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Re: spanning-tree extend system-id posted 11/26/2003
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ccie2b, et. al:

I'll take a stab at adding to the discussion:

The Mac Address Reduction feature is a way to allow up to 4096 different
bridge ID numbers, 1 for each possible vlan on a switch.  Instead of using a
set of incrementing the mac address field of the bridge ID, the MAR feature
cuts the priority field into a 4 bit priority and a 12-bit "system-ID",
which is actually the vlan ID.  When MAR is enabled the mac address portion
of the bridge ID remains unchanged.

The MAR bridge priority only uses the upper 4 bits of the former 16 bit
field.  In order to be compatible with non-MAR systems, the 4-bit priority
is evaluated as if it had 12 zeros to the right:

 PRI      VLAN ID         16-bit #
1000-000000000001  is 32769   vlan 1 with MAR priority 32768  (default)
0001-000000000002   is 4098    vlan 2 with MAR priority 4096
0010-000000000003   is 8195    vlan 3 with MAR priority 8192

As an experiment, I trunked my default 3550 to my default 6000 in hybrid
mode and trunked the vlans above to see how each interpreted the bridge IDs:

Here are partial results for "show spanning-tree vlan x" on the 3550 showing
the BIDS notice that the mac-address portion does not change:

Vlan 1:
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     000b.469d.b980
Vlan 2:
  Bridge ID  Priority    4098   (priority 4096 sys-id-ext 2)
             Address     000b.469d.b980
Vlan 3:
  Bridge ID  Priority    8195   (priority 8192 sys-id-ext 3)
             Address     000b.469d.b980

Here is how the Cat6k saw the same vlans:

Vlan1:  The Cat is the root.  The priorities are at the default, and the 6k
has the lower mac address. Are the priorities equal??
Designated Root             00-01-63-2c-5c-00
Designated Root Priority    32768
Designated Root Cost        0
Designated Root Port        1/0
Root Max Age   20 sec   Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-01-63-2c-5c-00
Bridge ID Priority          32768

Vlan 2:  The 3550 is root, and the 6k sees that priority as the full 16-bit
value, 4098.  So maybe the priorities were not equal above??
Designated Root             00-0b-46-9d-b9-80
Designated Root Priority    4098
Designated Root Cost        19
Designated Root Port        3/1
Root Max Age   20 sec   Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-01-63-2c-5c-01
Bridge ID Priority          32768

Vlan 3: 3550 is again root.  Notice that the mac-address portion of the 3550
bridge ID stays the same across vlans.  The mac-address portion of the
bridge ID on the Cat6k increments from 00  to indicate the vlan ID.
Designated Root             00-0b-46-9d-b9-80
Designated Root Priority    8195
Designated Root Cost        19
Designated Root Port        3/1
Root Max Age   20 sec   Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-01-63-2c-5c-02
Bridge ID Priority          32768

So, the MAR allows for close to 4096 vlans without requiring 4096 unique mac
addresses on the switch.  One thing to look out for, however, is that by
default, your nice new MAR switch is unlikey to become root, because the
non-MAR switches interpret its priority as PRI plus VLAN ID > 32768.

My $.03,


-Bob Sinclair
 CCIE #10427, CISSP, MCSE
 bsinclair@xxxxxxxxxxxxxxxxxx

----- Original Message ----- 
From: "ccie2be" <ccie2be@xxxxxxxxxx>
To: "Group Study" <ccielab@xxxxxxxxxxxxxx>; "iwan" <iwan@xxxxxxxxxxx>
Sent: Tuesday, November 25, 2003 6:13 PM
Subject: Re: spanning-tree extend system-id


> At first, I also found this command to be a bit confusing, but really it's
> quite simply.
>
> 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.
>
> dt
> ----- Original Message ----- 
> From: "iwan" <iwan@xxxxxxxxxxx>
> To: <ccielab@xxxxxxxxxxxxxx>
> 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
> > system-id)
> >
> > 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...
> >
> > Thanks,
> >
> > Iwan Hoogendoorn
> > MCSA,MCSE, MCDBA, CCA, CCNA
> >
> > _______________________________________________________________________
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