This document describes the Nx Networks implementation of the Internet Group Management Protocol (IGMP). It includes the following sections:

Introducing IGMP

IP supports dynamic multicast through multicast extensions to OSPF (Open Shortest Path First) and DVMRP (Distance Vector Multicast Routing Protocol). Nx Networks routers running OpenROUTE 5.6 also support static multicast routes on a standalone IP interface.

The Internet Group Management Protocol (IGMP) is useful by itself or as an MOSPF (Multicast Extension to OSPF) or DVMRP extension that lets an IP host participate in IP multicasting. IGMP lets a router keep track of IP group membership on its local LANs by sending IGMP Host Membership Queries and receiving IGMP Host Membership Reports. An MOSPF or DVMRP router then distributes information on the location of groups throughout its routing domain. This enables routers to efficiently send a multicast datagram to its multiple destinations.

Normally, IP sends no IGMP queries on a router interface unless that interface is a LAN interface and the interface is running either DVMRP or MOSPF. However, if Standalone IGMP is enabled, IGMP can run on any physical interface (even on WANs). Any entries in the IGMP database on the interface directly contribute to the IP multicast forwarding table of the router, even if MOSPF or DMVRP are not running on the interface.

You can enable or disable standalone IGMP on an interface anytime, and you can configure the multicast IP address in the IGMP database for the interface. This has the effect of permanently configuring the multicast IP address in the IGMP database for the interface as if an end station on the interface were permanently reporting the multicast address to IGMP.

Displaying the IGMP Prompts

Displaying the IGMP Prompts from DVMRP

Config>protocol dvmrp
DVMRP Config>igmp

Internet Group Management Protocol Configuration
IGMP Config> 

Monitor>protocol dvmrp
DVMRP>igmp
Internet Group Management Protocol Console
IGMP> 

Config>protocol ospf
Open SPF-Based Routing Protocol configuration console

OSPF Config>igmp
Internet Group Management Protocol Configuration

IGMP Config> 

OSPF>igmp
Internet Group Management Protocol Console

IGMP>

Config>protocol ip
IP Config>igmp
Internet Group Management Protocol Configuration

IGMP Config> 

IP>igmp
Internet Group Management Protocol Console

IGMP>

For static IGMP to run on an interface, you enable standalone IGMP on an IP interface using the enable standalone-IGMP command. You permanently configure a multicast IP address for the interface using the join interface command.

The default IGMP settings work for most configurations.

IGMP Commands

Press Space twice after you type a command to display the available parameters for each command. Enter help for information about using the command line interface.

[C] means the command is available at the IGMP Config> prompt.

[M] means the command is available at the IGMP> prompt.

See Displaying the IGMP Prompts.

Table 17 IGMP Commands

Command	Description
Clear-Config [C]	Resets the IGMP configuration to standard defaults.
Clear-Counters [M]	Resets all IGMP counters to zero.
Disable IGMP [C] [M]	Turns off dynamic IGMP for an interface.
Disable Standalone-IGMP [C] [M]	Turns off standalone IGMP for an interface.
Enable IGMP [C] [M]	Turns on dynamic IGMP for an interface.
Enable Standalone-IGMP [C] [M]	Turns on standalone IGMP for an interface.
Join Interface [C] [M]	Permanently configures the multicast IP address of a standalone interface in the IGMP database.
Join Router-Internal [C] [M]	Establishes the router as a member of a multicast group.
Leave Interface [C] [M]	Removes the interface's membership in a multicast group.
Leave Router-Internal [C] [M]	Removes the router's membership in a multicast group.
Mgroups [M]	Displays group membership information on the router's interfaces.
Restore [M]	Restores current configuration from permanent memory.
Save [M]	Saves changes you make at the IGMP> prompt to permanent memory.
Set [C] [M]	Sets various IGMP parameters.

Clear-Config [C]

Example: clear-config

Delete IGMP config entries(Yes or [No]): y
IGMP config memory deleted.  IGMP standard default values will be used.
Restarting the router, or doing IGMP>restore, then IGMP>save
will recreate the config records with the default values. 

Example: clear-counters

Disable IGMP [C] [M]

Syntax: disable igmp

Interface number [0]? 

Syntax: disable standalone-IGMP interface#

Enable IGMP [C] [M]

Note: For IGMP to run on an interface, you must configure either MOSPF or DVMRP on the interface.

Interface number [0]?

Syntax: enable standalone-IGMP interface#

Join Interface [C] [M]

You can enter join interface at either the configuration prompt or the monitoring prompt. There are three differences when you enter join interface at the monitoring prompt:

• The effect on group membership is immediate. You do not need to restart the router.

• The effect on group membership is temporary. Unless you explicitly save changes you make at the IGMP> prompt, the router does not save them when you restart the router.

• The command keeps track of the number of times the interface joins a particular group.

Join Router-Internal [C] [M]

You can enter join router-internal at either the configuration prompt or the monitoring prompt. There are three differences when you enter join router-internal at the monitoring prompt:

• The effect on group membership is immediate. You do not need to restart the router.

• The effect on group membership is temporary. Unless you explicitly save changes you make at the IGMP> prompt, the router does not save them when you restart the router.

• The command keeps track of the number of times the router joins a particular group.

Group address [0.0.0.0]? 128.185.0.0

• The effect on group membership is immediate. You do not need to restart the router.

• The effect on group membership is temporary. Unless you explicitly save changes you make at the IGMP> prompt, the router does not save them when you restart the router.

• The command does not delete group membership until the number of leaves executed equals the number of joins previously executed.

Leave Router-Internal [C] [M]

You can enter leave at either the configuration prompt or the monitoring prompt. There are three differences when you enter leave at the monitoring prompt:

• The effect on group membership is immediate. You do not need to restart the router.

• The effect on group membership is temporary. Unless you explicitly save changes you make at the IGMP> prompt, the router does not save them when you restart the router.

• The command does not delete group membership until the number of leaves executed equals the number of joins previously executed.

Enter the address to be deleted [0.0.0.0]? 128.185.0.0

List [C]

Syntax: list

all

interfaces

joins

Example: list all

             Stand                        General   General   Leave
    IGMP     Alone            Leave       Query     Query     Query
Ifc Enabled  IGMP    Version  Processing  Interval  Attempts  Attempts
0   Y        N        2        Compatible   125        2         2

Multicast       Joined
Group           Interface
224.1.4.5       0

interfaces

Example: list interfaces

             Stand                        General   General   Leave
    IGMP     Alone            Leave       Query     Query     Query
Ifc Enabled  IGMP    Version  Processing  Interval  Attempts  Attempts
0   Y        N        2        Compatible   125        2         2

Example: list joins

Multicast       Joined
Group           Interface
224.1.4.5       1

Syntax: list

all

config

interfaces

Example: list all

                                            General   General   Leave
                              Leave         Query     Query     Query
Interface   Enabled  Version  Processing    Interval  Attempts  Attempts
1(Eth/0)    Y        2        Compatible    125       2         2    

Interface                    Active Querier      Querier  Entries:
IP Address    Interface  Up  Users  IP Address   Age      In-Use  Freed
128.185.36.1  2(Eth/1)   Y   M    128.185.36.1   0        5       32  
 
Active User Flags: M=MOSPF D=DVMRP

Interface       Interface  Queries  Queries  --Queries Rcvd--  Reports
IP Address      Name       Sent     Rcvd     Unexp    DiffVer  Rcvd
128.185.36.1    2(Eth/1)   12775    26611    26611    26611    53112 

Interface	The number and type of interface.
Enabled	If Y, IGMP runs on this interface as long as you have configured a multicast protocol, either MOSPF or DVMRP, on the interface. If N, IGMP does not run on this interface and the interface does not detect multicast hosts.
Version	Shows which version of IGMP, version 1 or version 2, is running. See set version.
Leave Processing	Compatible, the default, means that the interface processes Leave Requests for a multicast group as long as there are no IGMP version 1 hosts in the group. See set leave processing.
General Query Interval	The rate, in seconds, at which this interface sends General IGMP Queries. See set query-interval-general
General Query Attempts	The number of General Queries the interface sends with no response before it ages out a multicast group. See set general-query-attempts.
Leave Query Attempts	The number of Specific Queries the interface sends with no response before it ages out a multicast group during Leave processing. See set leave-query-attempts.

The remainder of the display shows the actual status of each IGMP interface that is running in the router. The following is a description of these fields.

Interface IP Address	The IP address that IGMP is using as its source address for this interface.
Interface	The number and type of the interface.
Up	If Y, IGMP is running on the interface. If N, IGMP is not running on the interface. There are a several reasons IGMP may not be running on the interface: 1. IGMP is disabled. 2. There is no multicast protocol, MOSPF or DVMRP, configured on the interface. 3. The interface is down.
Active users	Shows what multicast protocols, M (MOSPF) or D (DVMRP), are actively using the multicast group information from the IGMP local group database. Note: MOSPF may not be actively using IGMP if the router is not the designated router. In this case, the software does not display an M.
Querier IP Address	The IP address of the querier for this interface. The querier is the router with the lowest IP address. A router assumes that it is the querier until it learns of another router with a lower IP address.
Querier Age	The number of seconds until IGMP assumes the querier is no longer present and ages out the querier. If the querier is another router, then it is aged so that if the querier is no longer present, this router assumes the role of querier. The age is refreshed each time IGMP receives a Query from the querier. If this router is currently the querier, this field has no meaning.
Entries In-Use	Multicast groups currently active on this interface. To display detail about the multicast groups, enter the mgroups command.
Entries Freed	Multicast groups that were previously active on this interface and have aged out. This number gives an indication of past IGMP activity.
Queries Sent	Queries sent, general or specific.
Queries Rcvd	Queries received, general or specific.
Queries Rcvd Unexp	Queries received while this router believed that it was the querier. Some of these occur at the normal processing of negotiation of the querier. A large number indicates a query negotiation problem.
Queries Rcvd DiffVer	Queries received that were a different version of IGMP.
Reports Rcvd	IGMP reports received, IGMPv1 or IGMPv2.

config

Example: list config

                                            General   General   Leave
                              Leave         Query     Query     Query
Interface   Enabled  Version  Processing    Interval  Attempts  Attempts
0(Eth/0)    Y        2        Compatible    125       2         2  

Example: list interfaces

Interface                 Active Querier         Querier  Entries:
IP Address  Interface  Up Users  IP Address      Age      In-Use  Freed
192.168.6.6  0(Eth/0)  N   M     192.168.6.6     0        2       3

Active User Flags: M=MOSPF D=DVMRP

Interface       Interface  Queries  Queries  --Queries Rcvd--  Reports
IP Address      Name       Sent     Rcvd     Unexp    DiffVer  Rcvd 
192.168.6.6     0(Eth/0)   12775    26611    26611    26611    53114

Syntax: mgroups

Local multicast group entries: in-use: 1  freed: 3
Multicast                            Reported       Expire  v1   Leave
Group           Interface  UpTime    Last By        Time    Age  Age
224.185.0.0     Internal   565       128.185.3.108  109     109  0 

Restore [M]

Note: This command does not affect the join configuration.

This will overwrite your existing runtime configuration.
Continue? (Yes or [No]): yes
Restore complete 

Note: This command does not affect the join configuration.

This will overwrite your existing saved configuration.
Continue? (Yes or [No]): yes 
Save complete 

Syntax: set

general-query-attempts

leave-processing

leave-query-attempts

query-interval-general

version

The group membership ageout time is the General Query Interval multiplied by the General Query Attempts, plus 10 extra seconds to ensure enough time to receive a response to the last query.

By default, IGMP sends two general query attempts at an interval of 125 seconds. The ageout time for a multicast group using the IGMP defaults is:

(125 seconds * 2 attempts) + 10 = 260 seconds

Example: set general-query-attempts

Interface number [0]? 
General Query attempts before expiration (2-20) [2]? 

There are three settings available. The default setting of Compatible should work for all situations since it is designed for backward compatibility. However, for workarounds, you can use the settings Always and Never described below.

Compatible	This is the default. It means that the interface processes Leave Requests for a multicast group as long as there are no version 1 hosts present in the multicast group.
Always	The interface always processes Leave Requests.
Never	The interface never processes Leave Requests.

Example: set leave-processing always

Interface number [0]?

The default is 2. You can increase the attempts if 2 is not enough retries for some hosts on the interface.

The interface sends Specific Queries at one second intervals. Therefore, once Leave processing starts, IGMP ages out the multicast group in approximately two seconds if no member of the multicast group responds.

Example: set leave-query-attempts

Interface number [0]? 
Specific Query attempts before LEAVE expiration (2-20) [2]?

The group membership ageout time is the General Query Interval multiplied by the General Query Attempts, plus 10 extra seconds to ensure enough time to receive a response to the last query.

By default, IGMP sends two general query attempts at an interval of 125 seconds. The ageout time for a multicast group using the IGMP defaults is:

(125 seconds * 2 attempts) + 10 = 260 seconds

Example: set query-interval-general

Interface number [0]? 
Query Interval (3-3600) [125]? 

Example: set version

Interface number [0]? 
Version (1-2) [2]?