6.3.5 Other Multicast Support
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_parmeters
traceoptions trace_options ;
} ] ;
igmp_interface_parmeters 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 April 27, 1997
gated@gated.merit.edu