Multicast PIM Accept RP

PIM Accept RP is a security feature on Cisco IOS routers that prevents unwanted rendezvous points or multicast groups to become activate in the PIM sparse mode domain. By default a RP will accept all multicast groups in the range (the entire class D range) but if we want we can configure our router to allow only PIM join/prune messages towards the groups that we want.

Let me demonstrate this feature using a very simple topology:

R1 R2 Fastethernet Interfaces

Only 2 routers, R1 will be our rendezvous point. Let’s configure this network so that PIM sparse mode is enabled and R1 becomes the RP:

R1(config)#ip multicast-routing          
R1(config)#ip pim rp-address
R1(config)#interface fastEthernet 0/0    
R1(config-if)#ip pim sparse-mode 
R2(config)#ip multicast-routing 
R2(config)#ip pim rp-address
R2(config)#interface fastEthernet 0/0
R2(config-if)#ip pim sparse-mode 

This is how we enable multicast routing, configure R1 as the RP and enable sparse mode. Let’s take a look what multicast groups R1 will serve:

R1#show ip pim rp mapping 
PIM Group-to-RP Mappings

Group(s):, Static
    RP: (?)
R2#show ip pim rp mapping 
PIM Group-to-RP Mappings

Group(s):, Static
    RP: (?)

Both routers agree that R1 is the RP for the entire multicast group range Let’s change it so that it only accepts multicast group

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Forum Replies

  1. Hi Hans, is the IP address of R1. I usually use subnets / IP addresses like this:

    192.168.XY.X where X = R1 and Y = R2.

    For loopbacks I use for R1, for R2 etc.


  2. Hi Jam,

    It doesn’t matter if you use a loopback or “regular” interface as the source as long the RPF check is correct. It’s difficult to tell what is wrong without seeing your config.

  3. Hi Stefanita,

    That’s right, thanks for letting us know. Just fixed this.


  4. We will also need to allow address, as this is used for RP.
    If not allowed below log is noticed.

    *Mar  1 01:41:51.139: %PIM-6-INVALID_RP_JOIN: Received (*, Join from for invalid RP

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