Re: Storm control in 3550 and 3560 switches

Other way around. Broadcasts are a SUBSET of multicasts.

At layer2, a multicast is defined by the I/G bit. The least significant
bit of the most significant byte of a MAC.

For example, all IP multicast at layer2 begins with 01-00-5E-xx-xx-xx.
01 is the most significant byte (left-most). In binary it's 0000 0001.
The 1 is the least-significant bit (right-most) of this byte. That's
the I/G bit. A 1 value indicates a GROUP address while a 0 value
indicates an INDIVIDUAL address (the I and the G).

A broadcast at layer2 is FF-FF-FF-FF-FF-FF. Or in binary, 1111 1111 in
the most significant byte. Being all-ones, it's a very specific notation.

Not all multicasts can be called broadcasts. But all broadcasts can
also be called multicasts (group addresses).

So if you have a storm control threshold of 10% for multicast, but 20%
for broadcast... Once you hit your 10% mutlicast mark, your broadcasts
will get throttled as well since they are part of that same group.

It's all about perspective (and a little history). ;)

HTH,

*Scott Morris*, CCIE/x4/ (R&S/ISP-Dial/Security/Service Provider) #4713,

JNCIE-M #153, JNCIS-ER, CISSP, et al.

JNCI-M, JNCI-ER

evil_at_ine.com

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Anantha Subramanian Natarajan wrote:
> Hi Darby,
>
> Thank you very much for the descriptive explanation on the topic.Not sure
> I understood the below caveat
>
> "If you enable broadcast and multicast traffic storm control, and broadcast
> traffic exceeds the level within a 1-second traffic storm control interval,
> traffic storm control drops all broadcast and multicast traffic until the
> end of the traffic storm control interval. "
>
> Is that mean that,if the broadcast traffic exceeds the threshold level,it
> will also drop multicast traffic,when the multicast didn't eceed the
> threshold defined on the later 2 interface ??/..Is it true for both 3550 and
> 3560 switches ?
>
> Thank you
>
> Regards
> Anantha Subramanian Natarajan
>
>
>
>
> On Mon, Sep 14, 2009 at 5:05 AM, Darby Weaver <darby.weaver_at_gmail.com>wrote:
>
>
>> Some caveats you may to take note of on Storm-Control:
>>
>> All Layer 2 Broadcasts are Multicasts.
>>
>> All Layer 2 Multicasts are *not* Broadcasts.
>>
>> Traffic storm control monitors the level of each traffic type for which you
>> enable traffic storm control in 1-second traffic storm control intervals.
>> Within an interval, when the ingress traffic for which traffic storm
>>
> control
>
>> is enabled reaches the traffic storm control level that is configured on
>>
> the
>
>> port, traffic storm control drops the traffic until the traffic storm
>> control interval ends.
>>
>> The following are examples of traffic storm control behavior:
>>
>> If you enable broadcast traffic storm control, and broadcast traffic
>> exceeds the level within a 1-second traffic storm control interval, traffic
>> storm control drops all broadcast traffic until the end of the traffic
>>
> storm
>
>> control interval.
>>
>> If you enable broadcast and multicast traffic storm control, and the
>> combined broadcast and multicast traffic exceeds the level within a
>>
> 1-second
>
>> traffic storm control interval, traffic storm control drops all broadcast
>> and multicast traffic until the end of the traffic storm control interval.
>>
>> If you enable broadcast and multicast traffic storm control, and
>> broadcast traffic exceeds the level within a 1-second traffic storm control
>> interval, traffic storm control drops all broadcast and multicast traffic
>> until the end of the traffic storm control interval.
>>
>> If you enable broadcast and multicast traffic storm control, and
>> multicast traffic exceeds the level within a 1-second traffic storm control
>> interval, traffic storm control drops all broadcast and multicast traffic
>> until the end of the traffic storm control interval.
>>
>> 1) One very important remark everybody seems to miss, is that storm-control
>> only works on INBOUND packets. It does not prevent a port from being
>> overwhelmed with broadcasts from the core or other access switches.
>>
>> 2) Watch out with stormcontrol on trunk ports! Includes any allowed or
>> non-allowed Vlan
>>
>> 3) The storm-control "drop filter" action will block on all VLANS.
>>
>> Finally:
>>
>> Storm control is supported only on physical interfaces; it is not supported
>> on EtherChannel port-channels or physical interfaces that are members of
>> port channels even though the command is available in the CLI. If a
>>
> physical
>
>> interface with storm control configured joins an EtherChannel, the storm
>> control configuration for the physical interface is removed from the
>>
> running
>
>> configuration.
>>
>>
>>
>>
>>
>> On Sun, Sep 13, 2009 at 8:01 PM, Anantha Subramanian Natarajan <
>> anantha.natarajan_at_gravitant.com> wrote:
>>
>>
>>> Hi All,
>>>
>>> My understanding on ,Multicast storm control on 3550 switch is as below
>>>
>>> "When the rate of multicast frames increased more than the specified
>>> rate
>>> on the Multicast storm-control configuration,it will block all multicast
>>> frames exceeded and also even all unicast/broadcast traffic is dropped
>>> until
>>> the multicast rate on the port is decreased than the threshold
>>> defined.Exceptions would be BPDU's and CDP frames".
>>>
>>> Is my above understanding right ?.If so ,my questions is
>>>
>>> Does the above statement is also true for 3560 switches.For some reason.I
>>> couldn't find a reference on the 3560 config guide,unless I missed it.
>>>
>>> Thank you for the assistance.
>>>
>>> Regards
>>> Anantha Subramanian Natarajan
>>>
>>>
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>>>
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>>>
>>>
>>>
>>>
>>>
>>>
>>>
>> --
>> Darby Weaver
>> Network Engineer
>>
>> 407-802-7394
>> darbyweaver_at_yahoo.com
>>
>
>
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Received on Mon Sep 14 2009 - 09:56:28 ART