Information Technology Reference
In-Depth Information
This section is not intended to be a complete LAN design guide, but just a list of recommendations that
will help in implementing a safe network as far as bridging is concerned. Assuming knowledge of the
protocol itself, we will introduce the following topics:
•
The reasons that can cause the STP to fail
•
What information to look for to identify the source of the problem
•
What kind of design minimizes spanning-tree risks and is easy to troubleshoot
Spanning-Tree Protocol Failure
A failure in the STA generally leads to a bridging loop (not a spanning-tree loop because you don't need
STP to have a loop). Most customers calling the TAC for spanning-tree problems suspect a bug, but
experience proves that it is seldom the case. Even if the software is at stake, a bridging loop in an STP
environment necessarily comes from a port that should block but that is forwarding traffic.
What can cause a blocked port to go to forwarding? Let's first recall why a port ends up in a blocking
state. Each LAN has a single designated bridge. This bridge is responsible for the connectivity of the
LAN toward the root bridge.
In Figure 23-9, Bridge B has been elected as the designated bridge, and Bridge C is blocking because it
is only providing an alternate path to the root. Why is Bridget C blocking, not Bridge B? This is
determined practically by the BPDUs that B and C exchange on the LAN. Here, Bridget B had a better
BPDU than Bridge C. Bridge B keeps sending BPDUs advertising its superiority over the other bridges
on this LAN. If Bridge C fails to receive these BPDUs for a certain period of time (called the max
age—20 seconds, by default), it would start a transition to the forwarding mode.
Figure23-9 Blocked Port on Bridge C Keeps Receiving BPDUs from Bridge B
A
Root
B
designated
C
blocked
BPDU
Important note:
A port must keep receiving superior BPDUs to stay in blocking mode.
The following subsections list the different situations that can lead the STA to fail. Most of these failures
are, in fact, related to a massive loss of BPDUs, causing blocked ports to transition to forwarding mode.
Duplex Mismatch
Duplex mismatch on a point-to-point link is a very common configuration error. This occurs especially
when one side of the link is hard-coded as full duplex. If you leave the other side in autonegotiation
mode, it will end up in half-duplex mode (a port with duplex hard-coded does not negotiate anymore).
