Information Technology Reference
In-Depth Information
The worst-case scenario is when a bridge sending BPDUs is configured for half-duplex operation on a
link, whereas its peer is configured for full-duplex mode. In Figure 23-10, the duplex mismatch on the
link between bridges A and B can easily lead to a bridging loop. Because Bridge B is configured for
full-duplex operation, it does not perform carrier sense when accessing the link. Bridge B will then start
sending frames even if Bridge A is already using the link. This is a problem for Bridge A, which detects
a collision and runs the back-off algorithm before attempting another transmission of its frame. The
result is that, if there is enough traffic from B to A, every single packet (including the BPDUs) sent by
Bridge A will be deferred or collisioned and eventually dropped. From an STP point of view, because it
does not receive BPDUs from Bridge A anymore, Bridge B has lost its root. This leads Bridge B to
unblock its port to Bridge C, hence creating the loop.
Figure23-10Bridging Loop Created by a Duplex Mismatch
Half-duplex:
Still runs carrier sense
and collision detection
Doesn't do
carrier sense
A (root)
Half-duplex
B
Full-duplex
Collision
BPDU lost
To be retransmitted
C
Unidirectional Link
This is a very frequent cause for a bridging loop. Unidirectional links are often caused by a failure not
detected on a fiber link, for instance, or a problem with a transceiver. Anything that can lead a link to
stay up while providing a one-way communication is very dangerous as far as STP is concerned. The
example shown in Figure 23-11 is very straightforward.
