Cisco IOS Show Interface Explained

Lessons Discussion
21

Hi Guys,

Why is the figure ‘255’ used for calculations of reliability and load?

Thanks,

Gareth.

22

Hi Gareth,

This is the only thing I could find:

They state:

reliability = number of errors / number of total frames

And:

The value is expressed as a fraction of 255

But there is no explanation of why they went with 255. Probably because they use an 8-bit value to store these values.

Rene

23

Thanks Rene. The 8-bit value explanation makes sense.

1 Like
24

Hi sir,

I have a several thing here which i would really like to know about and how and when this things counts
0 packets input, 0 bytes
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out

Thanks you

25

Hello Akash

It looks like you’re examining the output of the show interface command. Take a look at this lesson which describes this output in detail, and if you have any more specific questions, feel free to let us know!

I hope this has been helpful!

Laz

26

Issue with switch port
Issue with switch port662×334 67 KB

Hello Dear Team of network lessons.
I have a cisco switch 3560.
When I want to plug my Cisco IP phone to its’ port 10 Gigabit, 12-Gigabit then it shows not connected please see the output, to other switches shows up and connected only to Cisco IP phone it’s mentioned ports shows not connected.
Cisco IP phone model: CP-8845
Please note that to other ports my phone is working and all the ports are in same VLAN, same speed, and same protocol, and there is no port security issues.
Thanks in advance for the help and support.

Regards,

Ajmal Ahmady

27

Hello Ajmal

So from my understanding, you connect the phone to other ports and it works fine, but when you connect it to Gi0/10 it shows not connected. The only thing that I can think of is that either PoE is not enabled on this port, or you may simply have a faulty port on the switch. When you plug the phone in, does its screen light up? Does it get power? If not, take a look at this troubleshooting guide for PoE:

If it does light up, and you still get not connected, then it is probably a fault with the switch itself. Can you connect a laptop to the port and see if you get connectivity? A laptop doesn’t use PoE, so if the laptop works, there may be a problem with the way the switch supplies PoE.

I suggest you go through these steps, and if there is still a problem, I believe that it is due to hardware.

I hope this has been helpful!

Laz

1 Like
28

![Capture|668x338](upload://ApVs1Dmm5tKBRqzyylKdTVIHsKo.
Liz Sir,
Thanks for sending me the link of troubleshoot, as I checked my switch has totally 12 ports and it’s port 9 & is designed or not supports POE device based on output.

Regards,

Ajmal

1 Like
30

Hi Rene / Team,

Just want to confirm if physical issues could cause an interface to have output errors? Just like input errors as well?

output errors: Same as the input errors counter but for sent packets. This counter increases when anything goes wrong with the transmission of the packet.

Thanks,
Ferds

31

Hello Ferdinand

Yes, output errors can be due to physical issues on the port. However, it is less likely that output errors will occur due to a fault on the cable connecting to the port itself. Typically, some things you should keep in mind:

  1. Check to see if the output errors and the total output drops are the same. If they are, then it is likely that the output buffer is being maxed out. When such a buffer is full, any additional packets that attempt to egress are dropped. This can take place if:
  • you have QoS policies that apply policing or shaping
  • you have congestion on the link, where more traffic than the link capacity can handle is being transmitted
  • the interface is part of a port-channel and the load balancing algorithm is sending too many packets via this particular link causing congestion and dropped packets.
  1. If the values of output errors and total output drops are different, then this could be a problem with the physical port itself. It may be a result of a short circuit on the pins of the RJ-45 port, on the cable being used, or on the transceiver being used in the case of a fiber optic link.

These are just some of the things you should keep in mind when dealing with such errors. These can often be difficult to diagnose, but with patience and a methodical troubleshooting plan, these can be dealt with. Let us know how you get along and if we can be of more help in the process.

I hope this has been helpful!

Laz

1 Like
32

Thanks for the response. You are right, the port is a member of a port channel. But its the only member port. I also saw some jumbo packets being dropped. I did not see any output drops though.

What do you think about the jumbo packets on port?

33

Hello Ferdinand

The fact that some jumbo packets were dropped does not necessarily mean that there is a problem with jumbo packets. Are all of them being dropped or just some? If all of them are, then you should check the MTU configurations. If only some are being dropped, then the issue is somewhere else.

Can you give us some more information about the problems you are facing? Can you share your show interface output?

I hope this has been helpful!

Laz

34

Hi Laz,
Sorry, I meant I saw jumbo packets but they are not being dropped. Looks like issue is now resolved. I think the output errors being dropped is because the connected server was actually still busy making changes on its side. You think this a possibility?

show int Eth1/31 | inc "error|jumbo"
    1071604936 jumbo packets  0 storm suppression packets
    0 input error  0 short frame  0 overrun   0 underrun  0 ignored
    2609863474 jumbo packets
    9926357 output errors  0 collision  0 deferred  0 late collision
35

Hello Ferdinand

First of all, it’s great that the issue has been resolved. I do see a lot of output errors on the interface as well as a lot of jumbo packets. These two may or may not be related. If you have configured jumbo frames on the device, then the jumbo frame count will simply increase, but that’s to be expected since this is normal behavior.

You can also check the value of Total output drops in the output of the interface. If that value is the same as the output errors, then the problem is with a misconfigured QoS queuing mechanism. However, since this port is connected to a server it is unlikely that QoS is configured on this port?

In any case, unless you can recreate the problem and diagnose it directly, it will be difficult to determine what went wrong. Until then, we can only speculate… :slight_smile:

I hope this has been helpful!

Laz

1 Like
36

No QoS configured on the port.
Yep, you are right, it’s now hard to troubleshoot since the issue has been resolved.
Thanks for the response!

1 Like
37

Hi
I have these errors reported on port’s counters.

FCS Errors: ................................... 0
Single Collision Frames: ...................... 0
Late Collisions: .............................. 0
Excessive Collisions: ......................... 0
Multiple Collisions: .......................... 0
Received packets dropped > MTU: ............... 0
Transmitted oversized packets: ................ 0
Internal MAC Rx Errors: ....................... 0
Received Pause Frames: ........................ 0
Transmitted Pause Frames: ..................... 0
Receive Packets Discarded: .................... 134
Transmit Packets Discarded: ................... 896

Does these discarded packets a symptom of a network congestion or it means that problem exists on the switchport?

Thanks

38

Hello Giovanni

Before I respond, can you tell us a little bit more about how you obtained this output? What devices is this on, and what command did you issue to get these results? What kind of interface are we looking at?

Let us know and we’ll be able to respond in more detail. Thanks!!

Laz

39

Hi @lagapidis ,

I typed the command:

show interface counters gigabitethernet 1/0/22

This is a switch TOR that connect customer’s server (Windows O.S.) to the public network in a datacenter.

My customer accused network speed issues, but I’m not so sure that the root cause of the problem is caused by a broken port of the switch.

If I clear the counters and retype the same command, I not get any errors.
Maybe a speed test can cause packets to be discarded?

Thanks

40

Hello Giovanni

Thanks for that! Now there are several reasons for getting the Transmit and Receive Packets Discarded counters up. In most cases, this has to do with queues that have been filled and surplus packets are being dropped as a result. Depending upon the algorithms set up in the device, packets may be discarded to free up buffer space or to prevent their delivery to a higher layer protocol, in an attempt to limit traffic flows. In such cases, there may be packet drops without any errors. An example of this is when employing WRED which helps to prevent congestion by randomly dropping packets.

You mentioned that this seems to be taking place when they’re doing speed tests, which would make sense, because it is only then that congestion occurs, and some mechanism may be dropping packets for the various reasons we mentioned before.

Also, you mentioned that this is a Top Of Rack switch? Is it a Nexus device? If so, Nexus devices have preconfigured QoS mechanisms that may result in such dropped packets if traffic approaches the port speed of an interface.

I hope this has been helpful!

Laz

41

Thanks.
The switch was a DELL N2048, an access level switch.
At the moment, we decided to change the port of the switch and monitor the new counters to see if the problem still exist.

But which of these counters can suggest me that probably the problem is hardware and not related to congestions? ( port broken o SFP broken o cable broken ecc ecc )

Thanks