From: Ronnie Angello (ronnie.angello@gmail.com)
Date: Tue Feb 20 2007 - 21:26:50 ART
I think that you're referring to the bridge identifier (bridge id), but the
answer to your question is...it depends!
Priority is just one part of the bridge id.
The bridge id with the extended system id disabled (no spanning-tree extend
system-id) is 8 bytes. It is made up of a 2 byte priority field (0-65535)
and the 6 byte system id (mac address).
The bridge id with the extended system id enabled is still 8 bytes.
However, the first 4 bits are the priority value (multiple of 4096) and the
system id extension makes up the next 12 bits. The sytem id extension holds
the vlan id. The last 6 bytes is the actual system id (mac address).
Ronnie
On 2/20/07, Gobind <gobind@inbox.com> wrote:
>
> So in no case this is true,right?:-
>
> Bridge priority = Default bridge priority - Vlan Number
>
> I heard it in CBT was got confused!
>
> > -----Original Message-----
> > From: louisccie_r_s@yahoo.com
> > Sent: Tue, 20 Feb 2007 11:21:11 -0800 (PST)
> > To: gobind@inbox.com, ccielab@groupstudy.com
> > Subject: Re: Bridge Priority
> >
> > Yes each vlan has its own root depending on the priority. If priorities
> > are the same then it uses the mac to select the root. Each vlan and
> > priorities are combined to create a unique ID. Check the floowing link
> > for a siminlar question and answers:
> >
> >
> http://www.velocityreviews.com/forums/t37177-bridge-priority-and-sysidext-number.html
> >
> > Gobind <gobind@inbox.com> wrote: If the sys-id-ext usually correlates
> > to Vlan Number,it means like for Vlan Number 1 the sys-id-ext would be
> > 1,right? it is taken automatically or manually? I am confused about the
> > part that the If in a switch every Vlan has its own priority then when
> > the Root Election occurs,does it occurs to every Vlan,lets say there are
> > 3 Vlans and what if it have different Priority then is it also possible
> > that on Switch would be Root Bridge for 1 Vlan and not other two(due to
> > higher Priority)?
> >
> >
> > -----Original Message-----
> > From: louisccie_r_s@yahoo.com
> > Sent: Tue, 20 Feb 2007 10:59:29 -0800 (PST)
> > To: gobind@inbox.com, ccielab@groupstudy.com
> > Subject: Re: Bridge Priority
> >
> >
> > I am not sure if you refering to a specific lab exercise, but in real
> > environments you can select which switch you want to be the Root Bridge
> > for specific vlans by lowering the priority from the default 32768. The
> > lower the priority the more prefered to be the root (example 32000). If
> > you want to FORCE a specific switch to be the root then you set priority
> > 0. For example:
> >
> > spanning-tree vlan 10-14 priority 0 (This will set this switch to be the
> > root for vlans 10,11,12,13,14)
> >
> > You can set a different root bridge for each group of vlans for load
> > balancing. You can direct certain vlans traffic through one distribution
> > switch and other vlans through the other distribution reduntant switch
> > for load balancing L2 traffic.
> >
> > Usually is good to set the L3 distribution switches as the Root bridges
> > for the vlans attached to the access layer switches attached to it.
> >
> > Thanks,
> >
> > Loizos Y.
> > CCIE#10702 R & S
> >
> >
> >
> > Gobind <gobind@inbox.com> wrote: Hi Guys,
> >
> > I have some confusion wiht the Bridge Priority.In Some CBTs I am getting
> > to hear that:-
> >
> > Bridge Priority is Default Priority(32768) - Vlan Number
> >
> > I asked my friend,he said Default Priority is Default Priority + Vlan
> > Number
> >
> > In one Cisco Dumentation it is written:-
> >
> > "Each VLAN on each network device has a unique 64-bit bridge ID
> > consisting of a bridge priority value, an extended system ID, and an STP
> > MAC address allocation"
> >
> > Source:- http://www.cisco.com/en/U
> >
> S/products/hw/switches/ps708/products_configuration_guide_chapter09186a008016
> > 0a5b.html#wp1040218
> >
> > I am really confused(maybe I am getting it wrong) what Priority(in STP
> > is),also I am not getting if the Priority is Default priority + Vlan
> > Number or - Vlan Number then what Vlan Number is it refering to? I mean
> > the Trunk belongs to every Vlan then which Vlan Number it takes while
> > making the election.(when it sends BPDU i.e.)
> >
> > Gobind
> >
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