When working with Cisco routers, many-a-times we wish
to boot the router from a local TFTP server. Here is a brief description of how
the router can be configured to boot from a TFTP server.
Step 1: Setting up
required variables in ROMMON
Whenever a Cisco router boots up, it initially goes into the ROMMON
mode and then boots up into the IOS from there. Now, inorder to boot up into
the IOS, it obviously needs access to the IOS from ROMMON. So, inorder to boot
from the TFTP server, we first need to ensure that the router has access to the
TFTP server from ROMMON mode, from where it can download the IOS each time that
the router needs to boot up.
There are some variables which we need to set in ROMMON mode, which
will ensure that the IOS gets downloaded from TFTP.
So, here are the sequence of events that we need to carry out to set
these variables.
- Power cycle the router, and during bootup, press ctrl + break continuously so that the router goes into ROMMON mode.
- Once in ROMMON, give the following commands:
rommon
1 > IP_ADDRESS=<ip address of router interface>
rommon
2> IP_SUBNET_MASK=<subnet mask for the above IP>
rommon
3 > DEFAULT_GATEWAY=<default gateway for router (can be same as TFTP
server IP)>
rommon
4 > TFTP_SERVER=<IP address of TFTP server>
rommon
5 > TFTP_FILE=<name of IOS to be used for bootup (including the .bin
extension)>
rommon
6> tftpdnld –r
(the
‘-r’ parameter ensures that when the IOS is downloaded, it is not stored in
flash, but is directly decompressed into the DRAM during router bootup. So the
flash is completely bypassed here.)
Once the above commands are entered, the router will start
downloading the IOS from the TFTP server. After the download is complete, the
router will reload automatically, and boot up into the IOS that was specified
above.
The ‘show version’ output will have the following statement:
System image file is "tftp://<IP of tftp server>/<IOS
image name>"
This step ensured that we have the correct variables set up in
ROMMON for the router to reach the TFTP server and download the IOS from there
on subsequent boot-ups.
Step 2: Changing boot
variable in the IOS
Now that the router has booted successfully into the IOS and all the
required variables have been set up in ROMMON, all that is left to do is to set
the boot variable to point to the TFTP server in the IOS, so that the router
looks for the TFTP server during bootup to find the IOS image.
This can be configured by the following command in global
configuration mode:
Router(config)# boot system tftp <IOS image name> <IP of
TFTP server>
Save the configurations.
We have now setup all the desired variables (in ROMMON and in the
IOS) to ensure that the next time that the router boots up, it will download
the IOS from the TFTP server, decompress it and then boot into the IOS.
Please note that since the IOS has to be downloaded from the TFTP
server each time that the router boots up, the boot up process takes a bit of
time.
It refers from:
https://supportforums.cisco.com/blog/150396/booting-cisco-router-tftp-server
More related:
How to Boot and Run IOS XE Software in Cisco 3850 Switch?
How to Recover from a 3850 Series Switch Boot Failure
How to Install the New License on the Device with “License Install” Command?
How to Boot and Run IOS XE Software in Cisco 3850 Switch?
How to Recover from a 3850 Series Switch Boot Failure
How to Install the New License on the Device with “License Install” Command?
