PVST+ alternate port role Query

[Palaniswamy P]

Hi Everyone,

In spanning-tree pvst+ mode I noticed one of the port is showing alt port role . while going through the forums i found that RSTP port roles might have been amended int the 802.1d(PVST+). Just to see the behavior  i shutdown the Gi0/1, which will generate the TC . 

In RSTP an alternate port will immediately transition to forwarding state. But in PVST+ even though it shows alternate blocking port, it will still transition from blocking - listening-learning-forwarding. so is there any use of showing a port role as alternate port in pvst+.

IOSVl2-4#sh spanning-tree vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 5e00.0000.0000
Cost 4
Port 2 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 5e00.0003.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1 Root FWD 4 128.2 P2p
Gi0/2 Altn BLK 4 128.3 P2p

IOSVl2-4#

IOSVl2-4(config)#int gi0/1
IOSVl2-4(config-if)#shut
IOSVl2-4(config-if)#end

IOSVl2-4#sh spanning-tree vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 5e00.0000.0000
Cost 8
Port 3 (GigabitEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 5e00.0003.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/2 Root LIS 4 128.3 P2p

IOSVl2-4#

IOSVl2-4#sh spanning-tree vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 5e00.0000.0000
Cost 8
Port 3 (GigabitEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 5e00.0003.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/2 Root FWD 4 128.3 P2p

IOSVl2-4#

Regards,

Palani

[Palaniswamy P]

as per the cisco documentation. in PVST+ to move the alternate port to transition to forwarding is only possible when we enable uplink fast. without which the port state will transition all the states till forwarding. i hope my understanding is correct

Understanding UplinkFast

if a switch loses connectivity, it begins using the alternate paths as soon as the spanning tree selects a new root port. By enabling UplinkFast with the spanning-tree uplinkfast global configuration command, you can accelerate the choice of a new root port when a link or switch fails or when the spanning tree reconfigures itself. The root port transitions to the forwarding state immediately without going through the listening and learning states, as it would with the normal spanning-tree procedures.

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/15-0_2_se/configuration/guide/3750x_cg/swstpopt.html