Testing multicast routing

Unanswered Question
Feb 23rd, 2009

What's the easiest way to test multicast routing to make sure multicasts are getting from Point A to Point B on routers/switches?

I have this problem too.
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Jon Marshall Mon, 02/23/2009 - 16:29


You can use

1) sh ip mroute - but this will only show pim dense mode by default. If you are using pim sparse mode and no client has requested the multicast stream then the command will not show any entres for that multicast group.

2) Easiest way i have found to test mutlicast is to use VLC which is available at videolan.org where you can setup a sender and receiver of multicast traffic.


ralphcarter Mon, 02/23/2009 - 16:39

You can generate a multicast stream using your switches on one end.

# ip sla 1

# udp-echo 12345 source-ip x.x.x.x control disable

# frequency 2

# timeout 0

# ip sla schedule 1 start-time now

Then on other end, join the multicast group

# int xxx

# ip igmp join-group

As Jon states with #1, if you use pim dense it will flood and you will see in mroute table.

I didnt know about videolan, will check it out.

mattkaya56 Mon, 02/23/2009 - 16:49

Hello Jon,

What configuration changes would you have to make on the router(s)? Can you elaborate please.


Jon Marshall Mon, 02/23/2009 - 16:54


What do you mean ? Are you talking about setting up multicast ?

I was assuming the OP had setup multicast and just wanted to test it was working.


mattkaya56 Mon, 02/23/2009 - 17:10

Yes Jon,

Any sample configuration that I can use as a basis. Looks very interesting.



Jon Marshall Mon, 02/23/2009 - 17:18


To setup multicasting across vlans involves only a few steps

1) enable multicast routing ie.

switch(config)# ip multicast-routing

2) enable multicast under each L3 interface that you have either

a) a multicast source OR

b) a multicast receiver


int vlan 20

ip pim dense-mode

Note that you can use either sparse mode or dense mode. If you use sparse mode additional config is needed but this is usually the recommended mode to use. To test basic multicast on a L3 switch LAN dense-mode is fine. Across a WAN you would definitely want to look at sparse-mode altho sparse-mode is also applicable to the LAN.

You then need a multicast application. As i said VLC is a good test tool. It allows you to stream an mpg across the network with multicast and then on a client you can run another copy of VLC to pick it up.

If you do use VLC and you are using multicast across vlans then make sure you set the TTL to more than 1. Each L3 hop will reduce the multicast TTL by 1.

IGMP which is used by hosts to register their interest in a multicast stream is enabled by default when you configure "ip pim sparse|dense mode" under the L3 vlan interface.

If you are not multicasting across vlans but merely within vlans then you may need to look at IGMP snooping if your switch supports it.

There is a fair bit of info above. Best thing to do is try and setup multicast in a lab environment with something like VLC and then come back to these forums if it isn't making sense :-)


pkurdziel Mon, 02/23/2009 - 17:51

I would use ip igmp join-group 224.x.x.x and ping then ping 224.x.x.x from the other end of the network.

I troubleshoot with:

*** (ping, sh ip mroute, sh ip rpf)

sh ip pim nei

sh ip pim int


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