IOS Update on 2811, help with choosing right IOS

[Predrag]

Hello guys, 

I have tried to search forum about my problem, but it was all confusing. I couldn't figure out solution to my problems. I hope you can help me.
At one costumer I have 2811 routers with very old IOSes, (c2800nm-advipservicesk9-mz.124-3i.bin, I found one post from 2009 where people sad that this IOS is old :D ). How can I determine which IOS is successor of this IOS and prerequisites for that IOS (can my router run it) 

Hardware of device:

Cisco 2811 (revision 53.51) with 223232K/38912K bytes of memory.
Processor board ID FTX1240A3FT
2 FastEthernet interfaces
1 Gigabit Ethernet interface
2 Serial(sync/async) interfaces
1 Virtual Private Network (VPN) Module
DRAM configuration is 64 bits wide with parity enabled.
239K bytes of non-volatile configuration memory.
62720K bytes of ATA CompactFlash (Read/Write)

P.S. Is there any "easy" guide through IOSes

Thanks a lot. 

Have a nice day, Predrag.

[Joseph W. Doherty]

Generally, if you go to Cisco software web page for 28xx, look for anything that will run with (your) 256 MB RAM and (your) 64 MB flash, providing advanced IP service.

Feature wise, higher IOS numbering would be "newer". Release dates should be listed. You may, though, because of maintenance, see "older" IOS versions with later release dates than some "newer" IOS versions.

Unfortunately, I don't know of an "easy" guide for IOS releases.

[Predrag]

Hello Joseph, 

So I go to   https://software.cisco.com/download/navigator.html, search 2811, software on chassis, and if I had advipservices I should search for newer version? Simple as that?

And one more, what is secret behind these two letters after 12.4  (pic on the link http://prntscr.com/hm2xao ).

Thank you. 

Best regards, Predrag.

[Joseph W. Doherty]

Yup, more or less.

Those two letters are special release trains, often provided to support new hardware or software feature not supported in the "frozen" mainline release.

[Leo Laohoo]

Read the Release Notes so you'll be able to determine what bugs are there on a chosen IOS version.