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RE: Bridge Priority posted 02/20/2007
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And forgive me if it was already mentioned in the thread....but WHY do
this? 

So you can use ONE MAC address to create a unique Bridge ID for a whole
bunch of VLANs. 

Anthony J. Sequeira
#15626

-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
Scott Morris
Sent: Tuesday, February 20, 2007 7:40 PM
To: 'Lou Ioanni'; 'Gobind'; ccielab@xxxxxxxxxxxxxx
Subject: RE: Bridge Priority

I think you'd have to sit there with a lot of math to verify that, but
it's
not accurate anyway, so save yourself some time.  :)

The old way for Bridge ID was 16-bit priority + MAC address = 64 bits.

Now, the bridge ID is still 64 bits, but instead of having 16 bits for
priority (who has 65000 switches anyway?), there are 4 bits used for
priority and then 12 bits used for VLAN ID.  So that's why you can get
some
strange values.

So the priority field is still 16 bits:

ppppvvvvvvvvvvvv

Where p = priority and v = vlan ID. 

HTH,

 
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
JNCIE
#153, CISSP, et al.
CCSI/JNCI-M/JNCI-J
IPexpert VP - Curriculum Development
IPexpert Sr. Technical Instructor
smorris@xxxxxxxxxxxx
http://www.ipexpert.com
 


-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
Lou
Ioanni
Sent: Tuesday, February 20, 2007 5:28 PM
To: Gobind; ccielab@xxxxxxxxxxxxxx
Subject: Re: Bridge Priority

Right. You do not deduct vlan number from default to get priority. What
confused you is the way they map vlans to priorities ex. 2-32768,
3-4094,
etc
   
  I never heard of such formula. I will be very surprised if there is
any.
   
  Loizos Y.
  CCIE#10702 R & S

Gobind <gobind@xxxxxxxxx> 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@xxxxxxxxx
> Sent: Tue, 20 Feb 2007 11:21:11 -0800 (PST)
> To: gobind@xxxxxxxxx, ccielab@xxxxxxxxxxxxxx
> 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-sysid
> ext-number.html
> 
> Gobind 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@xxxxxxxxx
> Sent: Tue, 20 Feb 2007 10:59:29 -0800 (PST)
> To: gobind@xxxxxxxxx, ccielab@xxxxxxxxxxxxxx
> 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 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_chapter09186
> a008016
> 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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