CEF essential for… routing?

By default, a 12.3T series IOS (yeah, yeah, I know that this entry could have been usable like 3 years ago) has CEF (Cisco Express Forwarding) disabled. There would be nothing wrong about it if… the router worked fine that way.

For example, through a Cisco SOHO 91 I couldn’t reach certain IP addresses. The router was “clean” – no access lists, no inspects, just pure traffic forwarding. The debugs showed the following while trying to ping some addresses:

IP: s=85.222.xxx.xxx (local), d=85.17.yyy.yyy, len 100, unroutable
    ICMP type=8, code=0
IP: s=85.222.xxx.xxx (local), d=85.17.yyy.yyy, len 100, unroutable
    ICMP type=8, code=0

After entering:

ip cef

the problem magically disappeared. So far I’ve got no clue as for what this all was about. Maybe some old bug? BTW, the problem persisted both with the WAN interface configured through DHCP and statically. Also, entering:

no ip cef

causes the problem to come back.

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