Earlier this year, Cisco announced replacement switches to the 1841/2800/3800 routers. The older routers have not entirely reached end of sale, but the newer routers (1941/2900/3900) offer big changes in router licensing.
Why is it different? Similar to a PIX or ASA, the new routers use license keys and a universal image to determine the available features. This new licensing scheme shows a clear difference between the version of IOS installed and the feature set installed. You can change them independently and they don't afect each other.
The license structure has been streamlined, as there are only 4 feature sets. None of the packages overlap. They are:
IP Base: The default feature set. It has support for the most common routing protocols (RIP, OSPF, EIGRP, BGP), common WAN technologies (MLPPP, MLFR), and common LAN technologies (802.1q trunking).
Data: The Data license adds support for MPLS, ATM, IS-IS, and non-IP L3 protocols.
Voice: The UC (Unified Communciations) feature set adds support for voice interfaces, CallManager Express, and Unity Express modules.
Security: The Security feature set adds IPSec VPN capabilities, SSL VPN capabilities, and IOS firewall features.
Cisco allows you to pick and choose better depending on your application. You will always need to be concerned with the amount of DRAM and Flash memory installed to make sure that you can run the proper features.
Also, check out the performance features on the new routers of upgraded ports and processor speed. And, as an alternative, consider the low price of the older 1841/2800/3800 series. A little thought can save some real dollars here.