PIM-SM Tunnel Interfaces
This will be a brief article covering PIM tunnel interfaces. Tunnel interfaces are used by PIM to encapsulate/decapsulate PIM Register messages.
Have you noticed the following output on a PIM router after you configure the RP address?
R3#
*Sep 19 13:59:43.417: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to upThis interface is automatically created on a PIM router when the RP address is known and is reachable. This is a special unidirectional tunnel interface which is used to encapsulate multicast traffic in a PIM Register message and unicast it to the RP. This process allows the RP to discover the source and then create a (S, G) tree rooted at the source. This tunnel interface is only used when the router is acting as a FHR.
If you think about it, the fact that this is a tunnel interface makes sense. The packet always has a single destination, the unicast address of the RP. The tunnel also adds a second header to the multicast packet. You could say it is PIM-in-IPv4 multicast encapsulation.
The outer header is destined for 6.6.6.6. The encapsulated packet is the multicast traffic
You can see details about this tunnel interface using the following show commands:
Output packets increment for each PIM Register that is sent. The input packets should never increment because it is a unidirectional tunnel.
RP Tunnel Interfaces
The RP creates two tunnel interfaces. The first is the default PIM Register tunnel which is actually to itself. The second is a PIM Register decapsulation tunnel. This is used to receive the PIM Register messages and decapsulate them to extract the multicast packet and forward it down the shared tree (if it exists).
Interestingly, the input packet counter is not incrementing even though the RP has decapsulated Register packets.
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