Unidirectional Link; Packet Corruption; Resource Errors - D-Link DGS-3308 Series User Manual

8-port Gigabit Ethernet Switch User's Guide
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 o ne 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.

Resource Errors

The DGS-3308FG/DGS-3308TG Layer 3 switch performs its switching and routing functions primarily in hardware, using
specialized ASICs. STP is implemented in software and is thus reliant upon the speed of the CPU and other factors to
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