switch cpu

Unanswered Question
Apr 27th, 2009

Hi all, Am I right in saying L2 switches still have a CPU ? how can multicast IGMP snooping affect the cpu ?

I have this problem too.
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Giuseppe Larosa Mon, 04/27/2009 - 07:22

Hello Carl,

yes L2 switches have a main cpu

igmp snooping doesn't affect switches with specialized HW ASIC chip for IGMP traffic analysis but older switches miss this ASIC chip.

For switches without ad hoc HW IGMP snooping causes an increase, also a huge increase in cpu usage

Hope to help


carl_townshend Mon, 04/27/2009 - 07:44

thanks for that, I had an odd switch on the network, a Nortel 460, IGMP was turned on for all vlans, this over the weekend bought down the who network, i did a capture and the switch was trying to perform Join requests, which inturn killed the cpu on the switch, so I gather on the cisco switches this would not of happend as its performed by a asic ?

Giuseppe Larosa Mon, 04/27/2009 - 08:32

Hello Carl,

yes it is likely that C2960 or similar have the ASIC to perform igmp snooping.

I don't remember an example of an older switch my guess is that C3524XL or 2924XL hadn't the ASIC and so it was better to use CGMP but this is Cisco proprietary.

about the nortel you should look at nortel website to find out if there is support in HW for IGMP snooping

Hope to help



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