So, does STP just not work on the Small Business Switches?

Question

Hi All,
I have an SG500-52P switch, and a catalyst 3650 switch.  I want to connect two links between the switches, and have STP block one of them for redundancy in the event of a link failure.  When I set this up, I got a duplicate IP address discovered message in the 500 switch, followed by the network crashing/locking up from what I can only assume was an STP loop.  Show spanning-tree on the 3650 before the crash showed that both of the ports were in FWD state - meaning that STP was not blocking redundant paths.
Verification that STP is running and BPDU's are flooding on the 500 can be found below:
SW500A#show spanning-tree
Spanning tree enabled mode RSTP
Default port cost method:  long
  Root ID    Priority    24577
             Address     a0:ec:f9:ef:6a:00
             Cost        20000
             Port        gi1/1/43
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32768
             Address     2c:3e:cf:ff:11:82
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
SW500A#show spanning-tree bpdu
Global: Flooding
SW3650#show spanning-tree
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     a0ec.f9ef.6a00
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     a0ec.f9ef.6a00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
SW3650#show spanning-tree  sum
Switch is in pvst mode
Root bridge for: VLAN0001, VLAN0003
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
The only thing I see is that the cost method in the 500 is long, while the cost method in the 3650 is short.
Am I missing something?

Answer 1

Hi
from your outputs seems that both switches understand each-other BPDU's (at least via one link!), because SW500A is not root bridge for that segment and accepts SW3650 to be root bridge.
> spanning-tree on the 3650 before the crash showed that both of the ports were in FWD state
I hope this is correct behavior as SW3650 is root bridge. This means that all ports on root bridge are designated ports and thus are in FWD state.
The switch-port which should be blocked must be on opposite (non-root) switch SW500A.
But be careful in your scenario: there could be potentially compatibility issue as:
SW500A is using Rapid STP (single - common - instance over all VLAN's)
SW3650 is using PVST+ (Per VLAN Spanning Tree)
the compatibility works in this scenario only in case, that all links between SW3650  and SW500A use VLAN1 as untagged VLAN.
That means: to allow Cisco SW3650 switch running rapid PVST+ or PVST+ to form a common spanning tree with SW500A switch running RSTP or STP, vlan1 (the native VLAN) must be configured as untagged on the SW3650 ports connected to SW500A switch.
If this is not met, it can leads to switching loops. And that could be your case.