So this is the conclusion of our Spanning Tree basics section, we just talked about the STP cost values for some of the more common port speeds and so let's carry on with our discussion. Now we mentioned earlier in this section some of the timers and most specifically the Forward Delay. We remember that it sits in the Listening or Learning State for 15 seconds and of course all these timers are customizable but again most people leave these at the defaults, it just makes troubleshooting easier if you have to bring in a consultant or you have to call Cisco for an issue. The two other timers that you'll need to look at and we'll see this in the lab as well are the Hello Timers, that's the time between the BPDU Hello Packets, defaults to 2 seconds. And the Max Age Timer which is how long a switch misses hellos before it says, hey that switch has gone down and I need to figure out a new path to get to that switch, 10 times the Hello Timer or 20 seconds. Now you can see if you add all these timers up it has to miss 20 seconds worth of hellos before the switch realizes, hey I need to bring up another port. At which point it brings that port up and it goes to Listening and then Learning, you add all this up, you could have 50, 60 seconds of down time in your network just for Spanning Tree to realize that something bad's happened and we need to find another path around the network. Now back in the olden days before you had Voice Over IP and these really latency and bandwidth intolerant applications, it was no big deal. You had an IP Stack and you know, hey if your file transfer timed out, well you just restart the file transfer. If your IP Stack was robust enough, you may just notice that your file transfer just stopped a third of the way through for about a minute and then it carried on and you may not even realize that there's a network problem. However if you've got a big Voice Over IP deployment and your entire LAN goes down for 50 seconds or 30 seconds or really even 10 seconds, your cell phones going to start ringing of the hook. Hey I was on this call and it dropped, every phone in the building just went offline, they're all registering again. If you've got monitoring software set up, you'll start getting alerts saying every phone in the building just unregistered, all kinds of nasty things happen on a modern network if you have a minute or two of complete network outage. And it's for this reason that the IEEE in conjunction with the major network vendors, Cisco, 3Com, HP et cetera, came up with RSTP or the Rapid Spanning Tree Protocol. Now we'll get into RSTP elsewhere in the course after we build regular Spanning Tree infrastructure in our lab and we see what happens when we run a ping and we just unplug one of the network cables. And I think we'll all agree that Rapid Spanning Tree is preferred in a modern network but for now that concludes our discussion of Spanning Tree basics.
|Course:||Implementing Cisco IP Switched Networks (642-813 SWITCH)|
|Duration:||8.5 hrs / 102 lessons|
Vista/XP/2000, OS X, Linux
QuickTime 7, Flash 8
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