There are plenty of tutorials on setting up AnyConnect on an ASA unit, and a handful of links noting that IOS 12.4(15) and later support AnyConnect, but I can't seem to find any good documentation about how to setup AnyConnect on IOS; most tutorials assume you only want a clientless VPN on IOS. the best I've found is this document on Cisco's site, but it's not working for me in practice - see below.

This is all on a Cisco 881W:

router#show version | include Version
Cisco IOS Software, C880 Software (C880DATA-UNIVERSALK9-M), Version 12.4(20)T1, RELEASE SOFTWARE (fc3)
ROM: System Bootstrap, Version 12.4(15r)XZ2, RELEASE SOFTWARE (fc1)

The old SSL VPN Client seems to install just fine:

router#show webvpn install status svc  
SSLVPN Package SSL-VPN-Client version installed:
CISCO STC win2k+ 1.0.0 
1,1,4,176
Thu 08/16/2007 12:37:00.43

However, when I install the AnyConnect client, after authenticating it hangs for a while during the self-update process, and stops with an error that the "AnyConnect package unavailable or corrupted."

When I try to install the AnyConnect package on the router, I'm told that it's an invalid archive:

router(config)#webvpn install svc flash:/webvpn/anyconnect-win-2.3.2016-k9.pkg
SSLVPN Package SSL-VPN-Client (seq:2): installed Error: Invalid Archive

Does anyone have a good sample on how to get the 2.x AnyConnect clients working with a Cisco device running IOS?