Unidirectional Link; Packet Corruption - D-Link DES-3350SR User Manual

D-Link DES-3350SR Standalone Layer 3 Switch
Full/Half Duplex Mismatch
A mismatch in the duplex state of two ports is a very common configuration error for a point-to-point link. If one port is configured
as a full duplex, and the other port is left in auto-negotiation mode, the second port will end up in half-duplex because ports
configured as half- or full-duplex do not negotiate.
In the above example, port 1 on B is configured as a full-duplex port and port 1 on A is either configured as a half-duplex port, or
left in auto-negotiation mode. Because port 1 on B is configured as a full-duplex port, it does not do the carrier sense when
accessing the link. B will then start sending packets even if A is using the link. A will then detect collisions and begin to run the
flow control algorithm. If there is enough traffic between B and A, all packets (including BPDUs) will be dropped. If the BPDUs
sent from A to B are dropped for longer than the MAX AGE, B will lose its connection to the root (A) and will unblock its
connection to C. This will lead to a data loop.

Unidirectional Link

Unidirectional links can be caused by an undetected failure in one side of a fiber cable, or a problem with a ports transceiver. Any
failure that allows a link to remain up while providing one-way communication is very dangerous for STP.
In this example, port 2 on B can receive but not transmit packets. Port 2 on C should be in the blocking state, but since it can no
longer receive BPDUs from port 2 on B, it will transition to the forwarding state. If the failure exists at boot, STP will not converge
and rebooting the bridges will have no effect. (Note: Rebooting would help temporarily in the previous example).
This type of failure is difficult to detect because the Link-state LEDs for Ethernet links rely on the transmit side of the cable to
detect a link. If a unidirectional failure on a link is suspected, it is usually required to go to the console or other management
software and look at the packets received and transmitted for the port. A unidirectional port will have many packets transmitted but
none received, or vice versa, for example.

Packet Corruption

Packet corruption can lead to the same type of failure. If a link is experiencing a high rate of physical errors, a large number of
consecutive BPDUs can be dropped and a port in the blocking state would transition to the forwarding state. The blocking port
would have to have the BPDUs dropped for 50 seconds (at the default settings) and a single BPDU would reset the timer. If the
MAX AGE is set too low, this time is reduced.
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