Introduction

Protocol Independent Multicast - Dense Mode (PIM-DM) is a data-driven multicast routing protocol, which builds source-based multicast distribution trees that operate on the Flood-and-Prune principle. It requires unicast-reachability information, but does not depend on a specific unicast routing protocol.

PIM Dense Mode (PIM-DM) is a significantly less complex protocol than PIM Sparse Mode (PIM-SM). PIM-DM works on the principle that it is probable that any given multicast stream will have at least one downstream listener. PIM-DM is ideal where many hosts subscribe to receive multicast packets, so most of the PIM Routers receive and forward all multicast packets.

Where PIM-SM only forwards a multicast stream when requested, PIM-DM always floods any new multicast stream that arrives at the PIM Router and only stops flooding the multicast stream on a given link if it is explicitly told to, by receiving a Prune message from the downstream PIM Router.

PIM-DM does not include the concepts of Rendezvous Points, which are used in PIM-SM. PIMSM explicitly builds unidirectional shared trees rooted at a Rendezvous Point (RP) per group. PIM-DM implicitly builds shortest-path trees by flooding multicast traffic domain wide, then Prunes back branches of the tree where no receivers are available. As with PIM-SM, so does PIM-DM also use Reverse Path Forwarding (RPF) to stop loops for packet forwarding for PIM Routers receiving multicast packets.

Below is an example PIM-DM configuration routing multicast between Vlan 1 and 2 on an AlliedWare Plus switch:

Move into global configuration mode:

awplus#configure terminal

Activate multicast routing:

awplus(config)#ip multicast-routing

Move into Interface Config mode for the Vlan that needs to be routed:

awplus(config)#interface vlan1 (Using 1 for example purposes)

Activate the IGMP Querier:

awplus(config-if)#ip igmp

Activate the PIM-DM routing function:

awplus(config-if)#pim dense-mode

Move back to global configuration mode:

awplus(config-if)#exit

Move into Interface Config mode for the Vlan that needs to be routed:

awplus(config)#interface vlan2 (Using 2 for example purposes)

Activate the IGMP Querier:

awplus(config-if)#ip igmp

Activate the PIM-DM routing function:

awplus(config-if)#pim dense-mode

Move back to global configuration mode:

awplus(config-if)#exit

Move back to privilege mode:

awplus(config)#exit

Remember to save your configuration:

awplus#copy running-config startup-config

Useful PIM Dense Mode Show Commands

There is several commands available to show you the status of your PIM Dense mode deployment.

• show ip pim dense-mode interface - The output of this command shows us the IP address andVLAN of the interfaces on the
  switch which have PIM-DM mode configured, as well as the PIMVersion number (v2) and the PIM Mode—D (Dense).
• show ip pim dense-mode interface detail - Additional information provided for above command.
• show ip pim dense-mode mroute - Here we can see the PIM-DM multicast routing table entries.
• show ip pim dense-mode neighbor detail - Neighbor information can be viewed with this command.
• show ip pim dense-mode nexthop - This command shows the next-hops of the unicast routes to the Sources and Receivers in
  the network.