Ran into an odd cisco IOS bug this morning after upgrading a cisco 7500 PPPoE DSL routers to 12.3(1a). 
Most PPPoE users would connect fine, but about 10 wouldn't. After cranking up debugging:

<small>
<pre>
debug aaa authentication
debug aaa authorization
debug ppp authentication
debug ppp error
debug ppp negotiation
debug radius authentication
debug vpdn call event
debug vpdn error
debug vpdn event disconnect
debug vpdn l2x-error
debug vpdn pppoe-error
debug vpdn pppoe-events

Aug 16 17:01:13: AAA/BIND(0000033C): Bind i/f Virtual-Template1 
Aug 16 17:01:13:  EVT: Dynamic Bind 0 0x44315440
Aug 16 17:01:13: ppp824 PPP: Phase is ESTABLISHING
Aug 16 17:01:13: ppp824 EVT: Set LCP Open 3 0x00000000
Aug 16 17:01:13: ppp824 LCP: I FORCED rcvd CONFACK len 15
Aug 16 17:01:13: ppp824 LCP:    MRU 1492 (0x010405D4)
Aug 16 17:01:13: ppp824 LCP:    AuthProto CHAP (0x0305C22305)
Aug 16 17:01:13: ppp824 LCP:    MagicNumber 0x47A8720D (0x050647A8720D)
Aug 16 17:01:13: ppp824 LCP: I FORCED sent CONFACK len 10
Aug 16 17:01:13: ppp824 LCP:    MRU 1440 (0x010405A0)
Aug 16 17:01:13: ppp824 LCP:    MagicNumber 0x0016E1B6 (0x05060016E1B6)
Aug 16 17:01:13: ppp824 PPP LCP not accepting sent CONFACK
Aug 16 17:01:13: ppp824 LCP: State is Closed
Aug 16 17:01:13: ppp824 PPP: Sending Acct Event[Down] id[33C]
Aug 16 17:01:13: ppp824 PPP: Phase is DOWN
Aug 16 17:01:13: ppp824 EVT: Free PPP 0 0x00000000
Aug 16 17:01:13: renshawr@pa.net Tnl/Sn 8718/822 L2TP: Destroying session, informational traceback below:
Aug 16 17:01:13: -Traceback= 40FE3824 40FE1F84 40FE1FB8 40FE6400 40FD3BB4 40FD3E20 40FD4168 40FD41E0 40FD4568
</pre>
</small> 

The router and the client appeared to be arguing over MTU negotiation and giving up. cisco has 
<a href="http://www.cisco.com/warp/public/471/l2tp_mtu_tuning.html">some information</a> on this, 
but the proscribed fix is non-functional. We had to set the virtual-template MTU to 1440:

<small>
<pre>
interface Virtual-Template1
 mtu 1440
 ip unnumbered Ethernet1/0/0
 ip access-group egads in
 ip mroute-cache
 peer default ip address pool local
 ppp authentication pap chap byradiusd
 ppp authorization byradiusd
</pre>
</small>

Hopefully, no users will require a smaller MTU. 

<p>
<hr align=center width=75% noshade>
<p>

IF David Lynch ambled onto a Steven Spielberg production and started taking still photos of things, they'd
probably be like <a href="http://www.amazon.com/exec/obidos/ASIN/0810910039/qid%3D1061068090/">Gregory 
Crewdson's Twilight</a> photos. Vivid dreams rock, it's just been too long since I've had one. 

<p>