17-3
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 17 Configuring IGMP Snooping and M V R Understanding IGMP Snooping
Figure 17-1 Initial IGMP Join Message
Router A sends a general query to the switch, which forwards the query to ports 2 through 5, all members
of the same VLAN. Ho st 1 wants to join multicast group 224. 1.2.3 and multicasts an IGMP member ship
report (IGMP join message) to the group with the equivalent MAC destination address of
0x0100.5E01.0203. When the CPU receives the IGMP report multicast by Host 1, the CPU uses the
information in the IGMP rep ort to set up a f orw ar d ing-tab le en tr y, as shown in Table 17-1, that in clude s
the port numbers of Host 1, the router, and the switch internal CPU.
Note that the switch hardware can distinguish IGMP information packets from other packets for the
multicast group.
•The fir st entry in the table tells the switching engine to send IGMP pack ets to only the switch CPU.
This prevents the CPU from be coming overloa ded with mu lticast fr ames.
•The secon d entr y tells the swit ching engi ne to send fram es addr essed to th e 0x0100 .5E01.02 03
multicast MAC address that are not IGMP packets (!IGMP) to the router and to the host that has
joine d the group.
If anothe r host (for exam ple , Ho st 4) send s a n unso lic ited IGM P j oin messa ge f or the sam e gro up (see
Figure 17-2), the CPU receiv es that mes sage an d add s the port nu mbe r of Host 4 to the forw ard ing tabl e
as shown in Table 17-2. Note that b ecause the for warding tabl e directs IGMP m essages to only the CPU,
the mes s age is not flooded to other ports on the switch. Any known mul ticast traffic is forwarded to the
group a nd not to t he CPU . A ny un known mul tica st t raffic is flood ed to th e VLAN an d se nt t o the CP U
until it becomes known.
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
IGMP report 224.1.2.3
VLAN
Switching engine
1
0
2345
45750
Table 17-1 IGMP Snooping Forwarding Table
Dest in atio n Addr ess Type of Pack et Ports
0100.5exx.xxxx IGMP 0
0100 .5e01 .0203 !IGMP 1, 2