Things I keep forgetting¶

Estimated time to read: 7 minutes

Last Updated: February, 2026

Troubleshooting interface problems¶

I've had a few issues in my lab network recently which have turned out to be caused by interface issues so I'm documenting a couple of NX-OS commands I find useful

show interface brief

Is the interface up?
Is the speed/MTU correct?
Are the transceivers supported and correct on both sides?
Is the interface mode (trunk, routed etc) correct?
Is there are VLAN required - does the VLAN exist (show vlan)?

show ip int brief

Shows the configured IPs and protocol/link/admin status
If the link's down check the interface status and look for physical connectivity problems (cable, transceiver, TX/RX etc)

show interface status

I use this to check the cable transciver/cable that's been used
Is the speed and duplex correct?
Do you see link-not-connected or notconnected in the status?
- Check the physical connections - cable/transceiver correct? RX/TX direction in the correct (i.e. is the cable plugged incorrectly)?

show interface status err-disabled

Any errors on the interface?

clear counters interface ethernet 1/1

show int counters errors non-zero

show int counters non-zero

Any errors on the interface?

show int e1/49 transceiver details

Is the transceiver working correctly?

Catalyst - Booting from TFTP server¶

This is the procedure I use when I need to boot a Catalyst switch (2960S in this case) from a TFTP server

Connect a cable to the Fa0 port on the switch
Connect to the console port (CLI)
flash_init - Initializes the flash file system and allows the switch to detect the files, including the IOS image to boot
mgmt_init - Initializes the Ethernet management port
set IP_ADDR 10.1.1.2/255.255.255.0
set DEFAULT_ROUTER 10.1.1.1
copy tftp://1.2.3.4/c2960s-universalk9-mz.152-2.E9.bin flash:

If you see Err: emac_phy_autonegotiation: timed out. then check management port connectivity

Nexus 9000 - ACI to NX-OS Mode¶

I've had to convert some ACI switches to NX-OS and use this process

Console/SSH to the switch
If it's a new switch running the ACI OS and hasn't been connected to a fabric you should see (none)#
New switches use admin as the username and no password
ifconfig eth0 10.237.102.34 netmask 255.255.255.224
route add default gw 10.237.102.33 eth0 cd bootflash wget http://10.237.97.136/software/nxos/nxos.9.3.12.bin clear-bootvars.sh setup-bootvars.sh nxos.9.3.12.bin
Verify the bootvars have changed
cat /mnt/cfg/0/boot/grub/menu.lst.local
cat /mnt/cfg/1/boot/grub/menu.lst.local
Type vsh
Type reload then y to reload

Use Standard Break Key Sequence Combinations for Password Recovery¶

https://www.cisco.com/c/en/us/support/docs/routers/10000-series-routers/12818-61.html

Upgrading APIC to new image without going through APIC upgrade UI - using SOL to copy image¶

I use this for new APIC that I need to wipe and upgrade. Since the APIC is an appliance running on a UCS server I find it's easier in some cases to upgrade the APIC by mapping the ISO and reinstalling through the CIMC. i.e. Not using the APIC UI

Copy the APIC ISO to an HTTP server
Open a virtual KVM to the server
SSH to the APIC CIMC - ssh admin@10.1.1.45
C225-WZP# scope vmedia
C225-WZP /vmedia # map-www apic http://prod-storage-filer-01.my-domain.com/aci-apic-dk9.6.1.2f.iso
Check the mapping status with show mappings detail
connect host to follow the install
Power cycle the server from the KVM
Pre F6 as the server boots to get into the server boot menu
Select the Cisco CIMC-Mapped vDVD
Monitor the progress as the APIC ISO is deployed

Note

If you see a message such as the following then make sure you complete the prompt through the Serial over LAN (SOL)

To speed up the install, enter iso url in next ten minutes:
? http://10.1.1.45/aci-apic-dk9.6.1.2f.iso
++ awk -F '/|:' '{print $4}'
+ urlip=10.1.1.45
+ '[' -z http://10.1.1.45/aci-apic-dk9.6.1.2f.iso ']'
+ '[' -z 10.1.1.45 ']'
+ break
+ '[' -n http://10.1.1.45/aci-apic-dk9.6.1.2f.iso ']'
+ set +e
+ configured=0
+ '[' 0 -eq 0 ']'
+ echo 'Configuring network interface'
Configuring network interface
+ echo 'type static, dhcp, bash for a shell to configure networking, or url to re-enter the url: '

You can enable the Serial over LAN port through SSH (ssh ssh admin@10.1.1.45) and access it via the KVM

scope sol
set enabled yes
set baud-rate 115200
commit
exit

Installing tcpdump 4.99 on Ubuntu 20.04 (Focal)¶

I needed version 4.99 so I could see the interface name

wget https://ftp.debian.org/debian/pool/main/libp/libpcap/libpcap0.8_1.10.0-2_amd64.deb
wget https://ftp.debian.org/debian/pool/main/t/tcpdump/tcpdump_4.99.0-2+deb11u1_amd64.deb
sudo dpkg -i tcpdump_4.99.0-2+deb11u1_amd64.deb libpcap0.8_1.10.0-2_amd64.deb
tcpdump --version

NX-OS Rollback and Config Diff¶

I knew NX-OS had a rollback/commit feature but never got around to playing with it until recently. I also found out about the checkpoint diff feature from a colleague when I needed to test some config and wanted to capture just the new config to put into documentation.

In general the rollback/commit feature allows you to save the current running config as a checkpoint and later revert (rollback) to that saved config if needed.

In this example I created a checkpoint without BGP, added some BGP config, showed the diff of the running config vs the initial_config checkpoint, and then rolled back to the initial_config (without the BGP)

checkpoint initial_config
show run bgp
conf t
router bgp 65500 ...
show diff rollback-patch checkpoint initial_config running-config
rollback running-config checkpoint initial_config atomic

Rollback Options

atomic will stop the rollback and revert to original configuration if any errors are discovered
best-effort skips errors and proceeds with rollback
stop-at-first-failure stops the rollback at the first error