Multicasting

PIM Modes

In actuality, PIM refers to a family of very similar routing protocols, but which each operate in a different mode. These are:

PIM Dense Mode (PIM-DM) (RFC 3973)
PIM Sparse Mode (PIM-SM) (RFC 4601)
Bidirectional PIM (BIDIR-PIM) (RFC 5015)

Router# show ip mroute 
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
   L - Local, P - Pruned, R - RP-bit set, F - Register flag,
   T - SPT-bit set, J - Join SPT, M - MSDP created entry,
   X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
   U - URD, I - Received Source Specific Host Report,
   Z - Multicast Tunnel, z - MDT-data group sender,
   Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
 Timers: Uptime/Expires
 Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 224.0.1.40), 00:07:04/00:02:49, RP 2.2.2.2, flags: SJPL
  Incoming interface: FastEthernet0/0, RPF nbr 10.0.12.2
  Outgoing interface list: Null

The first two steps are accomplished with single commands in global configuration on all routers in the multicast domain:

 R1(config)# ip multicast-routing
 R1(config)# ip pim rp-address 172.16.34.1

Note that there exist other means of configuring RP routers, namely Cisco's proprietary Auto-RP and PIMv2's Bootstrap Router (BSR) methods. In our example, only manual configuration is used.

PIM is enabled per interface:

 R1(config)# interface f0/0
 R1(config-if)# ip pim sparse-mode

Believe it or not, this is all the configuration necessary to get a bare bones multicast network up and running. After enabling PIM, routers will form adjacencies with other PIM routers and multicast routes will be exchanged:

R1# show ip pim neighbor
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
  S - State Refresh Capable
Neighbor          Interface                Uptime/Expires    Ver   DR
Address                                                            Prio/Mode
10.0.12.2         FastEthernet0/0          00:06:39/00:01:30 v2    1 / DR S
10.0.14.4         FastEthernet0/1          00:06:40/00:01:30 v2    1 / DR S