Original subtitles

Here’s router 1, which is this device

and I'll ping router 2 from router 1 to see if ping succeed.

So ping 10.1.1.2 ping succeed.

But notice we are getting a message that there's a duplex mismatch

between the router and the 2950-1 switch.

So we’ve been told that there’s a duplex problem here and on the switch

we're seeing that there’s a duplex mismatch on F0/3 to the router.

So we’ve been told that there’s a duplex problem here

but we're also told that there’s a duplex issue on F0/4 to the 2950-2 switch on F0/4 10

so there’s a problem here as well. 11

Now you may not be physically connected to the console 12

and hence you may not see this issues 13

that’s a good reason to use a syslog server or a management application 14

from a company such as SolarWinds. 15

So let’s ping router 2 again. 16

Ping succeeds, so pings aren't necessarily a good way to test for duplex mismatches. 17

However I'll do an extended ping to that router so let’s try that again 10.1.1.2 18

I'll send a thousand pings and I’m gonna make the datagram size, a large size 19

so we wanna send a large of traffic unto the network and hopefully we'll encounter 20

problem and there you go, we’ve got a message saying a late collision has occurred. 21

So this is a problem you my encounter. 22

One side may tell you that there are late collisions as you can see over there. 23

Were getting a number of late collisions, on the other side were not seeing that. 24

so on the switch sh int f0/3 we're not seeing a lot of late collisions 25

the configuration of that interface is set to a speed of 10 meg 26

and duplex are full but notice on this side sh running-config int f0 27

this side has its speed sets to 100 lets confirm that. 28

sh int f0 notice the duplex is half, speed is set in this example to 10 Mbps 29

so the router because it can't negotiate properly with the other device in this case 30

the 2950 switch has set its speed to a value of 10 meg half duplex. 31

So the point to remember is when there is a mismatch 32

the side that is set for half duplex will see a late collisions on the connection 33

but this only occurs when you're sending enough traffic. 34

so a standard ping doesn’t show that problem 35

but when a large amount of traffic is sent on this half duplex connection 36

we're getting a late collisions on the switch side sh int f0/3 37

notice the speed is full duplex 10 Mbps where's in the router it's half duplex 10 Mbps. 38

even though I configured it to use 100 Mbps, its using a failed back position 39

of the worst case scenario which is 10 meg half duplex 40

and in this case we're having that problem. 41

Notice this is 100 bases TXFX where's this side is 100base TX. 42

So in this example the router has gone for half duplex 10 Mbps 43

where's the switch is is using 10 meg full duplex 44

and hence we’ve got a duplex mismatch. 45

So in summary we get told by CDP that there is a duplex mismatch. 46

The local interface is not using full duplex 47

where’s the other side is using full duplex. 48

Pinging a device doesn’t tell us that there’s a problem 49

but when large packets are sent 50

It uses main counter drops for throughput and you may see late collisions. 51

So let me exaggerate that link again, send a thousand pings 52

large data gram size, sh int f0/3 53

Notice we are now seeing the late collisions taking place on the interface. 54

So on a half duplex side, we see this output of late collisions. 55

On the interface, sh int f0 notice the number of late collisions has increased. 56

On the full duplex side we make it input errors such as CRC errors or runt frames. 57

so in this example we’ve got 28 input errors, 15 CRC's 58

were aren’t seeing any runts but we are receiving duplex mismatch messages.