6.3.4
Other Multicast Support
6.3.4.1 Internet Group Management Protocol (IGMP)
IGMP was primarily designed for hosts on multi-access networks to inform
locally attached routers of their group membership information. This is
performed by hosts multicasting IGMP Host Membership Reports. Multicast
routers listen for these messages and then can exchange group membership
information with other multicast routers. This allows distribution trees
to be formed to deliver multicast datagrams. The original version of IGMP
was defined in Host Extensions for IP
Multicasting.
Extensions to IGMP have been developed and released in later releases
of the IP Multicast distribution from Xerox PARC. These extensions, known
as IGMP version 2, include explicit leave messages for faster pruning and
multicast traceroute messages. GATED is aware of both IGMP Version 1 and
Version 2 messages.
The IGMP Statement
igmp yes | no | on | off [ {
query-interval sec ;
max-resp-time sec;
robustness sec ;
interface interface_list igmp_interface_parameters
traceoptions trace_options ;
} ] ;
igmp_interface_parameters are any of:
[enable] [disable] [version [1 | 2]]
[query-interval sec] [max-resp-time sec]
[robustness value]
The igmp statement enables or disables the IGMP protocol. If
the igmp statement is not specified, the default is igmp
off; If enabled, IGMP will default to enabling all interfaces
that are both broadcast and multicast capable. These interfaces are identified
by the IFF_BROADCAST and IFF_MULTICAST interface flags. IGMP must be enabled
before one of the IP Multicast routing protocols are enabled.
The options are as follows:
- query-interval sec
- The interval between General Queries sent by Querier. IGMP Host Membership
Queries are sent if this router is elected the designated querier for the
LAN. If not specified, the default value is 125 seconds. By varying the
query interval, an administrator may tune the number of IGMP on the subnet.
The larger values of query interval cause IGMP Queries to be sent less
often.
- interface interface_list
- Enables or disables IGMP on this interface or list of interfaces.
- Currently, the gated implementation of IGMP cannot detect when there
are multiple addresses configured on the same physical interface. It will
send IGMP messages to each logical network. This should be fixed in a future
release.
- The possible parameters are:
- disable
- Specifies that IGMP packets received via the specified interface will
be ignored. The default is to listen to IGMP on all interfaces.
- enable
- This is the default. This argument may be necessary when disable
is used on a wildcard interface descriptor. Enable the interface for IGMP
protocol.
- version [1 | 2]
- Version 1 is the default set. Version 2 is the default.
- queryinterval sec
- Query interval in seconds between General Queries sent by the Querier.
IGMP querier to transmit Group Specific queries or for version 1 query
cycle.
- max-response-time sec
- This is the longest interval that a group will remain in the local
group database without receiving a Host Membership Report. If not specified,
the default value is 270 seconds. It is calculated as 2*(query-interval
+ 10). The 10 seconds is the variation that hosts use when sending Host
Membership Reports. If the value of query-interval is specified and the
timeout interval is not specified, GateD will use this formula to calculate
an appropriate timeout interval. However, if a timeout interval is specified,
it will override any calculated value.
- robustness value
- Robustness variable allows tuning for the expected packet loss on a
subnet. If a subnet is expected to be lossy, the Robustness Variable may
be increased. IGMP is robust to (robustness-1) packet losses. The robustness
variable <strong> must not </strong> be set to one. Default
value is 2.
traceoptions trace_options
Specifies the tracing options for IGMP. (See Trace
Statements and the IGMP specific tracing options below.)
Tracing options
Packet tracing options (which may be modified with detail,
send or recv):
- packets
- All IGMP packets.
- query
- IGMP Host Membership Query packets
- report
- IGMP Host Membership Reports and New Host Membership Reports
- leave
- IGMP Host Leave messages
- mtrace
- IGMP Multicast trace route request and responsepackets
plus Cisco multicast trace messages.
Sample IGMP configurations
See the sample multicast router configurations.
Last updated November 22, 1997
gated@gated.merit.edu