When we configure PIM on our routers, we will establish PIM neighbor adjacencies, and the PIM hello messages are also used to elect a designated router for each multi-access network. The DR is the router that will forward the PIM join message from the receiver to the RP (rendezvous point).
Since the DR is used to forward PIM join messages to the RP, it doesn’t do much good for multicast dense mode where we don’t have an RP. The only exception is when you use IGMPv1…in that case, the PIM DR will work as the IGMP query router because IGMPv1 doesn’t have a query router election.
Let’s take a look at the following topology to see how the DR works:
Above, we see a small network with 4 routers. R1 is our RP, and R4 is the receiver. As you can see, R2, R3, and R4 are connected to the same multi-access network (switch). When R4 sends a PIM join message, both R2 and R3 would receive it and forward it to R1. This would mean that we have 2 multicast streams which results in duplicate packets and wasted bandwidth.
To avoid this problem, we will elect the DR. R2 or R3 will become the designated router, and only one of them will forward the PIM join message to our RP.
Let’s configure this small network and take a close look at how the DR works:
R1(config)#ip multicast-routing R1(config)#ip pim rp-address 184.108.40.206 R1(config)#interface loopback 0 R1(config-if)#ip pim sparse-mode R1(config)#interface fastEthernet 0/0 R1(config-if)#ip pim sparse-mode R1(config)#interface fastEthernet 0/1 R1(config-if)#ip pim sparse-mode
We will use the loopback interface on R1 to advertise as the RP.
R2(config)#ip multicast-routing R2(config)#ip pim rp-address 220.127.116.11 R2(config)#interface fastEthernet 0/0 R2(config-if)#ip pim sparse-mode R2(config)#interface fastEthernet 0/1 R2(config-if)#ip pim sparse-mode
R3(config)#ip multicast-routing R3(config)#ip pim rp-address 18.104.22.168 R3(config)#interface fastEthernet 0/0 R3(config-if)#ip pim sparse-mode R3(config)#interface fastEthernet 0/1 R3(config-if)#ip pim sparse-mode
R2 and R3 are configured for sparse mode and a static RP, R4 is only a receiver, so we don’t need PIM.
R2#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 192.168.12.1 FastEthernet0/0 01:24:25/00:01:28 v2 1 / S 192.168.234.3 FastEthernet0/1 01:24:29/00:01:26 v2 1 / DR S
R3 has been elected as the Designated router on this segment. Why? because by default, the highest IP address will determine who becomes the PIM DR. Now let’s enable a debug to see what the designated router really does for us:
R2#debug ip pim PIM debugging is on
R3#debug ip pim PIM debugging is on
We will use debug ip pim on R2 and R3. Now we will join a multicast group on R4:
R4(config)#interface fastEthernet 0/0 R4(config-if)#ip igmp join-group 22.214.171.124
R4 will join multicast group 126.96.36.199. Now let’s see what R2 and R3 think of this:
R2# Check RP 188.8.131.52 into the (*, 184.108.40.206) entry
Above, you see that R2 doesn’t do much with it, it does add 220.127.116.11 as the RP for multicast group 18.104.22.168. You can see it here:
R2#show ip mroute 22.214.171.124 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 (*, 126.96.36.199), 00:11:38/00:02:19, RP 188.8.131.52, flags: SP Incoming interface: FastEthernet0/0, RPF nbr 192.168.12.1 Outgoing interface list: Null
Above, you see that it created an entry for the 184.108.40.206 group address with 220.127.116.11 as the RP. Now let’s take a look at R3: