RSTP port role election

[s-santhosh]

Hello,

Recently implemented a branch office LAN with dual core switches (core a and core b) all access switches connect to core a and core b.

core a is root bridge with priority 0, b is as secondary root bridge with priority 4096  and rest of the switches with defaulf priority.

when a access switch is connected to core a and core b. RSTP converged but core b elected as Alternate role instead as designated on access switch side to core a as root port and core b as designated.

Core B

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Gi1/1/1             Altn BLK 20000     128.49   P2p

Core A

Te1/1/1             Desg FWD 2000      128.53   P2p

Access Switch

Te1/1/1             Root FWD 2000      128.53   P2p

Gi2/0/1             Desg FWD 20000     128.55   P2p

--------------------------------------------------------------------------------------------------

core02

Port 49 (GigabitEthernet1/1/1) of VLAN0001 is alternate blocking

   Port path cost 20000, Port priority 128, Port Identifier 128.49.

   Designated root has priority 1, address e02f.6d46.8880

   Designated bridge has priority 32769, address acf2.c5b2.3380

   Designated port id is 128.55, designated path cost 2000

   Timers: message age 15, forward delay 0, hold 0

   Number of transitions to forwarding state: 0

   Link type is point-to-point by default

   Loop guard is enabled on the port

   BPDU: sent 4, received 2417

sera

Port 53 (TenGigabitEthernet1/1/1) of VLAN0001 is root forwarding

   Port path cost 2000, Port priority 128, Port Identifier 128.53.

   Designated root has priority 1, address e02f.6d46.8880

   Designated bridge has priority 1, address e02f.6d46.8880

   Designated port id is 128.53, designated path cost 0

   Timers: message age 16, forward delay 0, hold 0

   Number of transitions to forwarding state: 2

   Link type is point-to-point by default

   Loop guard is enabled on the port

   BPDU: sent 21, received 2837

sera

Port 55 (GigabitEthernet2/0/1) of VLAN0001 is designated forwarding

   Port path cost 20000, Port priority 128, Port Identifier 128.55.

   Designated root has priority 1, address e02f.6d46.8880

   Designated bridge has priority 32769, address acf2.c5b2.3380

   Designated port id is 128.55, designated path cost 2000

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   Loop guard is enabled on the port

   BPDU: sent 2665, received 4

core01

Port 53 (TenGigabitEthernet1/1/1) of VLAN0001 is designated forwarding

   Port path cost 2000, Port priority 128, Port Identifier 128.53.

   Designated root has priority 1, address e02f.6d46.8880

   Designated bridge has priority 1, address e02f.6d46.8880

   Designated port id is 128.53, designated path cost 0

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   Loop guard is enabled by default on the port

   BPDU: sent 5566, received 26

---------------------------------------------------------------------------------------------------------

How can I influence access switch port Gi2/0/1 to be elect as Alternate port and Core B port G1/1/1 as Designated port..?

Thanks in advance

[m.glosson]

Greetings. This question is a bit confusing because you refer to coreA and CoreB but then in your STP output we have core01 and core02. I assume core01 is coreA and core02 is coreB...

Anyway, it might be helpful if you included the output of:

     show run | include spanning

on both core switches, as well as the output of

     show spanning-tree

on both core switches, but just include the top sections (Root ID and Bridge ID information).

Also indicate in your drawing which ports are connecting the switches together.

Matt

[Peter Paluch]

Hello Matt and santosh,

While some additional information may be useful, please note that a designated port is elected as a port that sends the best BDPUs on the segment. BPDUs are compared in this sequence of fields:

1. Root Bridge ID (including the priority)
2. Root Path Cost
3. Sending Bridge ID (including the priority)
4. Sending Port ID (including the priority)
5. Receiving Port ID (including the priority; not a field of BPDU, evaluated rarely)

If we can trust santosh that the core switches have priorities of 0 and 4096, respectively, it follows then that the CoreB switch's Root Path Cost must be higher than the Root Path Cost of the access switch. Simply put, the access switch appears to be on a faster link towards the CoreA than the CoreB.

Best regards,

Peter

[s-santhosh]

Hi Matt, please consider switch names as corea and coreb, Thank you

coreb# sh run | in spann

spanning-tree mode rapid-pvst

spanning-tree loopguard default

spanning-tree portfast bpduguard default

spanning-tree extend system-id

spanning-tree pathcost method long

spanning-tree vlan 1-4094 priority 4096

---------------------------------------

coreb#sh spann

VLAN0001

  Spanning tree enabled protocol rstp

  Root ID    Priority    1

             Address     e02f.6d46.8880

             Cost        10000

             Port        488 (Port-channel1)

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

  Bridge ID  Priority    4097   (priority 4096 sys-id-ext 1)

             Address     e02f.6d2f.8380

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

             Aging Time  300 sec

__________________________________________________________________________

core a# sh run | in spann

spanning-tree mode rapid-pvst

spanning-tree loopguard default

spanning-tree portfast bpduguard default

spanning-tree extend system-id

spanning-tree pathcost method long

spanning-tree vlan 1-4094 priority 0

---------------------------------------

corea#sh spann

VLAN0001

  Spanning tree enabled protocol rstp

  Root ID    Priority    1

             Address     e02f.6d46.8880

             This bridge is the root

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

  Bridge ID  Priority    1      (priority 0 sys-id-ext 1)

             Address     e02f.6d46.8880

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

             Aging Time  300 sec

Diagram with interface details

[Peter Paluch]

Hello,

If I am interpreting your output correctly, the distance of your CoreB to CoreA (root) is 10000 while the distance of the Access switch to CoreA is 2000. That would explain why CoreB blocks its port towards the Access switch.

The solution should be fairly easy here - on CoreB Port-channel1 interface (the current root port), use the spanning-tree cost 1 command or any other value that is less than 2000.

Best regards,

Peter

[s-santhosh]

Hi Peter, It worked very fine, but same is not getting effect for other 2 more access switches connected to coreA and coreB switches.

They are also connected like same tengig to coreA and gig to CoreB with same configured as access switch 1.

Thanks for your support

[Peter Paluch]

Hello Santhosh,

Please post the entire show span vlan 1 output from core-a, core-b and the two access switches that you need to tweak. Thanks!

Best regards,

Peter