Original subtitles

In this package to trace a lab we're going to discuss spanning tree port font what is a spending tree

port Fost and why is it important.

Why do we use spanning tree port Fost in switched networks now on Cisco's Web site where they discuss

various options that you can configure with spanning tree including Port fast and they say that port

font causes a switch or trunk port to enter the spanning tree forwarding state immediately bypassing

the listening and learning states.

You can configure spanning tree port fast on switch or trunk ports that are connected to a single workstation

switch or server to allow those devices to connect to the network immediately instead of waiting for

the port to transition from the listening to learning to the forwarding state.

But you need to be careful with spanning tree port fast if you enable spending to report fast on a port

that's connected to another layer to device such as a switch you may create network loops.

So let's see the problem practically in this network.

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

prompt and use the command IP config.

Notice these ports are orange on the switch.

So if I use the command IP conflict slash renew the P.C. is not going to get an IP address because these

ports are currently blocked by spending three under the switch show spanning tree.

Notice of the ports are currently in the learning state spending trees just gone green.

But notice in the output of my switch I see that they are currently in the learning state so this command

shows us the state of ports do that again.

We can see that the ports are now forwarding but while the switch was transitioning from blocking to

listening to learning to forwarding the piece he wasn't able to get an IP address so would end up using

an address in this range later on the piece he would get an IP address if it continues to send DCP request

messages.

But you can have problems with applications on your pieces if it takes 30 seconds for a P.C. to receive

an IP address.

So here again the piece he has got an IP address.

But if I power cycle these devices while the switches booting up and while the ports are transitioning

through the spending tree states like they are now.

So again back on the switch the switch has no configuration show spanning tree.

Port is currently in the listening state and then should go to the learning state as you can see there

and then we'll eventually go to the forwarding state.

But during that time pieces will not be able to receive IP addresses or forward traffic windows P.S.

as an example won't be able to log on to the domain it's using one of these IP addresses instead of

the IP address allocated by the DHB server now 169 dot to 5 4 dot something that something is a IP version

4 linked local address.

In other words an IP address in this range 169 dot to 5 4 slash 16 is an IP version 4 link local address.

This is similar in concept to FTD colon colon slashed 10 in IP version 6.

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

thus range so that two pieces can still communicate with one another if they for instance connected

to a hub or a switch and there's no DHB server on the network so in other words that's what this piece

has done.

It used an IP address in this range but that means that it won't be able to communicate with other servers

on the network unless it continues to send a DCP requests and then gets an IP address in the correct

range as it did here.

The P.C. won't be able to communicate with other devices such as domain controllers DNS servers or even

connect it to the Internet.

This is a non readable IP address.

So how do we fix this.

The answer is to use Port Fost so can you configured this network so that the pieces receive IP addresses

from the DHB survey immediately.

The server here is configured as a DCP server so the pieces should receive IP addresses from the DHB

server as soon as they boot up.

So can you complete this lab yourself.

Download the packet trace a file and see if you can complete to the lab.

Otherwise continue watching as I complete to the lab.