According to Cisco feature navigator, ios version 12.4(24)T (c1841-ipbasek9-mz.124-24.T.bin) can run in the 1841 and supports BGP (note that feature set is IPBASE):
1) It is common that a version with an IPBASE feature set supports BGP (some docs says that bgp support is included in "SP" services feature set)?
Moving BGP into the IP Base feature set is a recent change. http://6200networks.com/2008/06/02/bgp-support-on-isr/
2) Will ios version 12.4(24)T run OK in the 1841? I think the response will be YES because Cisco feature navigator says that it is supported and the router has the DRAM and FLASH needed by 14.4(24)T.
Yes. Although I wouldn't necessarily use 12.4(24)T, the latest maintenance build of 12.4(15)T should be more stable. http://6200networks.com/2008/09/23/extended-support-for-cisco-ios-software%C 2%AE-release-12415t/
3) Must the customer pay in order to download 12.4(24)T or he can download if he has a valid Cisco maintenance contract for that router?
It's my understanding that you can download any software of the feature set you're entitled to use if you have SmartNet. Hope it helps Ivan http://www.ioshints.info/about http://blog.ioshints.info/