User Tools

Site Tools


latex2wiki:user_manual:igmp_and_mld

IGMP and MLD

Terminology and Concepts

When a receiver joins a multicast group, the multicast routers serving that receiver’s subnet need to know that the receiver has joined so that they can arrange for multicast traffic destined for that group to reach this subnet. The Internet Group Management Protocol (IGMP) is a link-local protocol for IPv4 that communi- cates this information between receivers and routers. The same role for IPv6 is performed by the Multicast Listener Discovery protocol (MLD).

The basic IGMP mechanism works as follows. When a multicast receiver joins a multicast group it mul- ticasts an IGMP Join message onto the subnet on which it is joining. The local routers receive this join, and cause multicast traffic destined for the group to reach this subnet. Periodically one of the local routers sends a IGMP Query message onto the subnet. If there are multiple multicast routers on the subnet, then one of them is elected as the sole querier for that subnet. In response to an IGMP query, receivers respond by refreshing their IGMP Join. If the join is not refreshed in response to queries, then the state is removed, and multicast traffic for this group ceases to reach this subnet. There are three different versions of IGMP:

  • IGMP version 1 functions as described above.
  • IGMP version 2 adds support for IGMP Leave messages to allow fast leave from a multicast group.
  • IGMP version 3 adds support for source include and exclude lists, to allow a receiver in indicate that it only wants to hear traffic from certain sources, or not receive traffic from certain sources.

XORP supports IGMPv1, IGMPv2, and IGMPv3.

MLD for IPv6 functions in basically the same way as IGMP. The functionality of MLDv1 corresponds with that of IGMPv2, and the functionality of MLDv2 corresponds with that of IGMPv3. XORP supports MLDv1 and MLDv2.

Standards

XORP complies with the following standards for multicast group membership:

  • RFC 2236: Internet Group Management Protocol, Version 2
  • RFC 3376: Internet Group Management Protocol, Version 3
  • RFC 2710: Multicast Listener Discovery (MLD) for IPv6
  • RFC 3810: Multicast Listener Discovery Version 2 (MLDv2) for IPv6

Configuring IGMP and MLD

IGMP and MLD only require the interfaces/vifs to be configured that are intended to have multicast listeners.

Configuration Syntax

 protocols {
    igmp {
       targetname: text
       disable: bool
       interface text {
         vif text {
            disable: bool
            version: uint(1..3)
            enable-ip-router-alert-option-check: bool
            query-interval: uint(1..1024)
            query-last-member-interval: uint(1..1024)
            query-response-interval: uint(1..1024)
            robust-count: uint(2..10)
         }
       }
       traceoptions {
         flag all {
            disable: bool
         }
       }
    }
 }

protocols {
  mld {
     targetname: text
     disable: bool
     interface text {
        vif text {
           disable: bool
           version: uint(1..2)
           enable-ip-router-alert-option-check: bool
           query-interval: uint(1..1024)
           query-last-member-interval: uint(1..1024)
           query-response-interval: uint(1..1024)
           robust-count: uint(2..10)
        }
     }
     traceoptions {
        flag all {
           disable: bool
        }
     }
  }
}
Keyword Description
protocols this delimits the configuration for all routing protocols in the XORP router configuration. It is mandatory that IGMP configuration is under the protocols node in the configuration.
igmp this delimits the IGMP configuration part of the XORP router configuration.
targetname this is the name for this instance of IGMP. It defaults to “IGMP”, and it is not recommended that this default is overridden under normal usage scenarios.
disable this takes the value true or false, and determines whether IGMP as a whole is enabled on this router 1 . The default value is false.
interface this specifies an interface to be monitored by IGMP for the presence of multicast receivers. Each interface to be monitored by IGMP needs to be explicitly listed. The value is the name of an interface that has been configured in the interfaces section of the router configuration.
       For each interface, one or more VIFs must be specified:
         vif: this specifies a vif to be monitored by IGMP for the presence of multicast receivers. Each vif
              to be monitored by IGMP needs to be explicitly listed. The value is the name of a vif that has
              been configured in the interfaces section of the router configuration (see Chapter 3).
              Each vif takes the following optional parameter:
                disable: this takes the value true or false, and determines whether IGMP is disabled on
                    this vif 2 . The default value is false.
                version: this directive specifies the protocol version for this interface/vif 3 . In case of IGMP
                    it takes a non-negative integer in the interval [1..3] with default value of 2. In case of MLD
                    the value must be in the interval [1..2] with default value of 1.
                enable-ip-router-alert-option-check: this directive specifies whether the router
                    should check that the link-local protocol packets received on this interface/vif have the IP
                    Router Alert option (see RFC-2213) in them 4 . If it is enabled, all link-local protocol packets
                    that do not contain the IP Router Alert option will be dropped.
                query-interval: this directive specifies the interval (in seconds) between general queries
                    sent by the querier on this interface/vif 5 . The default value is 125 seconds.
                query-last-member-interval: this directive specifies the maximum response time (in
                    seconds) inserted into group-specific queries sent in response to leave group messages on
                    this interface/vif. It is also the interval between group-specific query messages 6 . The
                    default value is 1 second.
                query-response-interval: this directive specifies the maximum response time (in sec-
                    onds) inserted into the periodic general queries on this interface/vif 7 . It must be less than
                    the query-interval. The default value is 10 seconds.
                robust-count: this directive specifies the robustness variable count that allows tuning for
                    the expected packet loss on a subnet for this interface/vif 8 . The robust-count specifies
                    the startup query count, and the last member query count. It is also used in the computation
                    of the group membership interval and the other querier present interval. The IGMP/MLD
                    protocol is robust to robust-count packet losses. The default value is 2.
traceoptions this directive delimits the configuration of debugging and tracing options for IGMP.
        flag: this directive is used to specify which tracing options are enabled. Possible parameters are:
                all: this directive specifies that all tracing options should be enabled. Possible parameters are:
                      disable: this takes the value true or false, and disables or enables tracing 9 . The
                         default is false.

Note that in case of IGMP each enabled interface must have a valid IPv4 address.

The configuration for MLD is identical to IGMP, except for the following:

  • The mld directive is used in place of the igmp directive.
  • The default value of targetname is ‘‘MLD’’ instead of ‘‘IGMP’’.
  • Each enabled interface must have a valid link-local IPv6 address.

Example Configurations

 protocols {
   igmp {
     interface dc0 {
        vif dc0 {
          /* version: 2 */
          /* enable-ip-router-alert-option-check: false */
          /* query-interval: 125 */
          /* query-last-member-interval: 1 */
          /* query-response-interval: 10 */
          /* robust-count: 2 */
     }
   }
 }
 protocols {
   mld {
     disable: false
     interface dc0 {
        vif dc0 {
          disable: false
          /* version: 1 */
          /* enable-ip-router-alert-option-check: false */
          /* query-interval: 125 */
          /* query-last-member-interval: 1 */
          /* query-response-interval: 10 */
          /* robust-count: 2 */
        }
     }
     traceoptions {
        flag all {
          disable: false
        }
     }
   }
 }

In the example configuration above, IGMP is enabled on two vifs on two different interfaces (dc0/dc0 and dc1/dc1). In addition, MLD is enabled on interface/vif dc0/dc0, and all MLD tracing functionality is enabled for diagnostic purposes.

Monitoring IGMP

The show igmp group command can be used to display information about IGMP group membership:

 user@hostname> show igmp group
 Interface      Group               Source          LastReported Timeout V State
 dc0            224.0.0.2           0.0.0.0         10.4.0.1         161 3      E
 dc0            224.0.0.13          0.0.0.0         10.4.0.1         159 3      E
 dc0            224.0.0.22          0.0.0.0         10.4.0.1         159 3      E
 dc0            224.0.1.15          0.0.0.0         10.4.0.3         160 2      E
 dc0            224.0.1.20          0.0.0.0         10.4.0.2           0 3      I
 dc0            224.0.1.20          1.2.3.4         10.4.0.2           0 3      F
 dc2            224.0.0.2           0.0.0.0         10.3.0.2         155 3      E
 dc2            224.0.0.13          0.0.0.0         10.3.0.1         157 3      E
 dc2            224.0.0.22          0.0.0.0         10.3.0.1         156 3      E

In the above example, Source refers to the multicast source address in the case of source-specific IGMP join entries, or it is set to 0.0.0.0 in case of any-source IGMP join entries. The LastReported field contains the address of the most recent receiver that responded to an IGMP Join message. The Timeout field shows the number of seconds until it is next time to query for host members (i.e., to send an IGMP Query message for this particular entry). The V field shows the IGMP protocol version. The State field shows the state of the entry:

  • I = INCLUDE (for group entry)
  • E = EXCLUDE (for group entry)
  • F = Forward (for source entry)
  • D = Don’t forward (for source entry)

The show igmp interface command can be used to display information about IGMP interfaces:

 user@hostname> show igmp interface
 Interface      State       Querier         Timeout Version Groups
 dc0            UP          10.4.0.1           None         3      5
 dc2            UP          10.3.0.1            136         3      3
 register vif DISABLED 0.0.0.0                 None         3      0

The information indicates whether IGMP is enabled on the interface and the IP address of the IGMP querier. If this router is the querier, then the time until the next query message is shown. Finally the number of multicast groups with receivers on this subnet is shown. Note that in the above example it is normal for the interface named register vif to be DISABLED. This interface has special purpose and is used only by PIM-SM. The show igmp interface address command can be used to display information about addresses of IGMP interfaces:

 user@hostname> show igmp interface address
 Interface     PrimaryAddr        SecondaryAddr
 dc0           10.4.0.1
 dc2           10.3.0.2
 register vif 10.4.0.1

As shown above, the PrimaryAddr per interface is the address used to originate IGMP messages, and all other alias addresses on that interface are listed as SecondaryAddr, with one address per line. The equivalent commands for MLD are:

  • show mld group
  • show mld interface
  • show mld interface address
latex2wiki/user_manual/igmp_and_mld.txt · Last modified: 2011/03/12 00:04 by Ben Greear