Troubleshooting Interfaces

(Rene Molenaar) #1

This topic is to discuss the following lesson:


Hi,My concern is I am looking for the topic ‘Configuring Ethernet switch’(Telnet,console and SSH). Can you please tell me where did you discussed this topic

(Rene Molenaar) #3


Here’s an example for the console and SSH:

Configuring Cisco router for the first time

Here’s an example to enable telnet server on your router or switch:

R1(config)#line vty 0 4
R1(config-line)#transport input telnet
R1(config-line)#password cisco123
R1(config-line)#transport input 

The configuration above will only ask for the password (cisco123). It’s also possible to use usernames/passwords instead:

R1(config)#username admin password cisco123

R1(config)#line vty 0 4
R1(config-line)#login local

And you can protect it with an access-list:

R1(config)#access-list 1 permit
R1(config)#line vty 0 4
R1(config-line)#access-class 1 in

Hope this helps.


(syedwaqas h) #4

What main items check when troubleshooting fiber based interface

(Lazaros Agapides) #5

Hello Syedwaqas.

When troubleshooting fibre based interfaces, there are some specific items that you should check:

First of all, all of the issues brought up by Rene in this lesson are applicable to fibre optic connections as well. Additionally, you should keep the following in mind when troubleshooting fibre based interfaces:

  1. Check the SFP or GBIC status by issuing the show inter status command. Here is an example from a production 3750 production switch that I have:
3750_DC_1#show inter status
Gi1/0/1   ***VOICE_SERVERS** connected    901        a-full a-1000 10/100/1000BaseTX
!-<output omitted>-!
Gi1/1/3                      notconnect   1            auto   auto Not Present
Gi1/1/4                      notconnect   1            auto   auto Not Present
Te1/1/1   ***LINK_TO_6506A_T connected    trunk        full    10G SFP-10GBase-LRM
Te1/1/2   ***LINK_TO_6506B_T connected    trunk        full    10G SFP-10GBase-LRM

Notice that interface Gi1/1/3 which is an SFP slot says Not Present. This means there is no SFP module installed. Also, notice on the 10 Gigabit interfaces Te1/1/1 and Te1/1/2 it says 10G SFP-10Base-LRM. It tells you what SFP module is installed. If you have a module installed and it says Not Present then it’s quite likely that there is a problem with the SFP or the port.

  1. When troubleshooting fibre interfaces, keep in mind that specific types of SFP or GBIC modules are compatible with specific types of fibre optic cabling. Make sure you are using the correct patch cord and SFP module for the type of fibre optic cabling installed in your structured cabling.

  2. Keep in mind that even if you use the wrong kind of cable (multi mode instead of single mode or vice versa) you MAY get connectivity, albeit unstable connectivity. (Believe me, it has happened to me!) This is where you’ll need to check errors on the interface using the show interface command. For example, on the same production switch:

3750_DC_1#show inter te1/1/1
TenGigabitEthernet1/1/1 is up, line protocol is up (connected) 
  Hardware is Ten Gigabit Ethernet, address is 2c54.2d79.881d (bia 2c54.2d79.881d)
  Description: ***LINK_TO_6506A_Te5/1***

  !-<output omitted>-!

  5 minute output rate 5470000 bits/sec, 1025 packets/sec
     9311415126 packets input, 7136165402633 bytes, 0 no buffer
     Received 96568047 broadcasts (90433453 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 90433453 multicast, 0 pause input
     0 input packets with dribble condition detected
     9561095975 packets output, 9508345166932 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out

Here you can see that there are no runts, giants or input errors, which is good. If there is a problem with media type mismatches, you will definitely get some errors.

  1. Further issues that may cause faults include incorrect TX and RX connectors. Try installing the connectors the other way around. Avoid this whenever possible as two emitters emitting at each other can damage the light sources. Just keep in mind that it can happen.

  2. Also, curving or coiling the fibre optic cable too tightly may cause excess refraction and cause errors or disconnection.

Most of the additional troubleshooting procedures for fibre optics beyond those explained by Rene have to do with the physical layer.

I hope this has been helpful!


(Mohammad Hasanuz Zaman) #6

Hi Laz,
According to point 3 as I understand …
If the SFP Module is Single Mode (Like GLC-LH-SM1 SFP-GE-L2) then we have to use Single Mode Fiber or vice versa , right ?? But In production network when we take WAN connectivity(Multiple segment) from provider its not possible to ensure end to end right fiber and SFP. Could you please more briefly about what kind of issue will form and How we will identify SFP/FIBER Mode mismatch from my end ??


(Lazaros Agapides) #7

Hello Mohammad

When preparing to connect a fibre connection from end to end, the first thing that must be done is to confirm that the fibre being used in all parts of the passive infrastructure (the fibre itself) is the same type. That is, the patch cords on either end and the infrastructure cable segments must be of the same type. You cannot use a single mode patch cord with a multi-mode infrastructure fibre. It just won’t work, OR, in rare cases, it might function, but with many errors (as I have seen from experience). Make sure the patch cords match the infrastructure fibre type.

Secondly, be aware of the SFP fibre module that you will use. Some modules support both Single and Multi-mode fibre. For example, the GLC-LH-SM1 that you mention supports single mode fibre of up to 10 km AND multi-mode fibre of up to 550m. Make sure you’re using the right SFP module for the right type of fibre and for the right lengths as well. Some SFP modules are compatible with different model SFP modules on the other end, but it is always a good idea to have the same SFP model on both ends.

When working with WAN connectivity, with multiple segments from the provider, what is important is not the same type of fibre from end to end, but the same type of fibre and compatible SFP modules for each active segment. The WAN connection that the provider gives you on one end may connect to a network device within the provider’s infrastructure. The SFP and patch cord you use should be compatible with that on the other end of the specific fibre segment. The provider is responsible for providing you with that information.

I hope this has been helpful!


(Mohammad Hasanuz Zaman) #8

Hi Laz,
Thanks for your explanation …
So we have to KEEP same type for per active segment , right ??


Suppose… Segment-1 : R1 to R2 [SFP Single Mode both end and Fiber also Single Mode ]
And Segment-2 : R2 to R3 [Multimode SFP both end and Multimode Fiber ]

The above is okk or not ?? Please correct me if I am wrong .Thx


(Lazaros Agapides) #9

Hello Mohammad

Yes, you are absolutely right!


(rosna s) #10

In the Half-Duplex Interface issue, why the Fast Ethernet interface not supporting half duplex? I believe the Fast Ethernets can support both Full and Half Duplex mode. Please clear my doubt.

(Lazaros Agapides) #11

Hello rosna

Yes, all Cisco devices have Ethernet ports that support both half and full duplex. In order for them to function correctly however, you require the appropriate configuration on both ends.

The following combinations of configurations will function correctly for duplex settings:

  1. Half duplex - Half duplex
  2. Full duplex - Full duplex
  3. Half duplex - Auto
  4. Full duplex - Auto

The only configuration that will not function is:

  1. Half duplex - Full duplex

If you have one of the first four configurations set up and it is still not working, then the problem is not with the duplex settings.

I hope this has been helpful!