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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 29 Configuring MLDv2 Snooping for IPv6 Multicast Traffic
Understanding How MLDv2 Snooping Works
Figure 29-1 Initial MLDv2 Listener Report
Multicast router A sends an MLDv2 general query to the router, which forwards the query to ports 2
through 5 (all members of the same VLAN). Host 1 wants to join an IPv6 multicast group and multicasts
an MLDv2 report to the group with the equivalent MAC destination address of 0x0100.5E01.0203.
When the router snoops the MLDv2 report multicast by Host 1, the router uses the information in the
MLDv2 report to create a forwarding-table entry, as shown in
Table 29-1
, that includes the port numbers
of Host 1, the multicast router, and the router.
The router hardware can distinguish MLDv2 information packets from other packets for the multicast
group. The first entry in the table tells the router to send only MLDv2 packets to the CPU. This prevents
the router from becoming overloaded with multicast frames. The second entry tells the router to send
frames addressed to the 0x0100.5E01.0203 multicast MAC address that are not MLDv2 packets
(!MLDv2) to the multicast router and to the host that has joined the group.
If another host (for example, Host 4) sends an unsolicited MLDv2 report for the same group
(
Figure 29-2
), the router snoops that message and adds the port number of Host 4 to the forwarding table
as shown in
Table 29-2
. Because the forwarding table directs MLDv2 messages only to the router, the
message is not flooded to other ports. Any known multicast traffic is forwarded to the group and not to
the router.
Forwarding
table
CPU
Host 1
Host 2
Host 3
Host 4
Router A
MLDv2 report
VLAN
PFC
1
0
2
3
4
5
130143
Table 29-1 MLDv2 Snooping Forwarding Table
Destination MAC Address
Type of Packet
Ports
0100.5exx.xxxx
MLDv2
0
0100.5e01.0203
!MLDv2
1, 2
