RE: STP Port Priority

Hi Garth,

The PVST and MST BPDUs contain two bridge IDs, the BID of the root and the
BID of the sending bridge. Given equal path costs, your SW3 should choose
its root based on Sending BID, regardless of sender port ID.

Below you see output from a CIERS2 lab I had handy. In this case SW4 is
dual-homed to an MST region. You see the root and sender bridge IDs are the
same whether SW4 is PVST or MST:

SW4 in PVST mode:

SW4#sh span int f0/20 det
 Port 22 (FastEthernet0/20) of VLAN0001 is blocking
   Port path cost 19, Port priority 128, Port Identifier 128.22.
   Designated root has priority 12288, address *0023.05c4.bb00*
   Designated bridge has priority 32768, address *0023.05c9.5e80*
   Designated port id is 128.22, designated path cost 0
   Timers: message age 3, forward delay 0, hold 0
   Number of transitions to forwarding state: 0
   Link type is point-to-point by default
   BPDU: sent 2, received 85

SW4#show span int f0/21 det
 Port 23 (FastEthernet0/21) of VLAN0001 is forwarding
   Port path cost 19, Port priority 128, Port Identifier 128.23.
   Designated root has priority 12288, address *0023.05c4.bb00*
   Designated bridge has priority 12288, address *0023.05c4.bb00*
   Designated port id is 128.23, 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 128

SW4 in MST mode:

SW4#show span int f0/20 det
 Port 22 (FastEthernet0/20) of MST0 is alternate blocking
   Port path cost 200000, Port priority 128, Port Identifier 128.22.
   Designated root has priority 12288, address *0023.05c4.bb00*
   Designated bridge has priority 32768, address *0023.05c9.5e80*
   Designated port id is 128.22, designated path cost 0
   Timers: message age 4, forward delay 0, hold 0
   Number of transitions to forwarding state: 0
   Link type is point-to-point by default, Boundary RSTP
   BPDU: sent 2, received 10
SW4#show span int f0/21 det
 Port 23 (FastEthernet0/21) of MST0 is root forwarding
   Port path cost 200000, Port priority 128, Port Identifier 128.23.
   Designated root has priority 12288, address *0023.05c4.bb00*
   Designated bridge has priority 12288, address *0023.05c4.bb00*
   Designated port id is 128.23, designated path cost 0
   Timers: message age 4, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default, Boundary RSTP
   BPDU: sent 4, received 15

So, if I understand you question, yes: the sending bridge puts its own ID in
the BPDU, but it does not replace the root BID.

HTH,

Bob Sinclair CCIE 10427 CCSI 30427
CIERS2 Online Instructor
www.tinyurl.com/ciers2online

> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> Garth Bryden
> Sent: Tuesday, September 07, 2010 5:00 AM
> To: Cisco certification
> Subject: 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 - 10:38:00 ART