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.

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

Suitable only for switch ports connected directly to a single host, PortFast allows a port running STP to go directly from blocking to forwarding mode.

A Cisco router will give you a warning when you configure PortFast:

SW1(config)#int fast 0/5

SW1(config-if)#

Not only will the switch warn you about the proper usage of PortFast, but you must put the port into access mode before PortFast will take effect.

Now, youd think that would be enough of a warning, right? But there is a chance – just a chance – that someone is going to manage to connect a switch to a port running Portfast. That could lead to two major problems, the first being the formation of a switching loop. Remember, the reason we have listening and learning modes is to help prevent switching loops. The next problem is that there could be a new root bridge elected – and it could be a switch that isnt even in your network!

BPDU Guard protects against this disastrous possibility. If any BPDU comes in on a port thats running BPDU Guard, the port will be shut down and placed into error disabled state, shown on the switch as err-disabled. A port placed in err-disabled state must be reopened manually.

BPDU Guard is off on all ports by default, and is enabled as shown here:

SW1(config)#int fast 0/5

SW1(config-if)#spanning-tree bpduguard enable

Its a good idea to enable BPDU Guard on any port youre running PortFast on. Theres no cost in overhead, and it does prevent the possibility of a switch sending BPDUs into a port configured with PortFast – not to mention the possibility of a switch not under your control becoming a root switch to your network!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!