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BlueWolf's Howl

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December 20, 2005

Catalyst Switching and Books

So now that the pressure is off and the BCRAN is finally out of the way... it's time to relax, right? Well, I have actually been relaxing more, but I still can't help myself... I started reading Cisco Catalyst LAN Switching by Louis Rossi.
Catalyst LAN Switching

I found this book in the 'on sale' pile at Microcenter. It's normally a $55 book - and I scooped it up for $3.99. The reason for the drastic price drop is that it is now out-of-print. It was written in 2000 and is a bit dated, but still a worthy read nontheless. It has been on my bookshelf as a 'want to get to it sometime' book for months. [There are a number of those books.]

I've found that the best way for me to read, understand and remember some of this stuff is to see it over and over. Reading 3 or 5 books on the same topic is much better (for me) than reading one book carefully and trying to remember it. Perhaps it's because I'm reading it 'recreationally' without pressure - and it tends to soak in more? Or perhaps it's the repetition that makes it stick? Either way, I know it's helpful. [YMMV]

Some of the things that I read before that finally stuck in my head while I was reading this one...

Portfast -- I love portfast. It's a command that you use on a port that is (and only will be) connected to an end user station. When a user boots up and the port activates, the user doesn't have to wait for the port to go through the spanning tree process to figure out that it's not connected to a switch. I have seen this cause some users to not be able to connect to the network properly - and normally it's the admin helper of the most powerful person at the site. One of the things that I didn't remember about it (or didn't know) was that portfast does not disable the spanning tree protocol for that port. It allows packets to be forwarded while the spanning tree algorithm is being run. If, by some odd chance, some bonehead actually put something there that would cause a loop, the STP algorithm would eventually detect it and put the port in blocking mode. So you would have a few looped packets that would eventually fade out and it would not allow someone to break your network on you.

Nice to know when you run into this in the field. Perhaps you notice that a network doesn't have portfast on the user ports. The admin is leary about putting something like this on his/her switch. Now I can explain it on a level where I know what to do, why to do it, and what will happen in any case.

Port mirroring or spanning -- I knew you could do this and found that you can set it to span TX | RX | Both. That part I didn't know. Luckily... if you don't specify transmit or receive, the default is Both... [Port spanning is used on a switch when you put a sniffer on it - so that it can see all the traffic from all the ports.]

Fast EtherChannel | Gigabit EtherChannel -- This is sooooo cool. The Spanning Tree Protocol (STP) will only allow ONE link to the root bridge. If you try to put a second link up for redundancy or more bandwidth, it will block it to prevent the loop. Fast EtherChannel allows you to bundle multiple links to get more bandwidth. Granted, it's not easy to do. You have to make sure that the ports you're using are on the same part of the switch backplane fabric or else it won't work. But, it is still possible to bundle a few ports together to get more bandwidth for your uplink. And the STP sees that bundle as ONE link. You might think this would be important at a large site where internal bandwidth would be needed. But, at such a place, it might be easier to justify (and afford) a larger/better switch. I can see this being important at a smaller site that's using older equipment and can't afford to buy an adequate switch. On top of that, such a place would probably have someone in charge of the switches that doesn't know much about networking and wouldn't know this exists, much less how to implement it. Nice idea to have in your bag of tricks. Someday it could be helpful.

Interface sl0 - I've seen this before. It's one of those ports that 'you just leave it alone'... Well, I finally found out that it's for a SLIP connection to the catalyst supervisor engine. The book only covered SE I's to SE III's -- and I've already dealt with a SE IV in the field. But, it's still good to know what the heck that sl0 is and what it's for....


Speaking of books at a great price... Check out CISCO PRESS!!! If you buy 3 books, you get all your books at half price! This offer ends on 31 Dec 05. Yes, I already stocked up on the books I wanted. I'm using the "buy now, read later" plan. In addition to the effort it takes to read all these books, it also takes quite a bit of coin. Computer books are usually in the $50 - $75 range. It's nice to see a deal like that - not to mention the current slim selection at the local bookstores... On top of that - after I submitted my order, I got a coupon for 35% off my next order. Sweeeet!

Posted by BlueWolf on December 20, 2005 02:19 PM