Re: STP Port Priority

thank you for labing that up Bob that definitely concludes how MST will react in the different scenarios I believe!

:-D

Sent from my iPad

On 08/09/2010, at 9:12 PM, "Bob Sinclair" <bob_at_bobsinclair.net> wrote:

> Hi Garth,
>
> In my previous email you saw that the BPDUs sent by the MST border switches
> to MST and STP switches do contain the sender BID. Here you see the same
> scenario with the non-MST switch in RPVST+ mode. F0/20 received designated
> bridge ID 32768.0023.05c9.5e80. F0/21, connected to the CIST root,
> received sender BID 12288.0023.05c4.bb00.
>
> RPVST+ switch connected to two switches that are in the same MST region.
> Does this scenario model your case?
>
> Here is the root:
>
> SW1#sh span
>
> MST0
> Spanning tree enabled protocol mstp
> Root ID Priority 12288
> Address 0023.05c4.bb00
> This bridge is the root
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID Priority 12288 (priority 12288 sys-id-ext 0)
> Address 0023.05c4.bb00
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Interface Role Sts Cost Prio.Nbr Type
> ---------------- ---- --- --------- --------
> --------------------------------
> Fa0/3 Desg FWD 200000 128.5 P2p
> Fa0/4 Desg FWD 200000 128.6 P2p
> Fa0/5 Desg FWD 200000 128.7 P2p
> Fa0/6 Desg FWD 200000 128.8 P2p
> Fa0/20 Desg FWD 200000 128.22 P2p
> Fa0/21 Desg FWD 200000 128.23 P2p Bound(PVST)
> Fa0/22 Desg FWD 200000 128.24 P2p
> Fa0/23 Desg FWD 200000 128.25 P2p
> Fa0/24 Desg FWD 200000 128.26 P2p
> --More--
>
> Here is the nonroot connected to the rstp SW4:
>
> SW2#sh span
>
> MST0
> Spanning tree enabled protocol mstp
> Root ID Priority 12288
> Address 0023.05c4.bb00
> Cost 0
> Port 25 (FastEthernet0/23)
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
> Address 0023.05c9.5e80
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Interface Role Sts Cost Prio.Nbr Type
> ---------------- ---- --- --------- --------
> --------------------------------
> Fa0/1 Desg FWD 200000 128.3 P2p
> Fa0/4 Desg FWD 200000 128.6 P2p
> Fa0/19 Desg FWD 200000 128.21 P2p
> Fa0/20 Desg FWD 200000 128.22 P2p Bound(PVST)
> Fa0/23 Root FWD 200000 128.25 P2p
> Fa0/24 Altn BLK 200000 128.26 P2p
>
> --More--
>
> Here is the RSTP switch. F0/20 connected to SW2 and F0/21 connected to SW1
>
> SW4#sh span
>
> VLAN0001
> Spanning tree enabled protocol rstp
> Root ID Priority 12288
> Address 0023.05c4.bb00
> Cost 19
> Port 21 (FastEthernet0/19)
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
> Address 0023.3307.7000
> Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
> Aging Time 300
>
> Interface Role Sts Cost Prio.Nbr Type
> ---------------- ---- --- --------- --------
> --------------------------------
> Fa0/10 Desg FWD 19 128.12 P2p
> Fa0/19 Root FWD 19 128.21 P2p
> Fa0/20 Altn BLK 19 128.22 P2p Peer(STP)
> Fa0/21 Altn BLK 19 128.23 P2p Peer(STP)
> Fa0/22 Altn BLK 19 128.24 P2p
>
> SW4#sh span int f0/20 detail
> Port 22 (FastEthernet0/20) of VLAN0001 is alternate 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 16, forward delay 0, hold 0
> Number of transitions to forwarding state: 0
> Link type is point-to-point by default, Peer is STP
> BPDU: sent 2, received 32
>
> SW4#sh span int f0/21 detail
> Port 23 (FastEthernet0/21) of VLAN0001 is alternate blocking
> 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 15, forward delay 0, hold 0
> Number of transitions to forwarding state: 0
> Link type is point-to-point by default, Peer is STP
> BPDU: sent 1, received 30
>
> I am not able to capture bpdus in this lab, but it seems to me this is
> pretty good evidence that the border switches are sending their unique BIDs
> to SW4. As you say, if this was not the case, how would the tie be broken
> if sending port ids are the same? The assumption in STP is that if sender
> BIDs are equal, then port IDs must not be.
>
> Best regards,
>
> 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: Wednesday, September 08, 2010 3:54 AM
>> To: bob_at_bobsinclair.net
>> Cc: Cisco certification
>> Subject: Re: STP Port Priority
>>
>> Hi Bob,
>>
>> Thanks for the assistance
>>
>> "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."
>>
>> I want to clarify what you mean here.
>>
>> In all versions of STP apart from MST this is definately true. Octets
>> 18
>> through 25 contains the senders bridge ID.
>>
>> In the case of MST Octets 18 through 25 contains the Regional Root ID
>> which
>> is the switch with the lowest cost the to CIST Root, as per the
>> standard
>>
>> *"Octets 18 through 25 shall take the value of the CIST Regional Root
>> Identifier when transmitted in RST and MST BPDUs, and the value of the
>> CIST
>> Bridge Identifier of the transmitting Bridge when transmitted in STP
>> Configuration BPDUs. On receipt of an STP Configuration or RST BPDU,
>> both
>> the CIST Regional Root Identifier and the CIST Designated Bridge
>> Identifier
>> shall be decoded from this field. On receipt of an MST BPDU, the CIST
>> Regional Root Identifier shall be decoded from this field."*
>> **
>> Then you have these new fields in octets 94 - 101
>>
>>
>> *"Octets 94 through 101 convey the CIST Bridge Identifier of the
>> transmitting Bridge. The 12-bit system id extension component of the
>> CIST
>> Bridge Identifier shall be transmitted as 0. The behavior on receipt is
>> unspecified if it is non-zero"*
>>
>> Now in my understanding octets 94 - 101 do not exist in RSTP BPDU and I
>> believe that from this, if we were interoperating with say RPVST+
>> switch that had two links into two different switches in the same MST
>> Region
>> that the senders bridge ID would be the same, because the standard says
>> that
>> the value of the CIST Bridge Identifier when transmitted in STP
>> Configuration BPDU's.
>>
>> I dont have a lab to test right now as they are all booked up, but then
>> if
>> we went back to my original question where "what if the ports
>> connecting to
>> the rpvst domain had the same port-id" what would be the tiebreaker?
>>
>> I Guess first off I have to see if running RPVST+ what the senders
>> bridge ID
>> will actually be in Cisco's implementation before getting to the second
>> part.
>>
>> Cheers,
>>
>>
>>
>> Garth
>> On Tue, Sep 7, 2010 at 10:38 PM, Bob Sinclair <bob_at_bobsinclair.net>
>> wrote:
>>
>>> 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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>>>>
>>>>
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Received on Thu Sep 09 2010 - 07:45:01 ART