Broadcast Storm Control

[nitass]

Hi everybody,

I’m suspected about broadcast storm control feature on switch. Could anyone please advice me?

1. When the broadcast storm control is triggered, can normal data packets (not broadcast packets) pass the switch?

2. If the network looping is occurred at unmanaged switch that doesn’t support spanning tree protocol and it connects to the managed switch that broadcast storm control is turned on, does it help this issue?

Managed switch

|

|

Unmanaged switch

||

\/<--- network looping

Thanks for advance,

Nitass

[ankurbhasin]

Hi Nitass,

When the broadcast strom is triggered it will allow the normal traffic whic is a unicast or multicast traffic.

You can define a strom control for broadcast, multicast as well as unicast traffic.

Have a look at this link for strom control and how to cofnigure it

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12119ea1/2950scg/swtrafc.htm#1063295

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12120ea2/3550scg/swtrafc.htm#wp1174641

HTH, if yes please rate the post.

Ankur

[arvindchari]

1. Unicast packets and multicast packets are not affected when u enable broadcast storm control. Multicast packets will be affected only if you enable multicast storm control on the switchport.

2. I have no experience in a setup such as this but the behavior of the storm-control broadcast level command suggests that the switch port will drop all broadcasts headed through the port (in both directions) for a specified period of time.

This however, still does not stop the source of the broadcast (i.e. the multiple links running to the un managed switch) so I would presume that the broadcasts might die down for a small period of time but they will resurface as the unmanaged switch would continue generating broadcast packets.

Thus the port on the managed switch would come back to normal state, only to go back into broadcast storm control state and stop all broadcasts all over again.

HTH

Please rate posts that help.

Regards

Arvind

[nitass]

Hi,

Thanks for all replies.

About the 2nd question, does it affect to other switch ports on managed switch? I mean they can work properly or not?

Thanks for advance,

Nitass

[ankurbhasin]

Hi Nitass,

It will not affect any other ports on managed switch, as strom control is applied on interface level so once you apply the broadcast strom control on the interface connected to the unmanaged switch any broadcast crossing the threshold coming from unmanaged switch will be dropped at that interface only and no other ports will be affected on managed switch,

HTH, if yes please rate the post.

Ankur

[arvindchari]

As Ankur has mentioned, other ports on the switch will not be affected but due to the way layer 2 technologies work, you might have serious issues relaying traffic to hosts.

For example, if the switch recieves an arp request for the mac address of the default gateway and does not have an entry in its CAM table, it usually broadcasts the frame on all switchports.

This might be blocked by a port in the broadcast storm control state and result in delays at the host end. In your case if the host was located on the unmanaged switch, he would never hear the broadcast frame and never be able to communicate with another host.

This is just one of several possible scenarios and you can save yourself a lot of trouble if you can disable or take down that redundant link on the unmanaged switch.

HTH

Please rate posts that help.

Regards

Arvind

[nitass]

Hi,

Thanks for all replies.

Do all of Catalyst switches have storm control for broadcast, multicast and unicast functionality? Does it depend on switch model?

And do you know these functions are standard for every switch brands?

Thanks for advance,

Nitass

[ankurbhasin]

Hi Nitass,

Yes nearly all the catalyst support this feature.

Like 2950,3550,3560,3750,4k and 6k support.

Not sure if other switch brand support it or not.

HTH, if yes please rate the post.

Ankur

[nitass]

Thank you very much.

Have a nice day,

Nitass