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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter24 Configuring IP Multicast Routing
Cisco Implementation of IP Multicast Routing
IGMP Version 2
IGMPv2 provides enhancements over IGMPv1. The query and membership report messages are
identical to IGMPv1 message with two exceptions. The first difference is that the IGMPv2 query
message is broken into two categories: general queries, which perform the same function as the IGM Pv1
queries, and group-specific queries, which are queries directed to a single group. The second difference
is that different type codes are used with IGMPv1 and IGMPv2 membership reports. IGMPv2 also
includes new features:
•Querier election process—IGMPv2 routers or multilayer switches can elect the query router without
having to rely on the multicast routing protocol to perform this process.
As each IGMPv2 router or multilayer switch starts, it sends an IGMPv2 general query message to
the all-host multicast group (224.0.0.1) with its interface address in the source IP address field of
the message. Each IGMPv2 device compares the source IP address in the message with its own
interface address, and the device with the lowest IP address on the subnet is elected as the querier.
•Maximum response time field—this field in the query message permits the query router to specify
the maximum query-response time and controls the burstiness of the response process.
This feature can be important when large nu m bers of groups are active on a subnet and you want to
spread the responses over a longer period of time. However, increasing the maximum response timer
value also increases the leave latency; the query router must now wait longer to make s ure ther e are
no more hosts for the group on the subnet.
•Group-specific query message—permits the query router to p erfor m the qu er y o pe rat ion o n a
specific group instead of all groups.
•Leave group messages—provides hosts with a method of notifying routers and multilayer switches
on the network that they are leaving a group as shown in Figur e 24-3.
Figure 24-3 IGMPv2 Leave Process
In this example, Hosts 2 and 3 are members of multicast group 224.1.1.1. Host 2 sends an IGMPv2 lea ve
message to the all-multicast-routers group (224.0.0.2) to inform all routers and multilayer switches on
the subnet that it is leaving the group. Router 1, the query router, receives the message, but because it
keeps a list only of the group memberships that are active on a subnet and not individual hosts that are
members, it sends a group-specific query to the target group (224.1.1.1) to determine whether a ny hosts
remain for the group. Host 3 is still a member of multicast group 224.1.1.1 and receives the
Host 1 Host 2 Host 3
224.1.1.1 224.1.1.1
Router
IGMPv2
querier
45146
1. Leave-group
message sent
to 224.0.0.2
3. IGMPv2
membership
report for
224.1.1.1
2. Send group-specific
query to 224.1.1.1