STP port role

[Ratheesh mv]

Dear Team ,

we are running pvst mode (Switch is in pvst mode) on WS-C3850-48T-E (Stack).In the show spanning-tree vlan xxx output I am seeing Backup port role(Gi2/0/22 )while running PVST mode .As per my understanding Backup port role is a one of the feature of RSTP mode .

Spanning tree enabled protocol ieee
Root ID Priority 379
Address 0042.5a43.df00
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 379 (priority 0 sys-id-ext 379)
Address 0042.5a43.df00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/22 Desg FWD 4 128.22 P2p
Gi2/0/22 Back BLK 4 128.86 P2p

===========================================================================================

Switch is in pvst mode
Root bridge for: VLAN0001, VLAN0011, VLAN0111-VLAN0113, VLAN0201-VLAN0206
VLAN0209-VLAN0211, VLAN0269-VLAN0270, VLAN0273, VLAN0282-VLAN0284
VLAN0290-VLAN0292, VLAN0333, VLAN0335, VLAN0377, VLAN0379, VLAN0381
VLAN0384, VLAN0387
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
EtherChannel misconfig guard is enabled
UplinkFast is disabled
BackboneFast is disabled
Configured Pathcost method used is short

Please help me to understand why is it showing Backup port role for PVSTP mode .

Thanks in advance ....

[balaji.bandi]

If you mentioned below on the post about Blocking :

Gi2/0/22 Back BLK 4 128.86 P2p

This is standard STP technique to minimize the Layer 2 Loops in the topology.

if 1/0/22 and 2/0/22  connected to same peer switch or in the network some where loop, that go in the BLK mode as part of STP steps. this is standard and expected, I have mentioned this based on the output.

if you do not like ports to be in blocking mode, if both the links going to same switch, then suggest to make them port bundling like port-channel - you have resilience of the link and forward the traffic  both the ports

if they are individual trunk, you like use both the link you need to allow only required VLAN in that trunk example : odd vlan 1/0/22 and even VLAN 2/0/22 - but again this is all my assumption based on the post.

if this is not the case , please provide what is your requirement and configuration to understand and suggest in better manner

is this make sense ?

[Ratheesh mv]

Thanks for your responds ..

I would like to get clarity on port role ....(not port state)....

Gi2/0/22 Back BLK 4 128.86 P2p ------>

Spanning tree enabled protocol ieee
Root ID Priority 379
Address 0042.5a43.df00
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 379 (priority 0 sys-id-ext 379)
Address 0042.5a43.df00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/22 Desg FWD 4 128.22 P2p
Gi2/0/22 Back BLK 4 128.86 P2p


"Back"--Why backup port is being appeared in the PVSTP mode .??


Thanks in advance ....

[Giuseppe Larosa · Accepted solution]

Hello @Ratheesh mv ,

according to the following output

Spanning tree enabled protocol ieee
Root ID Priority 379
Address 0042.5a43.df00
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 379 (priority 0 sys-id-ext 379)
Address 0042.5a43.df00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/22 Desg FWD 4 128.22 P2p
Gi2/0/22 Back BLK 4 128.86 P2p

I think that these two ports are connected together with a cable. Being this switch the root bridge for Vlan 379 it should have all ports in Designated state forwarding. However, if two ports of this switch are connected between them the port with lower ID 128.22 < 128.86 is preferred.

Instead of a simple cable the two ports may be connecting to a non Cisco switch that does not understand PVST BPDUs and just propagate them as L2 multicast traffic the end result is the same.

About the use of Role and Backup terms that are more appropriate for Rapid STP and Rapid PVST in recent IOS versions Cisco have done this "mixing" of terms as a way to have a more uniform output regardless of STP mode PVST or Rapid PVST.

You are right about this.

Hope to help

Giuseppe

[Ratheesh mv]

This is a stack switch ....Gi1/0/22 is stack master port  Gi2/0/22 is stack member port .Could you please tell me that BPDU is sent over only master port or members port too before electing the root bridge??? .. 

[Giuseppe Larosa]

Hello @Ratheesh mv ,

in a stack the master switch will provide the bridge ID for STP BPDUs.

STP BPDUs are sent out of all member switch ports using the same bridge ID.

From the point of view of outer world the stack is a single entity.

In your case gi1/0/022 is Designated so it sending out BPDUs every two seconds

gi2/0/22 listens to the messages sent out by gi1/0/22 but does not send out its own messages because it is backup on the segment.

So the reason that stopped gi2/0/22 to send out BPDUs is the fact that is connnected to a shared segment (either a cable or non cisco switch).

If you would have one gi2/0/x connecting to another device you should see that port in DESIGNATED state forwarding and sending out BPDUs every two seconds.

stopping to send BPDUs on ports of member switches (non master ) would be dangerous.

Hope to help

Giuseppe

[Ratheesh mv]

If my remote end is a shared medium , STP port type would be shared right...(As per my understanding) ....But in my output it is P2p.

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/22 Desg FWD 4 128.22 P2p
Gi2/0/22 Back BLK 4 128.86 P2p

[Giuseppe Larosa · Accepted solution]

Hello @Ratheesh mv ,

p2p is assumed on full duplex port.

However, it does not create any issue in your scenario because gi2/0/22 is in blocking state.

Declaring the shared state would be important in Rapid STP , because this version of the protocol tries to use handshake of proposal / agreeement BDPUs on p2p links instead of relying on timers. But here you are using PVST so you rely on timers in any case.

Hope to help

Giuseppe