IP Multicast S,G question.

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May 17th, 2007
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I am having a problem with multicast streams not flowing and have started troubleshooting.

On one of the routers the stream passes through I did a 'sh ip mroute'

and got this output:

ACN-RT7206-CORE#sh ip mroute

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

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

Timers: Uptime/Expires

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

(*,, 03:55:21/00:02:36, RP, flags: SF

Incoming interface: FastEthernet5/0, RPF nbr

Outgoing interface list:

GigabitEthernet0/3, Forward/Sparse, 03:55:21/00:02:36

(,, 00:02:51/00:00:38, flags:

Incoming interface: FastEthernet5/0, RPF nbr

Outgoing interface list:

GigabitEthernet0/3, Forward/Sparse, 00:02:51/00:02:36

(,, 00:02:58/00:00:31, flags: T

Incoming interface: GigabitEthernet0/3, RPF nbr

Outgoing interface list:

FastEthernet5/0, Forward/Sparse, 00:02:58/00:02:30


The source of the stream is which shows up in the list. But the other S,G entry that shows up, (, showing a router interface,, as the source.

ACN-RT7206-CORE#sh run int g0/3

Building configuration...

Current configuration : 271 bytes


interface GigabitEthernet0/3

description To ACN-6506B-CD-CR port g1/24

ip address

ip pim sparse-mode

ip route-cache flow

load-interval 30

duplex full

speed 1000

media-type gbic

negotiation auto


Is this possible? What would cause a router interface to be the 'S' in the S,G field?

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carenas123 Thu, 05/24/2007 - 05:21
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router receiving a source announcement from one peer, will forward it to its other peers. In this way, the source announcements can be flooded throughout a network of peers. When an RP receives a source announcement for a group with local interest, which means that someone has previously sent a (*,G)-join to this RP, it will send (S,G)-join towards the source S, building a SPT from the source in the other domain. Data received on the SPT can then be forwarded as usual.

jkeeffe Thu, 05/24/2007 - 05:58
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Is this then a temporary, short-lived S,G with a router interface as the source?


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