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Multicast Auto-RP Notes

Multicast Auto-RP

 – Cisco proprietary

 – Uses a Mapping Agent (MA) to distribute RP information

 – RP informs the MA about themselves

     -> Announce message

 – MA sends a Discovery message to all routes

 – Announce message are multicast messages sent by RPs to the destination

     ->, UDP port 496

     -> Only MAs join the group and will receive the message

 – Discovery messages are multicast messages sentby the MA to

     ->, UDP port 496

     -> All routers join this group and will receive the message

 – If multiple RPs exist, only the higher IP address RP information is sent by the MA

 – The election of a specific RP is done only if all RPs are claiming to service the same group range


 – If RPs are claiming the service groups, then MA sends the information of all RPs to everyone

e.g. –

R2 -> –

R3 -> –

     -> Both RPs will be used

Scenario – R2 ( -> –

R3 ( -> –

     -> Overlapping range

          -> –

          -> R3 will be used for the RP because of the higher IP address

If multiple RPs are claiming the same range with different prefix-lengths, then the longest matching prefix will be used

Configuring the RP

ip pim send-rp-announce <int> scope <ttl> [group-list <acl>] [interval <sec>]

 – The interval default is 60 seconds

 – If group-list is not definded, the RP will claim to serve all multicast IP addresses

Configuring the MA

ip pim send-rp-discovery <int> scope <ttl> [interval <sec>]


 – The interval default is 60 seconds

R2(config)# access-list 1 permit

R3(config)# access-list 1 permit

 access-list 1 permit

     -> This ACL covers the entire multicast IP address range

On any router

sh ip pim rp mapping

Configuring Auto-RP

 1. Activate multicast routing

 2. Configure PIM sparse mode on all interfaces

 3. Configure RP and MA

Join messages have to be sent to

 – Accounce ->

 – Discovery –

 – Problem

     -> The router cannot join without knowing where to send the join messages

     -> The links are pruned

     -> This means that announce and discovery messages cannot be sent by default

 – Solution

     -> Send messages in dense mode and flood the messages

          -> Change all ports to sparse-dense-mode

                    – or –

          -> Use sparse-mode and configure all routers with ip pim autorp listener

               -> This commands converts the annouce and discovery groups to dense mode

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