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IP Mroute Expires Timer - PIM Dense Mode

Hi all,

please refer to the drawing attached below:

supprot.jpg

R1-R3: Cisco Router 1841

R2-R4: Cisco Router 3945

All the routers are configured with PIM Dense Mode and OSPF.

PC1 is generating multicast data flow x.x.x.x and PC2 PC3 receive that multicast. They send IGMP join packet to the routers (PC2 to R2 and PC3 to R4).

The strange behaviour that I noted is the follow:

I simulated a fault disconnecting SW1 (switch off): on R2 and R4 I observed that "Expires time" of multicast route ("show ip mroute") is setted to 3 minutes and started the countdown. I expected that after 3 minutes the multicast route disappeared from multicast routing table but that is not happened; because after a random time the timer during the countdownad restarts again from 3 minutes and the countdown starts again. After 3 minutes the "Expires time" arrives finally to zero and the multicast route disappear from routing table.

Someone could help to find out the event that restart the expires timer to 3 minutes?

Considering that:

- PC1 is the only device that send multicast packets

- PC2 and PC3 are only receiver, they just send IGMP joint group messages

- All routers are configured with PIM-DM without any static igmp configuration

- I runned "debug ip pim x.x.x.x" on R2 and R4 but PIM messages are not related with timer restarting. I noted that the timer was resetted to 3 minutes without receiving any PIM packets

- I runned "debug ip igmp x.x.x.x" on R2 and R4 but IGMP packets are not related with timer restarting. I noted that the timer was resetted to 3 minutes without receiving any IGMP packets.

I hope I have made myself clear with the explaination of that issue.

Thank you

Claudio Minnetti

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