So we now need to fix this link between 2951 and 2952.
Both switches are showing.
Messages that there's a duplex problem.
So CDP does a nice job here.
Of showing us that there's a problem.
Rather show run interface.
f0/4.
We can see the speed and duplex are hardcoded.
Show run interface if zero.
Slash four.
On this side.
It's one half duplex.
On this side, it's 100 full duplex.
So that's a simple fix of setting the duplex to full.
The interface has gone down and comes up again.
And hopefully that's resolved the problem.
We could clear the counters as an example.
On if 0/4 and do the same here.
Clear counters if 0/4.
And then we could, after a while, look at those counters and see if we find any problems.
But that should have solved the problem of a duplex mismatch.
To confirm it.
Show Interface f04.
Right at the top here.
We can see.
So here's the command.
Here's the speed and duplex of the interface.
Interface is up, up, and it's connected.
Show interface if 0/4 on switch to.
He has the command once again.
Duplex and speed are shown here as well.
Interfaces up, up and it's connected.
We're not seeing any errors on the interface and on this one we are also not seeing any errors.
So that hopefully has resolved the problem on this link and this link.
So be careful.
Hard coding, speed and duplex auto negotiation may negotiate correctly, but it may not generally today
with gigabit and foster links.
Your problems are not as bad as they used to be.
On ten 100 links.
Let's see what happens if we configure speed and duplex on one side and leave the other side for auto
negotiation.
So on 2951, we will configure this interface.
With a speed of 100.
Duplex of full interface has gone down.
Interface will hopefully come up and there you go.
Here is the.
3750.
And we can see that it's complaining about a duplex mismatch.
So sho run interface f zero.
If 103 no duplex or speed has been configured on this interface show interface f103 the site is using
100 meg Hoff duplex whereas this site.
Is using 100 meg full duplex.
So if you have one side configured for auto negotiation and the other side is hardcoded.
You may have a problem because the site is not negotiating with the other side and the side may take
a fallback position.
Sometimes it's able, especially on gigabit and later interfaces, to still work out what the other
side is using and set its duplex and speed accordingly.
But here it wasn't able to do that because the 2950s are older switches.
So to resolve this, let's configure the 2954 auto negotiation.
So if 0/1 this interface speed.
Auto.
Duplex.
Order.
Interface has gone down.
Hopefully it'll come up.
And hopefully they'll be able to negotiate successfully.
So 3750 is showing up.
2950 showing up.
Show interface f103.
Full duplex 100 meg.
So that site is 100 meg.
Full duplex to show interface f01.
On this side, full duplex 100 meg.
So they've negotiated successfully.
So moral of the story is either configure both sides for auto negotiation or hard code.
Both sides.
Generally today, you're going to allow both sides to want to negotiate.
So run interface f101.
Show run interface f101.
Now this is the link between the two 3750s f101.
This interface is and currently connected.
So we've got one interface f101 between the two 3750s.
One side is not set for speed and duplex, it's set for auto.
The other side is using 100 half.
But in this case.
The site set for auto negotiation has reverted to 100 Meg Hoff duplex.
So that's worked out okay in this example.
However, if I change this to have a duplex or full, the negotiation will probably fail and we'll have
a duplex mismatch.
Interfaces have come up.
Let's see what it's done.
In this example it's negotiated correctly full duplex 100 meg the side DU Show Interface F1 zero one.
Full duplex a100 meg.
So we manually configured the side with speed and duplex this side.
Has been able to work out what the speed and duplex should be.
So let's set the speed here to ten and the duplex to half.
And let's see if the switches can successfully negotiate that.
So interfaces come up.
Show interface f101.
And I'll simply include the duplex so we don't see all the output.
Notice here it's negotiated properly.
So on this switch.
It's been manually configured for half duplex ten meg and this switch has successfully negotiated that.
If I change the duplex here to full, let's see what happens now when the interface comes up.
So here it's ten meg full duplex.
On this side it's also ten meg full duplex.
So the 3750s are negotiating correctly without any problems.
But that wasn't happening between a 3750 and a 2950.
So let's set the speed to 100.
So show interface and look at the duplex that's been set.
Try that again.
100 full duplex on the side.
100 full duplex.
So in this case, when I set the one side, it's successfully negotiating with the other side.
But we had problems with negotiation between the 3750 and 2950.
So generally the moral of the story is or to negotiation on both sides or hard code on both sides.
Mixing and matching can cause problems.
Today the general recommendation is use auto everywhere except perhaps between core links where you
want a hard code, both sides to increase the speed of that interface coming up.