PVST+ Lab

Per VLAN Spanning Tree Plus (PVST+) provides the same functionality as PVST using 802.1Q trunking technology rather than ISL. PVST+ is an enhancement to the 802.1Q specification and is not supported on non-Cisco devices.

We will need to get the values of 3 switches  where we are applying spanning-tree

We are going to check our actual configuration.


Let's check switch B

We will see different things:

1.-We see the Vlan 110 when we used "show mac address-table" command

2.-We see that we are not the root bridge

3.-The root bridge has a mac address of  000c.ce17.df80
4.-Our Mac address is 000d.28f5.9180
5-We see the priority of 32878= 32768 + VLAN 110
6.-The ROOT Port is Gi11

Let's  go to Switch A

Let's check the same on VLAN 50 and VLAN 80

As we predicted all ports are designated ports

In SWITCH B it's connected to SWITCH A through the interface G11..and that ends in interface fa0/15
Let's go back to SWITCH_B

 

The value 000c is less than the value 000d

Now let's check the SWITCH C

The port identifier of F/10 is the root ( the lowest interface from where it's sending): 128 + 0/10+2       = 128.12

This is our results

But what happens in case we want to manipulate the information and make int fao/10 of Switch C the blocked port instead of fa0/15?

Let's go back to Switch A

We can not manipulate the physical interface (3,10,15) .but we can manipulate 128 (the port priority)

So we will use a multiple of 16.We will use 224

Manipulation number 1 -Port Priority

 Doing this , we are making the link of fa0/3 of Switch A less attractive

Let's go back to SWITCH C to see the results

Now we have this topology

Manipulation number 2- Cost

Let's go to Switch C
We will manipulate the cost  to the value of 16

_________________________________________________________

Now let's take advantage of PVST+ and we will make SWITCH B the Primary root for

•  VLAN50
• VLAN 80      

And SWITCH B the Secondary root for 

• VLAN110

 On SWITCH C  we will the following changes:

 he Primary root for

• VLAN110

And SWITCH B the Secondary root for 

• VLAN 80
•  VLAN50    

Let's start with SWITCH B

 

Now let's do it with SWITCH C

Now let's do SWITCH B

Switch B is announcing that it's VLAN 50  primary root..same Mac address as well

 We are NOT the root here  with VLAN 110..

Now let 's switch back to SWITCH C

 ____________________________________________________________________

 

Summary:

What we learned:

• We could verify our paper calculation of the ports
• We could verify the root bridge, the root switch, the designated port, the blocking ports
• We could have two ways to make a root bridge
• We could set a priority as a numeric value or set a root secondary or primary
• We can go with the lowest port  cost which by default  is the lowest interface speed(but we can manually configure that)
• We can find different ways to calculate which path a switch can go back to the root
• We were able to manipulate  no the hardware value but the port priority
• etc, etc, etc 

This was a good lab..I hope you enjoy , share and comment!