Internet Group Management Protocol (IGMP) | 215
Leaving and Staying in Groups
Figure 12-5 shows how multicast routers track and refresh state changes in response to group-and-specific
and general queries.
1. Host 1 sends a message indicating it is leaving group 224.1.1.1 and that the included filter for 10.11.1.1
and 10.11.1.2 are no longer necessary.
2. The querier, before making any state changes, sends a group-and-source query to see if any other host
is interested in these two sources; queries for state-changes are retransmitted multiple times. If any are
interested, they respond with their current state information and the querier refreshes the relevant state
information.
3. Separately in Figure 12-5, the querier sends a general query to 224.0.0.1.
4. Host 2 responds to the periodic general query so the querier refreshes the state information for that
group.
Figure 12-5. IGMP Membership Queries: Leaving and Staying in Groups
IGMP Snooping
Multicast packets are addressed with multicast MAC addresses, which represent a group of devices, rather
than one unique device. Switches forward multicast frames out of all ports in a VLAN by default, even
though there may be only some interested hosts, which is a waste of bandwidth. IGMP snooping enables
switches to use information in IGMP packets to generate a forwarding table that associates ports with
multicast groups so that when they receive multicast frames, they can forward them only to interested
receivers.
1/1
Querier
Non-Querier
Membership Queries: Leaving and Staying
Type: 0x17
Number of Group Records: 1
Record Type: 6
Number of Sources: 2
Multicast Address: 224.1.1.1
Source Addresses: 10.11.1.1, 10.11.1.2
IGMP Leave message
Type: 0x11
Group Address: 224.1.1.1
Number of Sources: 2
Source Address: 10.11.1.1, 10.11.1.2
GMP Group-and-Source
pecific Query
Type: 0x22
Number of Group Records: 1
Record Type: 2
Number of Sources: 0
Multicast Address: 224.2.2.2
Type: 0x11
Group Address: 224.0.0.1
Number of Sources: 0
IGMP General
Membership Query
IGMP Membership Repor
Interface Multicast Group Filter Source Source
Address Timer Mode Timer
1/1 224.1.1.1 Include
10.11.1.1 LQMT
10.11.1.2 LQMT
224.2.2.2 GMI Exclude None
Queries retransmitted Last Member
Query Count times at Last Member
Query Interval
1
2
4
3
2/1
Non-querier builds identical table
and waits Other Querier Present
Interval to assume Querier role
Host 1 Host 2
To Next Page
Loading...
