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PIM sparse mode registration unicast packet source address

Multicast source connects to LAN port of both routers,

with an 819H the PIM registration packet tp the RP  uses the GRE tunnel interface address for registration

with a 1921 with no GRE tunnel the LAN Interface address is used for registration


any pointers to clarify why this is the case.

VIP Purple

Hello Whats your question?



What type of method of RP have you specified? - Anything other than Static RP requires a mapping agent to be able to list the  best currently available RP for the pim routers to join




Please don't forget to rate any posts that have been helpful. Thanks.
Community Member

Static RP with anycast, there

Static RP with anycast, there are 4 RP sitting in the Core

just for good measure there are 2 frame relay pvcs , one for each multicast stream.

However only static default route is used and both sources will use the same PVC for registration,

so i also get asymetric routing  for the RP response for one of the multicast streams, which is is a pain as there are firewalls in the way as well.



Hall of Fame Super Silver

Hello R-godden,my

Hello R-godden,

my understanding is that there is a configured GRE tunnel between the 819H router and the RP, because a non multicast IP network is in the middle


If so, the packet sent out the WAN interface of the 819H should be:

A GRE packet that contains the register message, that is another GRE packet that contains the (S,G) multicast frame.


The external GRE header will be like the one you have seen, but another GRE header should be inside it with source address the LAN interface and destination address the RP address.


Have you got packet captures or you are using ACLs to check the register message format ?

A packet capture would clarify what the 819H is doing.


The C1921 instead has a multicast capable network between itself and the RP so its register message contains only one GRE header not two nested.

In any case the RP would be able to build the source based tree towards the source because the original (S,G) packet is decapsulated at the RP.



Hope to help







Community Member

Thanks for the replies. The

Thanks for the replies.


The question is what source IP address should the first hop router use for the PIM register.

Just a bit more info :

The 1921 has 2 multicast sources on different LAN interfaces,

1 st source used the outgoing Frame Relay Interface address as the source address for the RP registration.

2nd source used the LAN interface address as the source address for the RP registration.


I can understand using the outgoing interface being used as the source IP address for the registration packet but need confirmation that this should always be case, or that the router just chooses randomly .

What logic is used to determine the source address to be used.






Community Member

PIM registration , how long

PIM registration , how long should the first hop router take to register source with RP


1. c800-universalk9-mz.SPA.152-4.M3.bin
This takes 13 minutes for the first hop router the 819H to send a rego packet to the RP
2. upgrade ios to c800-universalk9-mz.SPA.152-4.M5.bin
and soft reload again it takes 13 mins for rego packet to be sent from first hop router to RP
3. powered off for 30 seconds c800-universalk9-mz.SPA.152-4.M5.bin
again it takes 13 mins for rego packet to be sent from first hop router to RP
4. powered off for 5 minutes c800-universalk9-mz.SPA.152-4.M5.bin, instantly comes back after 2-3 mins of the reload
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