Original subtitles

So we now need to fix this link between 2950-1 and 2950-2

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 sh run int f0/4

We can see the speed and duplex are hard coded.

Sh run int f0/4 on this side it's 100 half duplex, on this side it's 100 full duplex.

So that’s a simple fix of setting a duplex to full.

the interface has gone down and comes up again and hopefully that’s resolve the problem

we could clear the counters as n example on F0/4 and do the same here

clear counters f0/4 and then we could after while look at those counters 10

and see if we find any problems 11

but that should have solve the problem of a duplex mismatch. 12

To confirm it sh int f0/4 rather the tap here we can see, so here’s the command 13

here’s the speed and duplex of the interface. Interface is up, up and it’s connected. 14

sh int f0/4 on switch 2. Here’s the command once again. 15

Duplex and speed are shown here as well. 16

Interface is up, up and it’s connected. 17

We’re not seeing any errors on this interface 18

and on this one, we are also not seeing any errors. 19

So that hopefully has resolve the problem on this link and this link. 20

So be careful hard codings speed and duplex. 21

Auto negotiation may negotiate correctly but it may not, generally today with gigabit 22

and faster links your problem are not as bad as they used to be on 10100 links. 23

Let see what happens if we configure speed and duplex on one side 24

and leave the other side for autonegotiation. 25

So on 2950-1 we will configure this interface with the speed of 100 26

duplex of full interface is gone down, interface will hopefully come up and there you go. 27

Here is the 3750 and we can see that’s it's complaining about a duplex mismatch. 28

So sh run int f0 f1/0/3 no duplex or speed is being configured on this interface. 29

sh int f1/0/3 this side is using 100 Meg half duplex 30

where's this side is using 100 meg full duplex. 31

So if you have 1 side configured for autonegotiation and the other side is hard-coded 32

you may have a problem because this side is not negotiating 33

with the other side and this side may take a full back decision. 34

sometimes it’s able especially on gigabit and later interfaces to still work out 35

what the other side is using and set its duplex and speed accordingly 36

but here it wasn’t able to do that because the 2950's are older switches. 37

So to resolve this, let's configured the 2950 for autonegotiation. 38

So F0/1 this interface speed, auto, duplex, auto interface has gone down 39

hopefully it will come up and hopefully they'll be able to negotiate successfully. 40

S0 3750 showing up, 2950 showing up sh int f1/0/3 41

Full duplex 100 meg, so that side is 100 meg full duplex. 42

do sh int f0/1 on this side full duplex 100 meg. 43

So they’ve negotiated successfully. 44

so moral of the story is either configure both sides for autonegotiation or hard code 45

both sides, generally today you’re going to allow both sides to auto negotiate. 46

sh run int f1/0/1 sh run int f1/0/1 47

now this is the link between the 2 3750's 48

f1/0/1 this interface isn’t currently connected, so we’ve got 1 interface F1/0/1 49

between the 2 3750's 1 side is not set for speed and duplex it's set for auto. 50

The other side is using 100 half but in this case 51

The side set for auto negotiation has reverted to 100 meg half duplex. 52

So that’s work out ok, in this example, however if I change this to have a duplex of full 53

the negotiation will probably fail and will have a duplex mismatch. 54

Interfaces have come up, let’s see what its done. 55

In this example it’s negotiated correctly full duplex 100 meg. 56

This side do sh int f1/0/1 full duplex 100 meg 57

So we manually configured this side with speed and duplex 58

this side has been able to work out what the speed and duplex should be. 59

So let’s set the speed here to 10 and the duplex to half. 60

And let's see if the switches can successfully negotiate that. 61

So interfaces is come up, sh int f1/0/1 and I'll simply include the duplex 62

so we don’t see all the output, notice here its negotiated properly. 63

So on this switch, it’s been manually configured for half duplex 10 meg 64

and this switch has successfully negotiated that if I change the duplex here to full 65

let's see what happens now when the interface comes up 66

so here it's 10 meg full duplex, on this side it's also 10 meg full duplex. 67

So the 3750 is on negotiating correctly without any problems 68

but that wasn’t happening between a 3750 and the 2950. 69

So let’s set the speed to 100. 70

So sh int and look at the duplex that’s been set 71

try that again 100 full duplex, this side 100 full duplex. 72

So in this case when I set the 1 side it’s successfully negotiating with the other side 73

but we have problems with negotiation between 3750 and 2950. 74

So generally the moral of stories is, autonegotiation on both sides 75

or hard code on both sides, mixing and matching can cause problems. 76

Today the general recommendation is use auto everywhere except perhaps between core links 77

where you wanna hard-code both sides to increase the speeds of that interface coming up.