Multicasting - Auto RP

Unanswered Question
Aug 15th, 2008

Dear All,

I have a query regarding Auto-RP. As per my understanding we must configure sparse-dense mode on interfaces in case of Auto RP and if we don't want to configure Sparse-mode only then we have another option of configuring Auto-RP Listner so that traffic for and groups can be flooded to all mutlicast routers.

Is there any scenario possible in which we will configure sparse-mode only without Auto-RP listner and multicast can also run properly ??



  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Giuseppe Larosa Sat, 08/16/2008 - 00:31

Hello Mujeeb,

the answer is yes.

PIM version 2 provides a more advanced mechanism of propagation of RPs to groups mappings that is called bootstrap protocol.

Boostrap messages are sent hop by hop using the PIM so using bootstrap you don't care of groups and anymore and you don't need to use sparse-dense mode on the interfaces.

Bootstrap advantages include:

election of the bootstrap router and bootstrap router redundancy, auto-rp mapping agent can't be changed dynamically and is not elected.

Mappings messages provide more complete information that allows every router to perform the RP to groups association locally instead of providing only the results of the mapping-agent action.

Hope to help


rmujeeb81 Sat, 08/16/2008 - 01:32


Thanks for the response. Apart from BSR if we use Auto RP and let suppose there are only 3 routers in multicast domain R1,R2,R3. R1 & R3 are spokes and R2 is hub and acting as RP and Mapping Agent. R3 is having some multicast group joined e.g and R1 is trying to ping this group. All interfaces of all devices are in sparse-mode only.

My question is that R1 and R3 are directly connected with RP/Mapping Agent so do we still need to configure sparse-dense mode instead of sparse-mode.



Giuseppe Larosa Sat, 08/16/2008 - 06:53

Hello Mujeeb,

if you use auto RP you need to solve the following problem:

how can routers learn what are the RPs to groups mappings, if this info travels inside specific multicast groups and that need to be multicast forwarded in the network ?

This is the reason for using sparse-dense mode on interfaces: they allow the groups and to be propagated using dense-mode. Once for these groups have been built their spanning-tree all the info can flow over them.

Being directly connected to the mapping agent/RP does not provide any advantage.

In a very simple scenario you can solve by using manual mapping instead of auto-RP by defining on R1 and R3 that a specific loopback of R2 is the RP address.

You could think to use manual mapping with an ACL that defines only groups and to be manually mapped.

Then all other groups mappings are dynamically learnt from auto-RP messages.

Hope to help



This Discussion