PVST Bridge ID and usage 1

[HungryHouse]

Hi All,

I'm trying my best to reexamine STP and its flavors and mechanisms again, and I am seeing when PVST is on, the Bridge ID increments by the VLAN#, hence VLAN1 Bridge ID shows 32769 using this new "extended system ID" added to the BPDU. I have a few questions regarding this and the usage of PVST:

1- Why does the output differ for show spanning-tree vs. show spanning-tree detail in this regard?
C2950SX#show spanning-tree detail

VLAN0001 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, sysid 1, address 000c.3068.db00

C2950SX#show spanning-tree

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000c.3068.db00

2- Does the BPDU used in PVST actually combine the ext-sys-id with the Bridge priority to form a new priority or is it a separate field thats added to the Bridge ID just like the MAC addy is?

3- I could see scaling issues with PVST with large VLAN DBs....With MSTP out there, is there any true advantage of using PVST?

If it appears I have a misunderstanding of how this works, please assist...I am certainly not an STP guru.

Thanks very much,
HH

 

[unclerico]

1) Why did the chicken cross the road?? Your guess is as good as mine. They both contain the same data it's just that the show spanning-tree output combines the sys-ext-id with the priority

2) Yes, sys-ext-id is added to the priority to make up the first 2 bytes of the Bridge ID field of the BPDU. Here's some from a packet capture for VLAN2:

Bridge Identifier: 32770 / 00:09:b7:51:4a:00

3) Yes, there can be scalability issues with PVST+ depending on how many VLANs you have defined. MSTP is standards based so that is a big plus, but it's not all roses when dealing with it. At least with PVST+/RPVST+ you can use VTP to propagate the VLANs from switch to switch (if this is something you use) as where MSTP you are manually hitting every switch. If you don't define all of your VLANs ahead of time with MSTP you could cause yourself a HUUUUUGE headache when you try to add them later. The reason is that the config name, revision, and VLAN hash must match for all switches in an MST region. When you add a VLAN after you are in production the first switch you add it to will be placed in a separate region and then you have STP reconverge right before your eyes.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[HungryHouse]

Thanks unclerico....I appreciate the answer(s), especially the practical uses of MSTP vs. PVST.

Have a great day.

-HH