Multicast Null Flag !!

Answered Question
Jul 9th, 2008

Hi every1, i was playing with PIM-SM, i am not going into the detail of my network but just showing you the output i want you guys to check.

R3#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

(*, 225.5.5.5), 00:07:55/00:03:28, RP 10.1.4.4, flags: SJC

Incoming interface: Serial0/1, RPF nbr 12.0.0.1

Outgoing interface list:

Ethernet1/0, Forward/Sparse, 00:07:55/00:03:28

(20.0.0.1, 225.5.5.5), 00:00:20/00:02:39, flags:

Incoming interface: Serial0/0, RPF nbr 0.0.0.0

Outgoing interface list:

Ethernet1/0, Forward/Sparse, 00:00:20/00:03:28

(*, 224.0.1.40), 00:07:59/00:03:27, RP 10.1.4.4, flags: SJCL

Incoming interface: Serial0/1, RPF nbr 12.0.0.1

Outgoing interface list:

Ethernet1/0, Forward/Sparse, 00:07:53/00:03:27

Serial0/0, Forward/Sparse, 00:07:59/00:01:57

Kindly look for this statement

(20.0.0.1, 225.5.5.5), 00:00:20/00:02:39, flags:

Why there is no flag set ?? let me tell you that R3 is connected to a multicast client directly on its lan interface and when i tested my multicast network, client was able to get it successfully, but i am confused why no flag is set ?

am i missing something here ???

Correct Answer by Harold Ritter about 8 years 7 months ago

Ovais,

You can use the mtrace command from the router connected to the receiver to find out to trace back to the source.

Regards,

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illusion_rox Wed, 07/09/2008 - 02:48

Guys, its done, i was missing a config :-), but it was an interesting observation, no flag, no rpf neighbour but still multicast was successful :)

Kevin Dorrell Wed, 07/09/2008 - 02:57

What exactly was missing from the config? (So I can add it in my experience bank.)

Kevin Dorrell

Luxembourg

illusion_rox Wed, 07/09/2008 - 19:21

Dear Kevin, actually on the SPT path, pim sparse wasnt enable, so thats why i was experiencing this issue, and 1 thing more, even after that i was seeing 2 flags against the same entry, "JT", can some 1 tell me how will you guys interpret this flag, T means that the router has switched over to SPT, whereas J means that its joining the SPT right ? how can they come both at the same time ?, i am really getting confused regarding the different flag combination, i referred to Beau famous book and no doubt its a great book but this area is really confusing me, does cisco gives any detail inside on what different combinations of flags mean ? ( any cisco doc ?? )

Kindly help me out in this.

Harold Ritter Wed, 07/09/2008 - 06:20

Ovais,

The mcast stream was simply forwarded down the shared tree as the source tree entry was imcomplete.

Regards,

illusion_rox Wed, 07/09/2008 - 19:22

Dear Hritter, can you tell me how to trace the multicast packet like we do for unicast packets ? i want to see that how it was going through the shared tree !

Thanks in advance and for clearing my confusion :)

Correct Answer
Harold Ritter Thu, 07/10/2008 - 09:57

Ovais,

You can use the mtrace command from the router connected to the receiver to find out to trace back to the source.

Regards,

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