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 224.0.0.0/4 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 192.168.12.1
R1(config)#interface fastEthernet 0/0    
R1(config-if)#ip pim sparse-mode 
R2(config)#ip multicast-routing 
R2(config)#ip pim rp-address 192.168.12.1
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): 224.0.0.0/4, Static
    RP: 192.168.12.1 (?)
R2#show ip pim rp mapping 
PIM Group-to-RP Mappings

Group(s): 224.0.0.0/4, Static
    RP: 192.168.12.1 (?)

Both routers agree that R1 is the RP for the entire multicast group range 224.0.0.0/4. Let’s change it so that it only accepts multicast group 239.1.1.1:

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Forum Replies

  1. Hi Hans,

    192.168.12.1 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 1.1.1.1/32 for R1, 2.2.2.2/32 for R2 etc.

    Rene

  2. Hi Rene,

    What if I sourced the Multicast traffic from R1 Loopback, will it accept, for my lab its not accept and showing RPF failed or RP failed from debug, even though R2 has correc RPF.

    Regards
    Jam

  3. 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.

  4. I believe the picture should be updated using 192.168.12.1 for R1 and .2 for R2.

    Thank you,
    Stefanita

  5. Hi Stefanita,

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

    Rene

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