Multicast Bidirectional PIM

(Rene Molenaar) #1

This topic is to discuss the following lesson:

(system) #2

Hi Rene

First of all thanks a lot or sharing your knowledge. Your way of explanation is very simple & concise. Before start learning any particular topics ya concept, i check ur blog posting includes that topic or not. Jst now i find, Bi Directional PIM. Its so nice to learn from ur blogs. Appreciate the efforts. Thanks a lotzzzzzz…

(system) #3

While labbing the same topology, i found something diff from ur configuration. Here it is:

  1. “ip pim bidir-enable” should be enable on R4 & R5 as well.
  2. “ip pim rp-address bidir” should be configured on R4 & R5 as well.

Then only i am getting the same output as u mention in your most. Let me know i have done mistakes anywhere ya i understood it wrongly.

1 Like
(system) #4


Thanks for the nice article.

(Paul B) #5

Shouldn’t the second-to-last R3 output only show f0/1 in the outgoing interface list? Since R5 is the only receiver?

(Rene Molenaar) #6

Hi Paul,

You mean this output?

(*,, 00:04:07/00:03:19, RP, flags: BC
  Bidir-Upstream: FastEthernet0/0, RPF nbr
  Outgoing interface list:
    FastEthernet0/0, Bidir-Upstream/Sparse, 00:04:07/00:00:00
    FastEthernet0/1, Forward/Sparse, 00:04:07/00:03:19


(Michael M) #7

Hi Rene,


Question on this part of the lesson: “to register sources to the RP. Each source is able to start sending to the source whenever they want.” Do you mean to say that the source can start sending to the RP whenever they want? Because there is no registration messages between the source and the RP multicast flows just continue until the source starts sending - Do I have this correct?

(Rene Molenaar) #8

Hi Michael,

With “normal” PIM the designated router on the segment will encapsulate the first multicast data packet in a register message and forwards it to the RP. When the RP doesn’t want to receive this traffic, it sends a register stop message back to the designated router.

It seems you understood it correctly. Any source can just start sending multicast traffic to the RP. There are no registration (stop) messages so it’s just being forwarded.


(Junior M) #9

Hi Rene,

I really enjoy the way you teach and the content is very well organized.
I have a question regarding PIM Bidirectional.
For have in all Pim enabled Router the *,G to the RP on downstream Router, we need to enable globally the pim bidirectional.
What if i have on third party router that doesnt’ support pim bidirectional? is it will be enable to from *,G to the RP?

(Rene Molenaar) #10

Hello Junior,

Today, it’s unlikely to find a network with multicast routers that don’t support bidirectional PIM. Back in 2007 however (when bidirectional PIM became a standard), most multicast routers didn’t support it so you needed something to make it compatible.

In the PIM hello packets, there’s an option that indicates whether the router supports bidirectional PIM or not. When the routers elect the designated forwarder, all routers on the subnet have to support bidirectional PIM. If one router doesn’t support, bidirectional PIM is disabled on that subnet.


(Kenneth G) #11

Hi Rene,

I do not understand based on the last output shown below
(Now we see FastEthenet0/0 as the Bidir-Upstream interface and FastEthernet0/1 in the outgoing list. This is something different than what you have seen before right?)

Isn’t your earlier output already showing FastEthenet0/0 as the Bidir-Upstream interface and FastEthernet0/1 in the outgoing list?

R3#show ip mroute    
(*,, 00:09:39/00:03:10, RP, flags: BC
  **Bidir-Upstream: FastEthernet0/0**, RPF nbr
  **Outgoing interface list:**
     **FastEthernet0/0, Bidir-Upstream/Sparse**, 00:09:39/00:00:00
    **FastEthernet0/1, Forward/Sparse**, 00:09:39/00:02:42


(Kenneth G) #12

Earlier output below:

R3#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

(*,, 00:04:07/00:03:19, RP, flags: BC
  **Bidir-Upstream: FastEthernet0/0**, RPF nbr
  **Outgoing interface list**:
    **FastEthernet0/0, Bidir-Upstream/Sparse**, 00:04:07/00:00:00
    **FastEthernet0/1, Forward/Sparse**, 00:04:07/00:03:19

Above you see that the FastEthernet0/0 interface is the upstream (towards the RP) and FastEthernet0/1 is the outgoing interface (towards R5). Now we will configure R4 to join the multicast group as well so that they can communicate with each other:

(Kenneth G) #13

Hi System,

I did the same below for the lab to work too. Let me know if you have found the answers to your query. Thanks.

“ip pim bidir-enable” should be enable on R4 & R5 as well.
“ip pim rp-address bidir” should be configured on R4 & R5 as well.
(Lazaros Agapides) #14

Hello Kenneth

You are correct in that the output was the same as that shown earlier in the lab. Of course, this is indeed different from the output shown in R2 which is shown just previously. I will ask Rene to clarify his statement.

Thanks for the heads up!