Multicast Addresses

Multicast address is -, within which - are used for LAN multicast. - are private multicast addresses similar to RFC 1918 addresses.

Multicast address are class D addresses, therefore there are 28 bit effective address space. However ethernet only allocates half of an OUI address space, which is 23 bits (01:00:5E:0xxx xxxx.xx.xx), for multicast. Therefore multiple multicast IP addresses can be mapped to the same ethernet MAC address.

IGMP v1 and IGMP v2

IGMP is used by hosts to report their group membership to the neighboring multicast routers. And IGMP Snooping and CGMP are used by the switches to determine the group membership of the switch ports.

* IGMP v1 Query-Response Process

1. ICMP Querier router periodically sends out membership query to All-Hosts address on the local subnet.
2. A host responds by sending membership report message to the group address it's a member of. And other hosts within the same group will suppress their report.

IGMP v1 doesn't have join message, it uses unsolicited membership report message to join a group. IGMP v1 doesn't have querier election process, the PIM or DVMRP designated router is automatically used as IGMP v1 querier router.

* IGMP v2 Leave Process

1. A host sends a leave group message to All-Routers address
2. The querier sends out a group-specific query to the group address.
3. If there is any other host left in the group, it will respond to the group-specific query.

IGMP v2 has Querier Election, Leave Group Message and Group-specific Query.

Multicast Routing Protocols

PIM, DVMRP, CBT and MOSPF are the multicast protocol used among the routers.

* Dense mode multicast routing protocols: DVMRP and PIM-DM.
* Sparse mode multicast routing protocols: PIM-SM and CBT.
* Link-State multicast routing protocols: MOSPF.

MOSPF is similar to Dense mode protocols. But instead of flooding and prunning multicast traffic, it floods out LSAs that identify the whereabouts of group members, i.e. receivers, in the network.

Cisco IOS does not support complete DVMRP, but PIM interoperates with DVMRP. And Cisco PIM routers will propagate DVMRP routes and use the DVMRP routing table for RPF.

Multicast RPF check is separate from the PIM, multicast routers may use unicast routing table, DVMRP multicast routing table or Multicast BGP routing table for RPF verification.

(S,G) is a source tree, also called Shortest Path Tree (SPT). It is used by Dense mode protocols.
(*,G) is a shared tree from the RP. It is used only by Sparse mode protocols.

PIM-DM Assert is similar to Spanning Tree's designated port election, it determines which PIM router on the LAN segment will be the forwarder for the incoming multicast traffic.

PIM DR (Designated Router) is different concept. It determines which PIM router on the LAN segment will be forwarding the join message to the RP to build the shared tree.

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