About DRAM and flash size requirements for IOS images

[Iulian Vaideanu]

I'm trying to get an idea about how "strict" those requirements are, as listed on Cisco's download pages.

Let's take the following example:  I have a Cisco 1841 router with 128MB of DRAM and 32MB of Compact Flash capacity, and I'm looking for an MPLS-enabled IOS image for it.  If I've done my research properly, MPLS would require at least a 12.4T IOS with SPSERVICES.  There seems to be no such thing for 128D/32F devices - however I noticed that c1841-spservicesk9-mz.124-15.T17.bin for example (128 / 64 required) fits just fine in the 32MB of flash if I delete the SDM-related files (which I don't need anyway).

Now, when it comes to RAM I always follow the requirement (the IOS would probably boot with less, but you never know when it might need more than the router has), but when it comes to flash capacity, I'm asking:  is there anything else to consider besides just enough having enough space available for the IOS image?  I mean, there's no such thing as "swapping" or other temporary use of flash space on Cisco routers, is there?

[Joseph W. Doherty]

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Posting

Crash files will want flash space.  Of course, if your system never crashes - it's a non-issue. 

A .bin (or any file) will require a tiny bit more space than the actual file size, as flash allocates space in blocks.  I.e. it's possible a file may appear to fit but it cannot (rather rare, though).

I've often totally erased/formatted flash, and just loaded the desired .bin.  I've also removed some "unnecessary" files (such as SDM files, as you also note) to free a little more space to allow a slightly larger IOS.

[Leo Laohoo]

The main thing to consider is your DRAM.  This is because it's very difficult to buy a compatible 3rd party DRAM.  You basically have to purchase the DRAM from Cisco.

Now when it comes to CF, you can go to any electronics shops and get any 3rd party CF.  The main issue for you is find a COMPATIBLE CF which you can use.  My recommendation is to go to an electronics shop and purchase two or three similar sized CF (up to 4 Gb) and try them out.  Return those that won't work and get a few more of the same ones that work.