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What is the significance of the expire timer on the main heading of an (S,G) entry on a router in dense mode?

Sam Brynes
Beginner
Beginner

Let's say we have this sample output from a PIM DM router:

 

rtr-b>show ip mroute
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, C - Connected, L - Local, P - Pruned
file:///V|/[%20CISCO%20PRESS%20]/Ciscopress-Developing.Ip.Multicast.Networks/di10ch10.htm (18 of 30)09/12/2003 01:11:54
Using PIM Dense Mode
R - RP-bit set, F - Register flag, T - SPT-bit set, J - Join SPT
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 224.2.127.254), 00:00:12/00:00:00, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial0, Forward/Dense, 00:00:12/00:00:00
(128.9.160.43/32, 224.2.127.254), 00:00:12/00:02:48, flags: PT
Incoming interface: Serial0, RPF nbr 198.92.2.1
Outgoing interface list: Null

The "00:00:12" part shows how long the entry has been up. What does "00:02:48" mean? This is the expiry timer, used to remove the entry if it does not receive multicast traffic for this (S,G) entry.

 

But in this case, the outgoing interface list is null, which means that none of our downstream neighbors or any hosts on our connected interfaces wants it. This router sent an (S,G) prune message to its RPF neighbor (198.92.2.1), so obviously, we won't be receiving multicast traffic for this (S,G) entry.

 

Given this information, what does the timer "00:02"48" mean in this context? Is it a count down timer for the router to send a state refresh or another prune message for this (S,G) entry, provided that the outgoing interface list remains null?

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