I remember reading back in my CCNA days that a 2500 (then state-of-the-art) is unlike other Cisco routers in that it both stores and executes the IOS in Flash memory, instead of copying it into NVRAM at bootup.
Is this true - well, at all, but also - is it true about the 2600 series?
I don't believe anything runs the SW from flash anymore! Running from flash was good in that it made the router quick to boot, but compared to DRAM flash is slow so it affected the performance of the router.
The other issue is the increasing size of IOS images. Back in the days of 11.0 an IOS image was reasonably compact, often only around 3-4M, but all it had to so was run a couple of routing protocols and forward some packets. Modern IOS is much more sophisticated with support for things like NAT and QoS commonplace, which makes the code much larger. I have IOS images >30M (and that is compressed). As a result of that the images now tend to be compressed, and expanded into DRAM at boot time.
Just take a look at the spec of a 2500 - 4M flash and 8M DRAM was not unusual, and compare that to a 2600 - http://www.cisco.com/en/US/products/hw/routers/ps259/prod_bulletin09186a00801c639b.html.
2600 series router decompresses the image and loads it into the DRAM as compared to loading the image directly from flash in case of 2500 series router.
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