1.6.a Layer 2 multicast

1.6.a i IGMPv2, IGMPv3

IGMP (Internet Group Management Protocol) is used to dynamically register individual hosts in a multicast group on a particular LAN. Hosts identify group membership by sending IGMP packets to their local multicast router.  Under IGMP, routers listen to IGMP packets and periodically send queries to discover which groups are active/inactive on any given subnet. 

IGMPv2 

In version 2, four types of IGMP message exist: 

• Membership query 
• Version 1 membership report 
• Version 2 membership report 
• Leave group 

IGMPv3 

Adds support for source filtering, which allows a multicast receiver to signal to a router the groups from which it wants to receive multicast traffic and from which sources this traffic is expected. 

Receivers signal membership to a multicast host group in two ways: 

• INCLUDE mode – Receiver announces membership to a host group and provides a list of source addresses (the include list) from which it wants to receive traffic. 
• EXCLUDE mode – Receiver announces membership to a host group a provides a list of source addresses (the exclude list) from which it does not want to receive traffic.  In order to receive traffic from all sources, a host uses an empty exclude list in EXCLUDE mode. 

1.6.a ii IGMP Snooping, PIM Snooping

IGMP Snooping is an IP multicast constraining mechanism that runs on a Layer 2 switch.  IGMP snooping requires the switch to examine/snoop on some Layer 3 information (IGMP join/leave messages) in the IGMP packets sent between the hosts and the router.  When the switch hears the IGMP host report message from a host for a particular multicast group, the switch adds the port number of the host to the associated multicast table entry.  When the switch hears the IGMP ‘leave group’ message from a host, the switch will remove the table entry of that particular host for that multicast group. 

IGMP Snooping is used for Host/Client PC multicast registrations, whereas PIM Snooping is used for Router multicast registration.  PIM snooping is dependent on IGMP snooping being enabled first. 

1.6.a iii IGMP Querier

When there is no multicast router in the VLAN to originate the queries, you must configure an IGMP snooping querier to send the membership queries. When an IGMP snooping querier is enabled, it sends out periodic IGMP queries that trigger IGMP report messages from hosts that want to receive IP multicast traffic. IGMP snooping listens to these IGMP reports to establish appropriate forwarding.

1.6.a iv IGMP Filter

It’s possible to filter IGMP messages on a router by utilizing ACL’s and deploying the ACL on an interface via  the command:

ip igmp access-group <acl name>

 On a switch, we must first create an IGMP profile:

 ip igmp profile <number>

Then define whether the traffic will be permitted or denied, then define the range of the multicast group.  Then the IGMP profile can be applied to a switch interface with the command:

ip igmp filter <number>

1.6.a v MLD

In IPv4 multicast group membership is managed through Internet Group Management Protocol (IGMP).Internet Group Management Protocol (IGMP) is a protocol that manages host membership in IP multicast group. A specific host listens on a specific IP multicast address and receives all packets to that IP address. In IPv6, multicast group management is accomplished with a set of ICMPv6 messages that comprise the Multicast Listener Discovery (MLD) protocol.

MLDv1 is similar to IGMPv2 and MLDv2 similar to IGMPv3.Like IGMP, MLDv2 is backward compatible to MLDv1.For cisco switches, MLD snooping provides the same functionality as IGMP snooping in IPv4.It provides information to the switch about which connected hosts are member of a specific group so that switch can make decision about whether, and on which interface, to allow traffic for that group.

MLD is automatically enabled when you perform one of the following configuration tasks on an interface:

• Enable PIM6
• Statically bind a local multicast group
• Enable link-local group reports