Re: ehter channel issue...

Hi Austin,

Perhaps the question should be which port it is sent on, since cabling could
also have been crossed between the switches.

The etherchannel is a logical layer 2 link, so uses a single mac address for
BPDUs. The destination mac address of STP is a multicast mac, so will not
be learned on any particular physical port of the channel (though macs are
associated with the logical channel in any case).

I had thought that the mac address is initially taken from the first active
port, so it might be worthwhile testing if bringing up the other port first
has an effect on the source of the BPDUs. I suppose there may might be
difference between the various aggregation protocols, so it would be
interesting to test. Will have a look later in the week.

If internally the source mac is still associated with a local physical port,
that port is likely used for transmission. If you fail that interface, as
well as learning the BPDUs on your second interface, I think your source mac
may also change. All things being equal, the lowest port number is used to
determine the STP DP, so if a brand new channel is negotiated with multiple
available interfaces (but no active member ports), selecting the lowest
numbered port for frame generation may help stability if a portchannel has
to later be torn down.

Transmitting copies of the same BPDU out each port would not achieve very
much, just cause additional load. Also link aggregation protocols will not
allow duplication of frames out each of the available ports. My *guess* is
that the port cisco choose to use for BPDUs is probably just an
implementation choice, not something which is mandated by the protocols.

Paul.

On Tue, Nov 3, 2009 at 1:59 PM, austin stone <amitkjoshi.cisco_at_gmail.com>wrote:

> Hello All,
>
> I have a query regarding ether channel layer 2.
>
> In The network (STP running on) I firm the the layer 2 ether channel in
> between switches. I want to know when the Root bridge sent the BPDU on
> which
> port it will receive because STp assume this is as one interface. as per my
> observation it sent the bpdu on first physical interfce in etherchannel
> group. why is it so.
> P1-- -- -- -- -- -- P1
> SW1 SW2
> P2-- -- -- -- -- -- P2
>
>
>
> Regards
>
>
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Received on Tue Nov 03 2009 - 21:57:03 ART