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New Member

High Number of Multicasts

Can anyone help with the following:

I am realitivley new to Cisco products.I work for an organisation that has a fully switched network of about 75+ sites. all devices are Catalyst 6000, 4000 and 3000. We have been experiencing performance problems and noticed that there is multicast traffic appearing all over the network.

I realise that by default routers do not forward multicasts. However, what can cause a high number of multicasts on LAN segments. As far as I am aware we have no applications that use multicasts.

What effect can a high number of multicats have on the network.

Are there any mainstream applications that use multicasting.


Re: High Number of Multicasts

Multicasts are not necessarily bad things to have, they are muc friendlier than broadcasts. OSPF and RIPv2 are examples of routing protocols that use multicast to communicate. What would be nice is to put a sniffer onto your network and see exactly what network addresses are being used for multicasting. By looking at the addresses you should be able to determine what is causing this another thing that may be showing up is BPDU traffic. Basically this is spanning tree traffic. Without more information it's difficult to help out on this one. If you get more information please feel free to post again.

New Member

Re: High Number of Multicasts

I am having a similiar problem, My 6509 started to broadcast until system halt.

Re: High Number of Multicasts

You can try "show multicast group" CLI command on your Cat4000, e.g., to see which multicast groups are present in your network.

One big source of multicasts I was dealing with was Symantec Ghost software.

To decrease the number of multicasts injected to your network from a particular port use

port storm-control multicast action filter

port storm-control multicast threshold rising xxx falling yyy

on your Cat3500 port, e.g.


Milan Kulik

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