Troubleshooting Interfaces

This topic is to discuss the following lesson:

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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

Hi,

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 
R1(config-line)#login

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 192.168.1.0 0.0.0.255
R1(config)#line vty 0 4
R1(config-line)#access-class 1 in

Hope this helps.

Rene

What main items check when troubleshooting fiber based interface

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
3750_DC_1#

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
3750_DC_1#

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!

Laz

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 ??

br//zaman

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!

Laz

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

R1----------R2------------R3

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

br//zaman

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Hello Mohammad

Yes, you are absolutely right!

Laz

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.

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!

Laz

I have switch SW-2 connected on interface f0/3 with switch SW-3 on f0/2.
Host A is connected with switch SW-2 on f0/21 (ip 10.10.10.101, net mask 255.255.255.0, def gateway 10.10.10.1)
Host B is connected with switch SW-3 on f0/21 (ip 10.10.10.102, net mask 255.255.255.0, def gateway 10.10.10.1)
Interface f0/3 on SW-2 & f0/2 on SW-3 are configured in trunk. Interface f0/21 on SW-2 & f0/21 on SW-3 are configured in access mode.
Host A cannot reach Host B.
Here are my show commands:

SW-2#show interfaces f0/21 switchport
Name: Fa0/21
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 10 (Dhjeta)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: All
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
SW-3#show interfaces f0/21 switchport
Name: Fa0/21
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: negotiated
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 10 (Dhjeta)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: All
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
SW-2#show interfaces f0/3 switchport
Name: Fa0/3
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Off
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 10 (Dhjeta)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: 10,20
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
SW-3#show interfaces f0/2 switchport
Name: Fa0/2
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Off
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 10 (Dhjeta)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: 10,20
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
SW-2#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/3 on 802.1q trunking 10

Port Vlans allowed on trunk
Fa0/3 10,20

Port Vlans allowed and active in management domain
Fa0/3 10

Port Vlans in spanning tree forwarding state and not pruned
Fa0/3 10
SW-3#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/2 on 802.1q trunking 10

Port Vlans allowed on trunk
Fa0/2 10,20

Port Vlans allowed and active in management domain
Fa0/2 10

Port Vlans in spanning tree forwarding state and not pruned
Fa0/2 10

Why hosts cannot reach each-other?

Hello Igli

At first glance I can’t say that I find something wrong with the information you have sent. One thing I do find unusual, although this should not cause a problem, is the fact that you are using VLAN 10 as the native VLAN on the trunk. You may want to change that and make VLAN 10 a tagged VLAN across the trunk. Secondly, just keep in mind that in order for the two hosts to communicate, no default gateway is necessary.

Even if this is not the issue, I suggest you work your way through the problem like so:

  1. Create an SVI for VLAN 10 on both switches, and use another address for these within the subnet, say 10.10.10.52 for SW2 and 10.10.10.53 for SW3. Try to ping 10.10.10.52 from host A and then 10.10.10.53 from host A. This way you can see if the problem is local to the host, local to the switch, or if the problem is on the trunk.
  2. Try pinging one SVI from the SVI of the other switch to see that the packets on VLAN 10 are indeed traversing the trunk.
  3. Then you can focus on troubleshooting either the trunk or the hosts themselves.

Hopefully this will give you a good start. Please share your results with us!

I hope this has been helpful!

Laz

A post was merged into an existing topic: VLAN Access-List (VACL)

Hi Guys - quick one… I’m sure I learned this way back in my CCNA studies but - what does ‘line protocol’ actually refer to? What can cause the interface to be UP but the line protocol DOWN? Is it something to do with encapsulation at layer 2? Thanks - Gareth.

Hello Gareth

You can think of the interface up/down as the status of the physical layer, and the line protocol up/down as the data link layer.

So if the interface is up but the line protocol is down, this usually means that the interface detects some electrical signal from the other end (in the case of a copper connection) showing that the physical layer is up, that is, that an electrical circuit has been established between the two endpoints. In the case of a fiber connection, this has to do with the initial exchange of light pulses to ensure that signals are indeed successfully being exchanged.

But, the data link layer connectivity is not established. This depends on what technology is being used at Layer 2. If it’s a serial link, this could be due to a mismatch in clock rates, or in encapsulation. If it is Ethernet, it could be a speed/duplex mismatch, or even a faulty cable. If it’s a fiber connection, it could be a problem in the transceiver or the fiber connectors.

I hope this has been helpful!

Laz

Thanks Laz - You would think that a faulty physical piece of equipment should be highlighted by the physical layer status and not the data link layer?

Thanks for the explanation and refresh. Sometimes it is easy to lose sight of the fundamentals of networking when studying higher up the OSI model. :slight_smile:

Hello Gareth

Yes, it can get confusing! Even so, if a cable or a transceiver (physical components) is faulty, it may be so on the data link layer. For example, a cable may physically connect the interfaces of two switches. However, it may be damaged in such a way so that electrical pulses are being transmitted, so the physical circuit is complete, but the framing is being disrupted, and Ethernet frames are not arriving successfully. Alternatively, a transceiver may be sending and receiving light pulses, but the fiber may be bent somewhere along its path to cause excessive light dispersion, causing fragmented or lost frames to the degree where no data link connection can be established.

Even though these are physical components, the resulting fault occurs on the data link layer.

No problem! You’re right, it’s easy to lose sight of the fundamentals. You have no idea how many times it’s happened to me!! Glad to be of help!

Laz

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