A number of years ago I had a customer who had a connection from the head office to a data center in
London and they raised a ticket stating that the connection to the data center was very slow.
They were connecting to a router with a 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 large files the connections were very very slow.
I went to site and on investigation I found that there was a duplex mismatch between a Cisco router
and a Cisco switch. Duplex mismatches used to cause a lot of problems in the past.
The negotiation of Duplex often failed on 10 100 links
and in those days it was recommended that you hard code speed and duplex on links between devices such
as Cisco routers and switches. Today, fortunately, things are a lot better today it's generally recommended
that you allow switches and routers to use auto-negotiation of duplex as well as speed.
Now in some cases, you may wannna hard code the speed and duplex between your switches and your routers
but be aware that if you configure it incorrectly you may end up with an issue such as the following.
Notice the switch is stating that there is a duplex mismatch on Fast
Ethernet 04 to another switch on FastEthernet 04 on the router.
We have a duplex mismatch on FastEthernet0 to the switch on Fast
Ethernet 0/3. In this video
I'm not using GNS3 I'm using physical switches and I'm simply using GNS3 to show you what the
topology looks like.
I have two routers each in turn connected to 2950 switches which are connected to 3750 switches and
I have a laptop connected on port F 0/5 to the first 3750 switch.
So let's have a look at some of the issues you may encounter with Duplex mismatches.
Remember that with full-duplex a device can transmit and receive at the same time.
Whereas with half-duplex only one side can send in other words a device cannot send and receive simultaneously
with half-duplex one party transmits and one party receives with full-duplex
both parties can transmit and receive at the same time half duplex was required for connections to hubs.
These days because of the use of Ethernet switches
we generally wanna use full-duplex which means as an example on this 100 megabits per second link
this switch can send traffic to the core switch at 100 megabits per second and at the same time the
core switch can send traffic to the 2950 switch at 100 megabits per second.
So we actually have a throughput in theory of 200 megabits per second 100 in each direction when using
full-duplex. Now I've cabled the link here to show you a problem with Duplex between two 2950s
and I'll also demonstrate auto-negotiation of speed and duplex between 3750s and 2950s
as well as a PC and a switch.
But before we do that let's have a look at some of the problems you may encounter when you have a duplex
mismatch.
As mentioned your throughput may be a problem.