Jack is very right that an IOS will come loaded on the routers that you purchase. Frequently you have an opportunity to specify what IOS should be on them. For the 1841s you will need something in the 12.3T train.
And Jack is very right that without a service agreement that you are not entitled to downloads of software for the routers.
The original post did not indicate the intended usage of the routers. Assuming that the intended usage is for IPSec connectivity then the Advanced Security feature set is very appropriate. I have a customer who is using 1841s for IPSec VPNs and they are doing well.
You probably want the 12.3(11)T8 version of the code (not the 12.3(11)T as referenced in the original post).
Depending on who you buy the equipment from you should be able to specify what IOS Feature set that you want when you order the 1841 routers. If you have already purchased 5 routers and they came with a feature set that does not meet your needs you may be able to negotiate something with whoever sold them to you to get the feature set that you need. But if you are not able to get whoever sold them to replace the IOS feature set then yes you may need to purchase the Advanced Security feature set.
Cisco offers a service contract on the equipment that they sell. If you order directly from Cisco it is called SmartNet. If you purchase from a reseller they may have their own name for the service contract.
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I am currently unable to specify "crypto keyring" command when configuring VPN connection on my cisco 2901 router.
The following licenses have been activated on my router :