Certification Tests
Cisco CCNP / BCMSN Exam Tutorial: BPDU Skew Detection And Cisco Switches
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.
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNP and CCNA tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.
You can also join his RSS feed and visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions! Details are on the website.
For a FREE copy of his latest e-books, ?How To Pass The CCNA? and ?How To Pass The CCNP?, just visit the website! You can also get FREE CCNA and CCNP exam questions every day! Pass the CCNP exam with The Bryant Advantage!
Chris Bryant
Similar articles
Cisco CCNA Exam Tutorial And Case Study: VLANs and IP Connectivity
In this CCNA case study, we'll take some basic switching and trunking theory and put it into action. We have two routers (R2 and R3) along with two switches (SW1 and SW2). Read more →Cisco CCNA Exam Tutorial: IGRP And Equal Cost Load Balancing
To pass the CCNA exam, you've got to know the role of the bandwidth command with IGRP and EIGRP and when to use it. In this tutorial, we'll configure IGRP over a frame relay hub-and-spoke network using the following networks:R1 (the hub), R2, and R3 are running IGRP over the 172. Read more →Cisco CCNP / BCMSN Exam Tutorial: The Core Layer Of Cisco's Three-Layer Model
In this section, you're going to be reintroduced to a networking model you first saw in your CCNA studies. No, it's not the OSI model or the TCP/IP model - it's the Cisco Three-Layer Hierarchical Model. Read more →Cisco CCNP / BSCI Certification: Introduction To ISIS Terminology
When you're studying to pass the BSCI exam and earn your CCNP certification, you're going to be introduced to ISIS. ISIS and OSPF are both link-state protocols, but ISIS works quite differently from OSPF. Read more →Aphorism
If there is technological advance without social advance, there is, almost automatically, an increase in human misery, in impoverishment.