Multicast and RPF Problem.

Unanswered Question
Dec 12th, 2008

We have a multicast source that sends out 4 streams. When I go to the recieving router and run the command below it get a high number of multicast drop. My video would play for about 5 minutes than hang. Is this drop related to my video hanging? What is the best way to troubleshoot this problem? Thanks

A26-LAN-R1#show ip mroute count

IP Multicast Statistics

270 routes using 501884 bytes of memory

24 groups, 10.25 average sources per group

Forwarding Counts: Pkt Count/Pkts per second/Avg Pkt Size/Kilobits per second

Other counts: Total/RPF failed/Other drops(OIF-null, rate-limit etc)

Group:, Source count: 1, Packets forwarded: 12196, Packets received: 24530

Source:, Forwarding: 12196/-14/1234/0, Other: 24530/0/12334****

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Giuseppe Larosa Sat, 12/13/2008 - 04:00

Hello David,

the show command tells the following:

for (S,G),

this is the only source seen

24530 packets have been received

but only 12196 have been forwarded

The OTHER counts says:

total 24530, dropped for RPF failure 0

dropped for other reasons 12334.

This includes OIF-null, rate-limit etc.

From this show I don't see an RPF problem.

you can verify if source passes the RPF with

show ip rpf

Have you implemented any form of multicast scoping, boundary or TTL limited ?

It looks like that or there is a form of rate-limit or is not accepting IGMP joins on interface


show ip igmp groups to verify also the age of groups.

If you see a reasonable age for group it narrows to scoping or rate-limiting issues.

Hope to help


davidhuynh5 Sat, 12/13/2008 - 07:14

Thanks G.

I don't have any rate-limit any of the configured items you mention. However, I'm a little confused about the packet forwarding and packet recieving.

If that output was from an SVI, where the multicast source ( was connected to, then should I have more packet forwarding and less receiving. Also, if multicast is udp, why am i getting recieved at all. Sorry, since new to multicasting.

Giuseppe Larosa Sat, 12/13/2008 - 11:38

Hello David,

no problems all of us are here to learn and share.

from the point of view of the multilayer switch SVI:

packets are received from the source via the multicast tree.

eventually they are forwarded out

the show is not specific of an interface it focuses on a multicast group and provides statistics for it.

Another important basic question is the type of IP multicast routing you have configured and achieved:

there are PIM sparse mode

PIM dense mode

in any case to have a multicast stream to be successfully forwarded from Vlan X to Vlan Y you need to enable ip pim on source vlan, destination vlan and if there routers in the middle in all transit links on the path from vlan x to vlan y

The basic difference between PIM dense mode and PIM sparse mode is:

PIM sparse mode requires explicit join to add an interface to the list of interfaces for a group (that is the OILIST)

PIM dense mode will forward the packets out of all multicast capable interfaces: downstream devices can signal they are not interested on the stream with an explicit Prune message for the group.

The pruned state is timed with a timer that can be of few minutes: after that the group is flooded again and if a new prune is received again forwarding stops.

so PIM DM uses a flood and prune method while PIM SM waits for an explicit join.

How have you configured multicast in your network ?

another aspect are parallel paths to the source.

If there are paralled paths to the source with equal unicast routing cost this can be a problem because RPF will choice only one interface as the one in which it expects to receive traffic from the source and if packets arrive on the other are dropped with cause RPF failure

This is not your case as I commented in my first post

To see what type of PIM is in use

show ip mroute multicast-group-address

to see Rendevous point mapping if any

show ip pim rp mapping

if you like you can post the output of these shows here.

Hope to help



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