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PVST Calculation

vishalpatil86
Beginner
Beginner

Dear All,

How PVST works?

can anyone explain woth the help of an example?

Thanks,

Vishal                  

1 Accepted Solution

Accepted Solutions

Hi,

acually, each switch believes to be an STP root until it receives a BPDU with better priority than its own one within the VLAN.

Is it clear now?

HTH,

Milan

View solution in original post

17 Replies 17

cadet alain
Mentor
Mentor

Hi,

PVST is just STP  but with an STP instance for each VLAN and by default all VLANs wiil follow the same STP topology but you can configure your switches so that there is a different STP topology per VLAN.

If you need more info on STP then we'll be glad to help.

Regards.

Alain

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Dear Alain,

Thanks for the reply!!!  Now suppose there are 10 switches running vlan 1, 10 and 20...

Now, for each vlan there is separate instance of STP on all switches or

each vlan has separate instace of STP on individual switches?

Hi,

a separate instance per VLAN on each switch.

Regards.

Alain

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Ok Alain, now we mover somewhat further. I have topology which is attahced here.

Just want to know how vlan 99 chosses the 223 switch over 450e core switch even if mac id of 4506e is lower than that of 223 switch?

Hi,

check the STP for VLAN 99 by sh spanning-tree vlan 99 commands on both switches.

You might see either of them having not the default STP priority configured for that VLAN.

Possibly your 223 swict had been configured as the STP root  for VLAN 99 in the past?

HTH,

Milan

hi milan,

all the values are at their deafault only

Can you post the output of the 'sh spanning-tree vlan 99' please.

Thanks,

Adam

Sent from Cisco Technical Support iPad App

Dear Adam,

Below is the output for show spanning tree for vlan 99 of all the switches

220

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f9ce.xxxx

219

VLAN0099

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f9ce.xxxx

238

VLAN0099

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

237

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 34bd.c8b6.xxxx

236

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 203a.07fb.xxxx

235

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 34bd.c8f6.xxxx

234

VLAN0099

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f98c.xxxx

227

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

228

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f9ce.xxxx

229

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

230

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

231

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

232

Address 0024.f98c.xxxx

Cost 32

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

233

Root ID    Priority 32867

Address 0024.f98c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

253

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 001d.7040.xxxx

221

Root ID    Priority 32867

Address 0024.f91c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f91c.xxxx

222

Root ID    Priority 32867

Address 0024.f91c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f98c.xxxx

223

Root ID    Priority 32867

Address 0024.f91c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f9c4.xxxx

223

Root ID    Priority 32867

Address 0024.f91c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f9c4.xxxx

225

Root ID    Priority 32867

Address 0024.f91c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

226

Root ID    Priority 32867

Address 0024.f91c.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

240

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address e8ba.7084.xxxx

241

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f91c.xxxx

244

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

245

spanning tree instance d oes not exist

246

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

247

spanning tree instance d oes not exist

248

Root ID    Priority 32867

Address 001d.7040.xxxx

Cost 4

Bridge ID  Priority 32867  (priority 32768

Address 0024.f9cd.xxxx

239

Root ID    Priority 32867

Address 18ef.6350.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 18ef.6350.xxxx

249

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f761.xxxx

250

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

242

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f91c.xxxx

251

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0025.4610.xxxx

252

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f761.xxxx

243

Root ID    Priority 32867

Address 001d.7040.xxxx

Bridge ID  Priority 32867  (priority 32768

Address 0024.f98c.xxxx

Hi,

which of the switches is your 4506?

Switch 253?

What does show spanning tree 99 show on it?

Istn't ther epossible youe VLAN 99 is discontinous somewhere?

In that case you would see two STP roots - each for one part of the discontinous VLAN 99!

HTH,

Milan

yes Milan, 4506e is 253. and there is discontinuity. But my doubt is root bridge is elected among all the switches.. am I right?

Please educate me!!!!

No,

the root is elected per VLAN among the switches which can "see" each others.

So for a discontinous VLAN you've got two roots finally - one per each continuous part of the VLAN.

HTH,

Milan

Dear Milan,

Instead of saying can see each other, can we say which are connected in loop or in chain?

Hi,

no we should say which see each other's BPDUs for the coresponding vlan instance.

Regards.

Alain

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Dear Alain,

please find attahced sheet. in this, there are two side of switch. one is looped connected to one gig port of core and other is chained connected to other gig port of core.  Now, you meant to say that these two sides of switched don't see each others BPDU??

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