I finished the STP chapter in the CCIE exam cert guide, and was almost disappointed. I have a hard time believing anyone could expect to be fully prepared for the written exam after reading that chapter. I would highly recommend that anyone taking the written give a strong look at the LAN Switching book from cisco press, Chapters 6 and 7, which both cover STP and Advanced STP. This book is FANTASTIC as far as STP theory goes. Blows the others out of the water. Granted, the config examples are done in CatOS, but ignore those..use the BCMSN book for configuration, LAN Switching for theory, and the CCIE exam cert guide for review!

I knocked out chapter 6 of LAN Switching, and part of chapter 7. Once I finish Chapter 7, I’m going to read up on a lot of MSTP, and do some STP labs in the next few days. I’d say that I’ll call STP “done” in about 3-5 days, after which it’ll be onto some IP Addressing review, and then EIGRP. If I can maintain this pace, I think I might be able to bump up my written date, which would give me more time to prep for the lab. We’ll see! Stay posted, I may put some STP notes up tomorrow..

Uplinkfast. How many of you can honestly say you’ve used it? Some of you? Most of you? Ok, well I guess I am the minority! I have seen spanning-tree uplinkfast used in configs, but never took the time to really understand the effects of it. The idea is simple enough. You have an access layer switch, with redunant links to the distribution layer. What if one of those links fail? Yes, the access switch will change it’s root port to the other (still working) link, but only after transitioning through the listening and learning states. The length of time it will take is based on the forward timer, which by default is 15 seconds. The default time to transition from blocking to forwarding can take up to 50 seconds. What uplinkfast will do is transition within 5 seconds from the failed link to the new link (changing it’s root port in the process). Much better!

Now, onto the actual effects of uplinkfast. First, uplinkfast will raise the STP priority from the default 32,768 to 49,152 for all VLAN’s. The best way to understand the logic behind this is to figure that by configuring uplinkfast on a switch, you are essentially saying “this switch is an access layer switch”..since uplinkfast is designed for access/edge devices. The higher priority will prevent this switch from becoming the root switch for the STP instance. As you can imagine, having an access-layer switch become the root switch is a very poor design!

The second effect of uplinkfast is that the STP path costs will be changed to 3000 + the current cost (assuming they are the default, we’ll say 19 for a 100 meg link)..this means the costs will be 3019. Why? This will also prevent the access switch from becoming somewhat of a transit point for traffic, because the cost will be so high that every other link into the core will appear much more attractive.

Finally, it’s worth mentioning that when uplinkfast transitions the new root port from blocking to forwarding, it will send out station-learning  multicast frames at a default of 150 packets per second. You can adjust this flood of multicast traffic in the following way: spanning-tree max-update-rate {0-32000}

Note: If you configure the max-update-rate to 0, no station-learning frames will be sent out. This will slow the convergence after a link failure.

There you have it! I was  talking to a buddy of mine about these bits of information regarding the path cost, and decided it’d be good to put it up for others to benefit from. Enjoy! Time to get back to STP and all the details.