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New Member

broadcast and unknown unicast packets

Hi all,

When the network looping occurs, what the most packets will be generated? Broadcast or unknown unicast packets?

If I want to control the number of unknown unicast packets, which storm control should be configured? Broadcast or unicast storm control?

Thanks a lot,

Nitass

9 REPLIES
Purple

Re: broadcast and unknown unicast packets

Depending on your situation, it could be either. However, broadcast storms are more likely. It is safe to configure broadcast storm control in most situations.

Hope that helps - pls rate the post if it does.

Pareshc

New Member

Re: broadcast and unknown unicast packets

Thanks for reply. I understood the broadcast packet and unknown unicast packet are different; broadcast packet is for ff.ff.ff.ff.ff.ff destination mac address but unknown unicast packet isn’t. Is it right?

And if I want to control the number of unknown unicast packets, which storm control should be configured? It should be unicast storm control, doesn’t it?

Thanks a lot,

Nitass

Bronze

Re: broadcast and unknown unicast packets

A network loop occurs primarily due to broadcast packets or unicast packets that are sent out of multiple interfaces to the same destination device.

for e.g if you had

PC1

|

Switch 1

| |

Switch 2

|

PC2

and you somehow assume that PC1 knew the exact mac address of pc2 and sent it a unicast frame, even then because STP is not running, it would cause a broadcast storm. This would be a unicast broadcast storm.

Broadcast storm control will only control packets that are designated as broadcast i.e. all 1's. If you suspect the storm is being caused by unicast packets you may have to enable unicast control.

HTH

Please rate posts that help.

Regards

Arvind

Purple

Re: broadcast and unknown unicast packets

That is correct - if your problem is with unicast packets, broadcast storm control is not going to help you.

Hope that helps - pls rate the post if it does.

Paresh

New Member

Re: broadcast and unknown unicast packets

Thanks for all replies.

Could you please correct me about the unknown unicast packet?

I understood that it’s a unicast packet which switch doesn’t have MAC address in CAM table. So the switch will send it out to every interface except the incoming. Is it correct?

Thanks a lot,

Nitass

New Member

Re: broadcast and unknown unicast packets

Sorry I forgot one more question.

And this unknown unicast packet is controlled by unicast storm control, not broadcast storm control. Is it right?

Thanks a lot,

Nitass

Re: broadcast and unknown unicast packets

It is obvious that broadcast control will have little effect on an unknown unicast.

http://www.cisco.com/en/US/products/hw/switches/ps5213/products_configuration_guide_chapter09186a00801ce1e8.html

Regards,

Leo

Re: broadcast and unknown unicast packets

This is correct. The switch will learn where a mac adress resides when it sees the adress as source of an ethernet datagram. Until then, the switch has no clue to where the adress is located and hence the packet is flooded to ensure delivery to the destination.

Regards,

Leo

New Member

Re: broadcast and unknown unicast packets

Thanks for reply.

And the unknown unicast packet is also controlled by unicast storm control, not broadcast storm control, isn’t it?

Thanks a lot,

Nitass

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