How is a *,G multicast entry created, and how to clear it.

Unanswered Question
Oct 16th, 2008

I am troubleshooting a multicast problem we are having and am using Microsoft's mcast.exe to generate a multicast stream. the group address I am using is 234.10.10.10. I have PIM sparse-mode configed on all L3 interfaces and am using Anycast-RP with MSDP between two RP's.

First questions is: If I just start up the mcast.exe on the sending PC, and don't have a receiving client - no one requesting to join the group - does a *.G entry for 234.10.10.10 get built at all?

This output shows a *,G entry for 234.10.10.10, but no clients are requesting the group at this point. The client that did join the group has been shut down for over 24 hours.

ACN-RT7604A-MDF#sh ip mroute 234.10.10.10

IP Multicast Routing Table

Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,

L - Local, P - Pruned, R - RP-bit set, F - Register flag,

T - SPT-bit set, J - Join SPT, M - MSDP created entry,

X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,

U - URD, I - Received Source Specific Host Report,

Z - Multicast Tunnel, z - MDT-data group sender,

Y - Joined MDT-data group, y - Sending to MDT-data group

V - RD & Vector, v - Vector

Outgoing interface flags: H - Hardware switched, A - Assert winner

Timers: Uptime/Expires

Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 234.10.10.10), 1d20h/00:03:14, RP 164.72.239.42, flags: S

Incoming interface: GigabitEthernet2/0/1, RPF nbr 164.72.188.10, RPF-MFD

Outgoing interface list:

GigabitEthernet1/2, Forward/Sparse, 1d12h/00:03:14, H

Here is the 'sh ip pim rp' output:

ACN-RT7604A-MDF#sh ip pim rp

Group: 239.255.255.254, RP: 164.72.239.42, v2, uptime 21w5d, expires never

Group: 239.255.255.253, RP: 164.72.239.42, v2, uptime 21w5d, expires never

Group: 239.255.255.250, RP: 164.72.239.42, v2, uptime 20w0d, expires never

Group: 235.80.68.83, RP: 164.72.239.42, v2, uptime 21w5d, expires never

Group: 239.192.10.1, RP: 164.72.239.42, v2, uptime 21w5d, expires never

Group: 238.23.13.5, RP: 164.72.239.42, v2, uptime 1d17h, expires never

Group: 234.10.10.10, RP: 164.72.239.42, v2, uptime 1d20h, expires never

Group: 224.0.1.24, RP: 164.72.239.42, v2, uptime 21w5d, expires never

Group: 224.0.1.60, RP: 164.72.239.42, v2, uptime 21w5d, expires never

Group: 224.0.1.40, RP: 164.72.239.42, v2, uptime 1y16w, expires never

Second question. How do I clear a *,G entry for 234.10.10.10? I tried 'clear ip pim rp-mapping 234.10.10.10' but it still shows up.

I have this problem too.
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Giuseppe Larosa Thu, 10/16/2008 - 08:49

Hello Jim,

the (*,G) entry is normal in PIM Sparse Mode it represents the Shared Tree that is rooted at the RP.

This entry is built to join the shared tree.

When a real source is present a source based tree is built and can take advantage of the shared tree.

(*, 234.10.10.10), 1d20h/00:03:14, RP 164.72.239.42, flags: S

Incoming interface: GigabitEthernet2/0/1, RPF nbr 164.72.188.10, RPF-MFD

Outgoing interface list:

GigabitEthernet1/2, Forward/Sparse, 1d12h/00:03:14, H

RP is 164.72.239.42 the interface to reach it is GigabitEthernet2/0/1, the oilist contains only GigabitEthernet1/2, Forward/Sparse and multicast is performed in hardware (H)

All good news. don't try to clear it it normal behaviour.

Hope to help

Giuseppe

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