Cisco Quick Tips

Compilation of my go to Cisco commands. I hope they are useful for you.

Show all ports on interface:

Show ip int brief

Show all ports with speeds:

Show int status

Show POE ports:

Show power inline

Show mac addresss:

Show mac address-table

Create an EtherChannel:

Channel-group 1 mode active

Show EtherChannel:

Show etherch sum

Show transceiver detail:

show int gi1/1 tran det     (Change gi1/1 to your interface)

Transmit power is typically good when it is in the 6 dB range between -1 and -7 dBm.

Receive power is normally expected between – 1 and -9.9 dBm. 

If either Tx or Rx is in the -25 or -30 dBm or lower range that’s usually indicative of there being no actual signal received and the transceiver is reporting the “noise floor” of the receiver stage. https://en.wikipedia.org/wiki/Noise_floor)

When Rx power is around -14 dBm or lower there is typically some sort of fault in the cable that’s causing excessive signal loss.

TDR:

Cisco switches have a TDR (Time-domain reflectometer). It basically means you can test how long the cable is, if it has a short, pair of wires open and any other problem. More info available here from cisco.

Switch# test cable-diagnostics tdr interface g0/1
TDR test started on interface Gi0/1
A TDR test can take a few seconds to run on an interface
Use ‘show cable-diagnostics tdr’ to read the TDR results.

Switch# sh cable-diagnostics tdr int g0/1
TDR test last run on: March 01 00:09:06

Interface Speed Local pair Pair length Remote pair Pair status
——— —– ———- —————— ———– ——————–
Gi0/1 auto Pair A 20 +/- 4 meters N/A Open
Pair B 20 +/- 4 meters N/A Open
Pair C 21 +/- 4 meters N/A Open
Pair D 20 +/- 4 meters N/A Open

Portfast:

PortFast causes a switch or trunk port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states.

This can be useful for example, if you have a desktop connected to a switch and it is logging on to an Active Directory domain and group policies are not updating correctly. When booting the PC and logging on to the domain the PC does not receive an IP address until the user has already logged on and can see the desktop. In the past before Windows cached logins the user may not be able to login to the domain as the port wasn’t activated/forward state.

Portfast is appropriate when you are sure that you are connecting to a single device that will not potentially bridge you to other ports. Portfast is fine to a server and desktop for example.

What happens if you turn on portfast on a trunk port? If you do have a loop and your switch interconnects have portfast enabled then the switch will not be able to detect the loop in time and therefore a loop will occur.

Enabling portfast on a port does not disable STP but it allows the port to forward immediately.

A loop is formed when you physically connect up the switches in a certain way. There is nothingwrong with having loops, in fact a lot of L2 networks have loops for redundancybut if there is a loop STP needs to block one of the ports to break that loop.

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