How to avoid flooding with broadcaste/multicaste frame

Unanswered Question
Jan 20th, 2009

How to avoid flooding with broadcaste/multicaste frame

Scenario: I would to distribute ghost image from a server in server vlan A to another end devices in same vlan B but in different Cisco switch. Do I have to be aware flooding issue in my network and in the case how can I avoid? Tanks in advance.


I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Giuseppe Larosa Tue, 01/20/2009 - 01:16

Hello Shanryar,

as explained in this recent thread

you need to enable :

ip multicast routing


ip pim sparse-dense mode

on L3 SVI interfaces Vlan A and Vlan B (or router subinterfaces)

on L2 only switches you need to enable ip igmp snooping

verify with

sh ip igmp snooping

if it is enabled on L2 switches

Hope to help


sfanayei Tue, 01/20/2009 - 04:29

Hi Giuseppe

Many tanks for your fast reply. Can you tell too that how I can to monitore the process?

I appreciat!


Giuseppe Larosa Tue, 01/20/2009 - 04:42

Hello Shanryar,

on the router L3 switch use

sh ip igmp groups

look at the time the group is joined

sh ip mroute multicast-group-address

check that the oilist contains Vlan A and/or Vlan B

on the L2 switch

check if IGMP is enabled with:

sh ip igmp snooping

you can see the multicast addresses in

sh mac-address-table int type x/y

there is a second section after unicast MAC addresses

Hope to help


mariov652 Tue, 01/20/2009 - 04:52


Giuseppe helped me out earlier with that post he recommended, so I'll give him a break and try to answer your question.

You should be able to check that IGMP snooping is active on the switch by typing "sh ip igmp snooping"

This gives you the IGMP snooping global settings as well as individual settings per vlan.

An easy way to check if broadcasts are only forwarded to the ports you want is to use a network sniffer (such as wireshark) on 2 or more systems in the same VLAN.

Start by monitoring both systems without the multicast requestor application running.

Then, on one system, start the application (and hence the multicast requests) - You should see multicast broadcasts only to that system and not the other.

I hope that helps.


This Discussion