[c-nsp] Why is CEF disabled on dynamic interfaces?

[Ed Ravin]

We have a 7206 non-VXR with an NPE-150 running IOS 12.2(15)T15.  As
previously discussed on this list, we can't upgrade it past 12.2T since
12.3 and onwards don't support the NPE-150 anymore.

We've got a bunch of DSL users coming in via ATM, using PPPoE or
PPPoA to authenticate.  The DSL users get a dynamically-created
interface based on a "Virtual-Template" description and the attribute-value
pairs sent by the RADIUS server.

The problem we're having is that CEF is disabled on the dynamically
created "Virtual-Access" interfaces.  This is indicated by the
"show cef int <interface>" command:

Virtual-Access93 is up (if_number 240)
  Corresponding hwidb fast_if_number 240
  Corresponding hwidb firstsw->if_number 240
  Internet address is 0.0.0.0/0
  Unnumbered interface. Using address of Loopback1 (209.166.192.42)
  ICMP redirects are always sent
  Per packet load-sharing is disabled
  IP unicast RPF check is enabled
  [...]
  Interface is marked as point to point interface
  Hardware idb is Virtual-Access93
  Fast switching type 7, interface type 21
  IP CEF switching disabled
  [...]

After some research, we discovered that if we removed "ip route-cache flow"
from the Virtual-Template, AND if we do NOT use a construction like
'cisco-avpair = "lcp:interface-config..."',  such as:
  cisco-avpair = "lcp:interface-config#1=description"
in the RADIUS entry, THEN and only then does CEF get enabled.

This is a bummer - we need CEF for performance, and we also need to
be able to push an interface description or route for a backend network.
Doing the latter via cisco-avpairs loses CEF.

Apparently this problem doesn't happen in 12.3T.  Any ideas for
workarounds, other than biting the bullet and upgrading the router
hardware so we can use a later IOS?

Thanks,

	-- Ed