Symptoms of STP failure and consequent switching loops include:

- Unusually high backplane utilization due to forwarding frames at line
speed.
- Rapid address re-learning, as the loop will allow a switch to see the same
source address entering on multiple ports.
- Rapidly incrementing frame counters on the affected ports as they receive
frames at line speed.
- Extremely poor link performance, high latency on ping
replies, and TCP timeouts, as this traffic has to compete with broadcast frames
caught in the loop.
- Broadcast storms within a Layer 2 domain. Since broadcasts will be
perpetuated by a loop, their effect will be felt throughout the switched
network.
How is it possible for the switch to stop receiving BPDUs while the
port is up? The most obvious answer is that the STP has been turned off.
Another common cause is the unidirectional link. A link is considered
unidirectional when:
- Links are up on both sides of the connection.
- Local side is not receiving the packets sent by remote side.
- Remote side receives packets sent by local side.
Consider the scenario in Figure
, the
arrows indicate the flow of STP BPDUs.
During normal operation, bridge B
is designated on the link B-C. Bridge B sends BPDUs down to C, which is
blocking the port. The port is blocked while C is seeing BPDUs from B on that
link.
Now, consider what happens if the link B-C fails in the direction
of C.
C will stop
receiving traffic from B, but B will still receive traffic from C.
C
will stop receiving BPDUs on the link B-C, and will age the information
received with the last BPDU. This will take up to 20 seconds, depending on the
maxAge STP timer. Once the STP information is aged out on the port, it will
transition from blocking state to listening, learning, and eventually to
forwarding STP state. This will create a forwarding loop, as there will be no
blocking port in the triangle A-B-C. Packets will cycle along the path (B still
receives packets from C) taking more and more bandwidth until the links will be
filled up completely bringing the network down.