Cisco 2960 Packet Loss Troubleshooting Commands
show clock – checking to see if the clock is right isn’t super important, but the next command is going to show you what events have happened recently, and if the clock is off, then the time stamps will be off…
show logging – Great place to start looking for anything happening out of the ordinary.
show proc cpu | include CPU – That’s going to show you the current CPU utilization. A 2960 should run in the 20 to 60% load.
show interface status – Displays the current link state and description of each interface in the switch.
***Look for anything linked up at half-duplex.
***Half-duplex is bad. Fix that.
show interfaces counters errors – That command is going to show you what interfaces are experiencing errors.
Since we are talking about packet loss, focus on OutDiscards.
An output discard occurs when the switch was trying to push a packet out an interface but there was too much congestion on that interface, so a packet had to be dropped.
show mls qos – That command will tell us if QoS is enabled or disabled.
How we try to address congestion will depend a bit on if QoS is enabled or not.
show interfaces flowcontrol – That command will show you which interfaces (if any) have flow control enabled.
Flow Control is kind of like QoS. It gives the switch the ability to either ask a sending device to shut-up and stop sending for a second, and/or it tells the switch to stop sending traffic if a receiving device asks it to stop.
As a general concept, Flow Control is dumb and should be disabled. But sometimes it can be useful in specific circumstances.
RxPAUSE is when the switch received a request from the directly connected device to stop sending packets.
TxPAUSE is when the switch asked the directly connected device to stop sending packets.
Both are clear indicators of congestion.
show mls qos interface buffers – That command will tell us how your packet buffer memory will be allocated to each interface if QoS is enabled.
show mls qos interface statistics – That command will help you understand the QoS markings of traffic entering or exiting the switch, and what kinds of packets are being dropped.
This can be a VERY powerful command.
If QoS is disabled, all packet drops will probably happen in Queue 3 (by design).
show version | include time – That command will show you the uptime of this switch.
This information can be useful to help you know if the switch recently rebooted (unexpectedly?).
show version | include reason – That command will tell you the reason for the last reboot.
Anything other than power-on is probably worthy of further investigation.
show version | include IOS – That command will tell us what version of software you are running.
A Catalyst 2960 is probably end of support, so we probably can’t upgrade to new software anyway.
But it might be interesting information.
show inventory – That command will tell us the specific hardware model of this switch, which might shed a clue about something.