Hi,In this article, I’ll talk about some problems you might encounter while working on a Cisco device:

When I plug a device on my Cisco CPE, I have some issues to get a DHCP lease.

This problem is mainly caused by the following points:

  • Your CPE ports are configured with the default spanning-tree behaviour. With the default behaviour, after being plugged, a switch port is temporarily put in a blocking state for 30 to 50 seconds, to acquire and calculate STP topology. If this behaviour is absolutely safe, some early network applications – like DHCP – will be subject to timeout.
  • When plugging your first equipment, if your CPE uses a level 2 switch, and has « vlan » interfaces , the vlan management interface takes some time to toggle from down/down to up/up state. The DHCP server is bound to this interface, so it won’t be able to process a request until the associated vlan interface is in up/up state.

To deal with the first issue, you have either to use the following commands to switch the level 2 port in STP portfast mode:

interface fastethernet X
 spanning-tree portfast

or to disable the spanning tree for your VLAN by typing the following in configuration mode:

no spanning-tree vlan <VID>

To mitigate the second issue, you must stick the vlan interface to the up/up state. You can do so using the following commands:

interface vlan X
 no autostate

I have screwed while flashing my Cisco device! Rommon tells me that it cannot find a bootable image.

keepcalm

Using tftpdnld, you can load a bootable image from a TFTP server from the rommon prompt. To do so, hook up your Cisco device to a network with a TFTP server, and type the following commands in the rommon prompt:

IP_ADDRESS=X.X.X.X
IP_SUBNET_MASK=X.X.X.X
DEFAULT_GATEWAY=X.X.X.X
TFTP_SERVER=<tftp_server_IPv4_address>
TFTP_FILE=<path_to_your_IOS_image_on_tftp_server>
tftpdnld -r

Once your image has booted, copy again your image from TFTP to flash, then check its integrity by computing and checking the resulting hash.

copy tftp://<tftp_server_addr>/<path_to_IOS_image> flash:<image_filename>
verify /md5 flash:<image_filename>

Then, configure the bootloader to load your image at boot time:

configure terminal
 boot system flash:<image_filename>
 exit
copy running-config startup-config

Now, you can reboot safely and enjoy your fresh IOS image.

I don’t remember my login/enable password, how to recover it?

To begin with, you need a serial console client which support the break signal: this is something you can’t emulate only with your keyboard. If you don’t know what the break signal is, please refer to the following document: Cisco Standard Break Key Sequence Combinations

As the recovery procedures is model dependent, visit the Password Recovery Procedures web page where you will find the detailed instructions for your device.