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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 17 Co nfiguring IGMP S noo ping and M VR
Unders tanding IGMP Snooping
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.
Note For mor e inf orm atio n on I P m ul tica st a nd IGM P, refer to RFC 11 12 and R F C 2 236.
The mu lti cast rou t er se nds ou t pe riod ic IGM P ge nera l q ue ries to al l VL A Ns. W he n IG MP sno oping is
enabled, the switch responds to the router queries with only one join request per MAC multicast group,
and the sw itch crea tes one e ntry per V LAN in the Layer 2 forwardi ng 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 ad de d to t he f orwa rd ing tabl e e nt ry.
Layer 2 multicast groups learned through IGMP snooping are dyna mic. However, you can statically
configure MAC multicast groups by using the ip igmp snooping vlan static gl obal c onfigura tion
comman d. If y ou spe cif y grou p m embe rsh ip f or a m ult icast group a ddr ess sta tica lly, your se tting
supersedes any automatic manipulation by IGMP snooping. Multicast group membership lists can
consist of both user-defined and IGMP snooping-learned settings.
If a port spanning-tree, a port group, or a VLAN ID change occurs, the IGMP snooping-learned multicast
groups from t his por t on t he V LAN are de lete d.
The switc hes suppo rt a maximu m of 255 IP mul ticast group s and suppor t both IGMP versi on 1 and
IGMP versi on 2.
In the IP multicast-source-only environment, the switch learns the IP multicast group from the IP
multicas t data stream and only f orwards traf fic to the m ulticast ro uter ports. The def ault learnin g method
is IP m ul tica st-sou rce- only learn i ng.

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. Alternativ ely, when the switch receiv es a general
query from the router , it f orwards the query to all ports i n the VLAN. Hosts w anting to join the multicast
group re spond by sending a join message to the switch. The s witch CPU creates a m ulticast
forwardi ng-t a ble e ntry for the gr oup if it i s not alre ady pr e sen t. Th e CPU also add s th e int erfac e wher e
the join message was received to the forwarding-table entry. The host associated with that interface
receives multicast traffic for that multicast group. See Figure 17-1.