Re: STP Port Priority

I have just built a topology based on the cabling infrastructure I have
available to me at the moment as below.

<SW1>----<SW3 >
    | /
    | /
    | /
<SW2>/

SW1 and SW2's link to SW3 is Port 16 (Both have Port ID 128.18) according to
SW3

SW3 link into SW 1 is Port 13

SW3's link into SW2 is Port 16

SW1 and SW2 are in an MSTP Region "12"

SW1 is the CIST Root

SW3 is running PVST+

As you can see,from the below output the

SW3# show spanning-tree detail
 VLAN0001 is executing the ieee compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 1, address 0013.c419.7b80
  Configured hello time 2, max age 20, forward delay 15
  Current root has priority 0, address 0019.55bb.8b80
  Root port is 13 (FastEthernet0/13), cost of root path is 19
  Topology change flag not set, detected flag not set
  Number of topology changes 0 last change occurred 00:00:59 ago
  Times: hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0, aging 300

 Port 13 (FastEthernet0/13) of VLAN0001 is root forwarding
   Port path cost 19, Port priority 128, Port Identifier 128.13.
   Designated root has priority 0, address 0019.55bb.8b80
   Designated bridge has priority 0, address *0019.55bb.8b80*
   *Designated port id is 128.18, designated path cost 0*
   Timers: message age 2, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default
   BPDU: sent 3, received 28

 Port 16 (FastEthernet0/16) of VLAN0001 is alternate blocking
   Port path cost 19, Port priority 128, Port Identifier 128.16.
   Designated root has priority 0, address 0019.55bb.8b80
   Designated bridge has priority 32768, address *001b.d4df.bf80*
   *Designated port id is 128.18, designated path cost 0*
   Timers: message age 2, forward delay 0, hold 0
   Number of transitions to forwarding state: 0
   Link type is point-to-point by default
   BPDU: sent 2, received 29

Does this mean that a when exiting a MSTP Region a switch will insert its
own Bridge ID in place of the "CIST Regional Root ID"?

Guess it'd make sense, there is not really any use for the CIST Regional
Root bits in the BPDU outside a region?

On Tue, Sep 7, 2010 at 4:41 PM, Garth Bryden <
hacked.the.planet.on.28.8k.dialup_at_gmail.com> wrote:

> Hello,
>
> I'm wondering how STP works out the Port ID part of the port priority...
>
> Say we have four switches
>
> <SW1>--------------- <SW3>
> | |
> | |
> | |
> | |
> <SW 2>---------------<SW4>
>
>
> Say SW1 + 2 are running MST Region name 12
>
> Say SW3 + 4 are running MST Region name 34
>
> The Intra Region Links on both switches are port fa0/1
> The Inter Region Links on both switches are port fa0/2
>
> The Port Costs between the regions are the same.
>
> SW1 is the CIST ROOT.
>
> Either SW3 or SW4 will elect the Boundary Root Port towards the CIST Root
> based on the lowest cost and become the CIST Regional Root. Now from what I
> understand that Port Cost for CIST is only cumulative between boundary ports
> and will not be adjusted within a region. So then if SW3 and SW4 have the
> same link costs there is going to be a tiebreaker, which would have to be
> broken
> based on Bridge ID.
>
> The Bridge ID in MST being the CIST Regional Root Bridge ID (Which I am
> assuming going off the MSTP BPDU which has the CIST Regional Root ID in
> place of where the Bridge ID used to be)...
>
> Further down, I then see that we have the "CIST Bridge ID" it looks as if
> these a additional fields for MST. So this would likely act as the
> tiebreaker for who becomes the CIST Regional Root in Region 34.
>
> Which brings me to my next question.
>
> If SW3 / SW4 was then running PVST+ it would not understand the "CIST
> Bridge ID" and Region 12 would look like a single Virtual Bridge. This is
> fine, except what if then my Port Priority and Port ID's are the same?
>
> What is the tiebreaker?
>
> Thanks,
>
> Garth

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Received on Tue Sep 07 2010 - 17:00:00 ART