Re: spanning-tree extend system-id posted 11/25/2003
At first, I also found this command to be a bit confusing, but really it's
First of all, realize that by default, this command is active on the 3550.
What this command does is rearrange the meaning of the bits in the BID
(Bridge ID). Originally. the BID consisted of 8 bytes of which the 2 high
order bytes were the priority and the remaining 6 bytes were a MAC address
that came from the backplane of the switch.
But with the system extend-id command, the 16 bits which were previously
used for priority are now split up into 2 sub fields: 4 bits for priority
and 12 bits for Extended System ID which is set equal to the vlan #.
Your next logical question (and mine) is why? On this I'm still kind of
fuzzy. I know that it has something do to with PVTP+ but I'm not sure
exactly how all the pieces fit together. It may also have something to do
with the number of vlan's supportable on the switch. Since each vlan
requires a unique BID and there may be a limit of how many mac addresses are
available, using this new setup may allow there to be a lower requirement on
the number of mac addresses required.
Maybe one of the more knowledgable people on GS will chime in here and
explain what's really going on.
----- Original Message -----
From: "iwan" <iwan@xxxxxxxxxxx>
Sent: Tuesday, November 25, 2003 4:15 PM
Subject: spanning-tree extend system-id
> Hi group,
> Can someone help me with this command ... (spanning-tree extend
> I don't understand the use of it.
> What I understand is that the Spanning tree normally uses 1 MAC address
> per VLAN to make a bridge ID unique.
> But with this feature it now uses the extended system ID AND the switch
> priority AND the spanning-tree MAC address to make the bridge ID unique
> for each VLAN or multiple spanning-tree instance...
> Is this correct???
> Please advise me...
> Iwan Hoogendoorn
> MCSA,MCSE, MCDBA, CCA, CCNA
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