.
Layer 2 multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure MAC multicast groups. If you specify group membership for a multicast group address
statically, your setting supersedes any automatic manipulation by IGMP snooping. Multicast group
membership lists can consist of both user-defined and IGMP snooping-learned settings.
IGMP Versions
The switch supports IGMP version 1, IGMP version 2, and IGMP version 3. These versions are
interoperable on the switch. For example, if IGMP snooping is enabled on an IGMPv2 switch and the
switch receives an IGMPv3 report from a host, the switch can forward the IGMPv3 report to the
multicast router.
Joining a Multicast Group
When a host connected to the switch wants to join an IP multicast group, it sends an unsolicited IGMP
join message, specifying the IP multicast group to join. Alternatively, when the switch receives a
general query from the router, it forwards the query to all ports in the VLAN. Hosts wanting to join
the multicast group respond by sending a join message to the switch. The switch CPU creates a
multicast forwarding-table entry for the group if it is not already present. The CPU also adds the
interface where the join message was received to the forwarding-table entry. The host associated
with that interface receives multicast traffic for that multicast group.
Leaving a Multicast Group

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