Some STP questions

[chinpohpang851]

I wonder why the priority is different in these 2 sh command in packet tracer?

Switch#sh spanning-tree vlan 1

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 000C.CF4B.914D

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 20

Switch#sh spanning-tree detail

VLAN0001 is executing the ieee compatible Spanning Tree Protocol

Bridge Identifier has priority of 32768, sysid 1, 000C.CF4B.914D

Configured hello time 2, max age 20, forward delay 15

Current root has priority 32769

Root port is 26 (GigabitEthernet0/2), cost of root path is 4

[Masoud Pourshabanian · Accepted solution]

Hello,

32769 comes from 32768 added to the VLAN ID which is 1.

Spanning tree operation requires that each switch have a unique BID. In the original 802.1D standard, the BID was composed of the Priority Field and the MAC address of the switch, and all VLANs were represented by a CST. Because PVST requires that a separate instance of spanning tree run for each VLAN, the BID field is required to carry VLAN ID (VID) information, which is accomplished by reusing a portion of the Priority field as the extended system ID.

Hope it helps,

Masoud

[chinpohpang851]

another question. If I have vlan 1, 2, 3, 4,  do i need to choose a specific VLAN as root switch or it is okay just use default VLAN 1?

[Masoud Pourshabanian]

If you are configuring PVST, you need to have a root bridge for each VLAN. For example you have two main switches in your network. Lets say SW1 and SW2.  For traffic sharing across different links, you can select SW1 as a root bridge for some VLANs(10, 11, 12,..) and SW2 for some other VLANs(21, 22, 23,...). You can nail this with changing the root bridge priority.

Masoud
 

[chinpohpang851]

why we need to do that? Can i just use vlan 1 for all?

[Masoud Pourshabanian]

You can not, because there is a separate instance of STP for each VLAN in PVST.

You may configure only one switch as a root bridge for your all VLANS.

Masoud

[chinpohpang851]

STP is enabled by default right? If I didn't alter the parameters like priority n STP cost, I think the Root switch should be the same as Vlan 1 root switch?

[Masoud Pourshabanian]

If you do not change any parameter, yes.

But it is going to be unstabe. If you add any other switch by accident, you may cause loop and also root bridge may change. And also your links would not be utilized. Some on of them would be blocked and some active.

I suggest you config it . The configuration is not difficult. a little more work but gives you confidence. It is worth it.

Masoud