Upload IOS via TFTP in ROMMON

It is recommended to maintain your network devices with updated IOS. However, it is highly recommended to plan & to prepare how to do a rollback in the worst case scenario, for example: if the router fails to boot properly due to corrupted image file or due to usage of new flash memory, etc.

To bypass this problem is just by let the router boot-up with the IOS from an TFTP itself.

First, make sure that the router is connected to a TFTP Server, also a valid ISO file is there.

During boot-up, press Ctrl+Break continuously so that the router goes into ROMMON mode.

Type the following command:

rommon 1 > IP_ADDRESS=172.16.1.1
rommon 2 > IP_SUBNET_MASK=255.255.255.0
rommon 3 > DEFAULT_GATEWAY=172.16.1.100
rommon 4 > TFTP_SERVER=172.16.1.100
rommon 5 > TFTP_FILE=C3640-Ix-Mz_20122-15_20T2.bin
rommon 6 > tftpdnld -r

IP_ADDRESS: 172.16.1.1
IP_SUBNET_MASK: 255.255.254.0
DEFAULT_GATEWAY: 172.16.1.100
TFTP_SERVER: 172.16.1.100
TFTP_FILE: C3640-Ix-Mz_20122-15_20T2.bin
TFTP_VERBOSE: Progress
TFTP_RETRY_COUNT: 18
TFTP_TIMEOUT: 7200
TFTP_CHECKSUM: Yes
TFTP_MACADDR: XXX.XXX.XXX.XXX
GE_PORT: Gigabit Ethernet 0
GE_SPEED_MODE: Auto
.
Receiving C3640-Ix-Mz_20122-15_20T2.bin from 172.16.1.100
...
...

• Command 1: IP address of router interface
• Command 2: Subnet mask for the above IP
• Command 3: Default gateway for router (can be same as TFTP Server IP)
• Command 4: IP address of TFTP Server
• Command 5: Name of IOS to be used for bootup (including the .bin extension)
• Command 6: Direct the router to load the IOS on memory rather than downloading it to the flash

Please notice, booting a Cisco router from TFTP not used only for this situation, can also be used for other stuffs such as the router’s RAM inadequate, security solution, flexibility to use different IOS.