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In this package to trace a lab we're going to discuss spanning tree port font what is a spending tree
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port Fost and why is it important.
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Why do we use spanning tree port Fost in switched networks now on Cisco's Web site where they discuss
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various options that you can configure with spanning tree including Port fast and they say that port
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font causes a switch or trunk port to enter the spanning tree forwarding state immediately bypassing
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the listening and learning states.
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You can configure spanning tree port fast on switch or trunk ports that are connected to a single workstation
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switch or server to allow those devices to connect to the network immediately instead of waiting for
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the port to transition from the listening to learning to the forwarding state.
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But you need to be careful with spanning tree port fast if you enable spending to report fast on a port
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that's connected to another layer to device such as a switch you may create network loops.
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So let's see the problem practically in this network.
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I'm going to power cycle the devices in the topology and then on P.S. 1 I'm going to open up a command
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prompt and use the command IP config.
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Notice these ports are orange on the switch.
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So if I use the command IP conflict slash renew the P.C. is not going to get an IP address because these
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ports are currently blocked by spending three under the switch show spanning tree.
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Notice of the ports are currently in the learning state spending trees just gone green.
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But notice in the output of my switch I see that they are currently in the learning state so this command
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shows us the state of ports do that again.
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We can see that the ports are now forwarding but while the switch was transitioning from blocking to
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listening to learning to forwarding the piece he wasn't able to get an IP address so would end up using
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an address in this range later on the piece he would get an IP address if it continues to send DCP request
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messages.
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But you can have problems with applications on your pieces if it takes 30 seconds for a P.C. to receive
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an IP address.
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So here again the piece he has got an IP address.
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But if I power cycle these devices while the switches booting up and while the ports are transitioning
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through the spending tree states like they are now.
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So again back on the switch the switch has no configuration show spanning tree.
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Port is currently in the listening state and then should go to the learning state as you can see there
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and then we'll eventually go to the forwarding state.
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But during that time pieces will not be able to receive IP addresses or forward traffic windows P.S.
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as an example won't be able to log on to the domain it's using one of these IP addresses instead of
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the IP address allocated by the DHB server now 169 dot to 5 4 dot something that something is a IP version
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4 linked local address.
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In other words an IP address in this range 169 dot to 5 4 slash 16 is an IP version 4 link local address.
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This is similar in concept to FTD colon colon slashed 10 in IP version 6.
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The idea is that if a P.C. doesn't get an IP address from a DHB server it will use an IP address and
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thus range so that two pieces can still communicate with one another if they for instance connected
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to a hub or a switch and there's no DHB server on the network so in other words that's what this piece
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has done.
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It used an IP address in this range but that means that it won't be able to communicate with other servers
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on the network unless it continues to send a DCP requests and then gets an IP address in the correct
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range as it did here.
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The P.C. won't be able to communicate with other devices such as domain controllers DNS servers or even
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connect it to the Internet.
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This is a non readable IP address.
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So how do we fix this.
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The answer is to use Port Fost so can you configured this network so that the pieces receive IP addresses
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from the DHB survey immediately.
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The server here is configured as a DCP server so the pieces should receive IP addresses from the DHB
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server as soon as they boot up.
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So can you complete this lab yourself.
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Download the packet trace a file and see if you can complete to the lab.
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Otherwise continue watching as I complete to the lab.
6310
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