Broadcom CFE backup
From DD-WRT Wiki
How to backup your CFE
So you're afraid you will ever need to use a backup of your CFE in case you brick your router sooner or later. There are two methods to do so, as follows:
by telnet/SSH
just enter the following command for all, except micro builds:
dd if=/dev/mtd/0 of=/tmp/cfe.bin
Then get the files with e.g. WinSCP from the /tmp directory.
On micro builds it's little more tricky:
cat /dev/mtd/0 > /tmp/cfe.jpg
Then point your browser to http://routerip/user/cfe.jpg and save as cfe.bin
You could do the same for mtd/1 (kernel, firmware), mtd/2 (jffs part if any) and mtd/3 (nvram)
by JTAG
this assumes you already have a JTAG cable ready to use on your router, use the following command:
wrt54g -backup:cfe