Original subtitles

In number of years ago, I had a customer who had a connection

from their head office to a data center in London and they raise the tickets

stating that the connection to the data center was very slow.

They were connecting to router with the VPN to the data center

and throughput was very poor, in other words they could ping the data center without

any problems but when they sent a large files, the connection were very, very slow.

I went to site and on investigation I found that there was a duplex mismatch

between the Cisco router and the Cisco switch.

Duplex mismatch is used to cause a lot of problems in the past. 10

The negotiation of duplex often failed on 10100 links and in those days 11

it was recommended that your hard code speed and duplex on links 12

between devices such as Cisco routers and switches. 13

Today fortunately things are lot better. 14

Today it’s generally recommended that you allow switches 15

and routers to use autonegotiation of duplex as well as speed. 16

Now in some cases you may want hard code the speed and duplex 17

between your switches and your routers but be aware 18

that if you configure it incorrectly you may end up with an issue such as the following. 19

notice the switch is stating that there’s a duplex mismatch on F0/4 20

to another switch on F0/4 21

On this router we have a duplex mismatch on F0 to this switch on F0/3. 22

in this video I’m not using GNS3 I’m using physical switches 23

and I’m simply using GNS3 to show you what the topology looks like. 24

I have 2 routers each and turn connected to 2950 switches which are connected 25

to 3750 switches and I have a laptop connected on port F0/5 to this first 3750 switch. 26

So let’s have a look at some of the issues you may encounter with duplex mismatches. 27

remember that with full duplex a device can transmit and receive at the same time 28

where’s with half duplex only 1 side can send, in other words a device cannot send 29

and receive simultaneously with half duplex 1 party transmits 30

and 1 party receives with full duplex both parties can transmit 31

and receive at the same time, half duplex was required for connections to hubs. 32

These days because of the use of Ethernet switches 33

we generally wanna use full duplex, which means as an example on this 100 Mbps link 34

the switch can send traffic to the core switch at 100 Mbps 35

and at the same time the core switch can send traffic to the 2950 switch at 100 Mbps. 36

so we actually have the throughput in theory of 200 Mbps, 37

100 in each direction when using full duplex. 38

now I've cabled this link here to show you a problem with duplex between 2 2950's 39

and I'll also demonstrate autonegotiation of speed and duplex between 3750's and 2950's 40

as well as a PC and the switch but before we do that let's have a look 41

at some of the problems you should may encounter 42

when you have a duplex mismatch as mentioned your throughput maybe a problem.