You may look at that feature's name and think, "What is a BPDU Skew,
and why do I want to detect it?" What we're actually attempting to
detect are BPDUs that aren't being relayed as quickly as they should be.
After the root bridge election, the root bridge transmits BPDUs, and
the non-root switches relay that BPDU down the STP tree. This should
happen quickly all around, since the root bridge will be sending a BPDU
every two seconds by default ("hello time"), and the switches should
relay the BDPUs fast enough so every switch is seeing a BPDU every two
seconds.
That's in a perfect world, though, and there are plenty of imperfect
networks out there! You may have a busy switch that can't spare the CPU
to relay the BDPU quickly, or a BPDU may just simply be lost in
transmission. That two-second hello time value doesn't give the
switches much leeway, but we don't want the STP topology recalculated
unnecessarily either.
BDPU Skew Detection is strictly a notification feature. Skew Detection
will not take action to prevent STP recalculation when BDPUs are not
being relayed quickly enough by the switches, but it will send a syslog
message informing the network administrator of the problem. The amount
of time between when the BDPU should have arrived and when it did
arrive is referred to as "skew time" or "BPDU latency".
A busy CPU could quickly find itself overwhelmed if it had to send a
syslog message for every BPDU delivery that's skewed. The syslog
messages will be limited to one every 60 seconds, unless the "skew
time" is at a critical level. In that case, the syslog message will be
sent immediately with no one-per-minute limit.
And what is "critical", according to BDPU Skew Detection? Any value
greater than 1/2 of the MaxAge value, making the critical skew time
level 10 seconds or greater.