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In number of years ago, I had a customer who had a connection
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from their head office to a data center in London and they raise the tickets
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stating that the connection to the data center was very slow.
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They were connecting to router with the VPN to the data center
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and throughput was very poor, in other words they could ping the data center without
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any problems but when they sent a large files, the connection were very, very slow.
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I went to site and on investigation I found that there was a duplex mismatch
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between the Cisco router and the Cisco switch.
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Duplex mismatch is used to cause a lot of problems in the past.
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The negotiation of duplex often failed on 10100 links and in those days
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it was recommended that your hard code speed and duplex on links
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between devices such as Cisco routers and switches.
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Today fortunately things are lot better.
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Today it’s generally recommended that you allow switches
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and routers to use autonegotiation of duplex as well as speed.
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Now in some cases you may want hard code the speed and duplex
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between your switches and your routers but be aware
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that if you configure it incorrectly you may end up with an issue such as the following.
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notice the switch is stating that there’s a duplex mismatch on F0/4
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to another switch on F0/4
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On this router we have a duplex mismatch on F0 to this switch on F0/3.
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in this video I’m not using GNS3 I’m using physical switches
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and I’m simply using GNS3 to show you what the topology looks like.
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I have 2 routers each and turn connected to 2950 switches which are connected
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to 3750 switches and I have a laptop connected on port F0/5 to this first 3750 switch.
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So let’s have a look at some of the issues you may encounter with duplex mismatches.
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remember that with full duplex a device can transmit and receive at the same time
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where’s with half duplex only 1 side can send, in other words a device cannot send
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and receive simultaneously with half duplex 1 party transmits
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and 1 party receives with full duplex both parties can transmit
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and receive at the same time, half duplex was required for connections to hubs.
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These days because of the use of Ethernet switches
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we generally wanna use full duplex, which means as an example on this 100 Mbps link
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the switch can send traffic to the core switch at 100 Mbps
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and at the same time the core switch can send traffic to the 2950 switch at 100 Mbps.
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so we actually have the throughput in theory of 200 Mbps,
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100 in each direction when using full duplex.
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now I've cabled this link here to show you a problem with duplex between 2 2950's
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and I'll also demonstrate autonegotiation of speed and duplex between 3750's and 2950's
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as well as a PC and the switch but before we do that let's have a look
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at some of the problems you should may encounter
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when you have a duplex mismatch as mentioned your throughput maybe a problem.
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