Troubleshooting Airport Extreme's wan port?

Question

I have an Airport Extreme base station that doesn't seem to be communicating through its WAN port.  Both Apple and Windoze computers can connect to it, wired or wirelessly, but I am unable to make the base station communicate with a gateway device, a (PPoE) DSL modem.  My brother (whose device this is) was having the same problem with his cable modem.
As the Airport boots, the link light on the WAN port blinks green a couple of times, only instantaneously, but otherwise remains dark.  Once it's booted, the status light blinks amber.  And of course there is no throughput, no connectivity to the Internet.
I also have a LinkSys WiFi router that works perfectly using the same gateway device and the same ethernet cable, which gives me to believe both the modem and the cable are in proper working order.  All the configuration information I'm entering into the Airport's web interface I'm cut and pasting from the same text file I habitually use to the LinkSys router (to cut down on clerical errors).  So presuming it's configured properly, presuming the cable and the gateway aren't at fault, I have to think the WAN port has given up the ghost.
So I thought to ask the Apple cognoscenti if they agree with my diagnosis.  Is there a diagnostic I'm overlooking?  Do the WAN ports on these devices have a history of being problematic?
TIA
Slanjevar!

Answer 1

I have discovered the chief problem was that the Airport doesn't take kindly to unresolved/unacknowledged errors/conditions under the Status button on the Summary page of the Airport utility.  It was regarding "extended mode" as an abnormality requiring acknowledgment, as well as my switching off all wireless security, as a temporary diagnostic measure.  Once I'd told it to 'Ignore' those conditions, I got both a solid green status light and a steady green link light on the WAN port.
However, it still isn't passing data.
As I was stumbling around in the Airport utility's GUI (before resolving the "start button" problem), I noticed it had an "Extend a wireless network" mode.  Since what my brother was after in the first place was getting stronger signal to the far end of the house, I asked if this wouldn't better suit his needs than substituting the Airport for his current (underpowered?) WiFi router.  He agreed, so I've changed my focus to setting up the Airport as an 'extender.'
Now, reconfigured in 'extender' mode, its 'Status' on the GUI's 'Summary' page shows 'Normal' and the status light is steady green.  The GUI for my WRT54 (with DD-WRT firmware) shows it is authenticated and joined to the network, as both a LAN and a wireless client.  And the Airport's logs concur: "Joined BSS [WRT54's MAC]" and "Installed unicast TKIP key for authenticator [WRT54's MAC]".
I've done this with the Airport either using DHCP or with a statically configured IP address to see if it made any difference.  It does not, either to its ability to join the network or to its (lack of) throughput.  So every indication I can find confirms it is a connected and fully functioning member of my network.
I also have an XPSP3 box connected to one of the LAN ports on the Airport.  The NICs on both ends show a solid green link light.  The Airport's logs acknowledge that the XP box is connected: "Connection accepted from [::ffff:192.168.1.3]".  And the XP box can 'ping' the Airport.  All of which spells "connected" to me.
However, the XP box can neither draw an IP address (the Airport's DHCP clients list remains empty) nor communicate with anything upstream of the Airport (which it only can do once I have given it a static IP address).  If I try to tracert or pathping from the XP box to the WRT54 (via the Airport), it stalls out at the Airport.  The response is the same whether I have the Airport's WAN port connected to a LAN port on the WRT54 or not.
Address scheme-wise, my WiFi router (WRT54) is on 192.168.1.1.  The XP box that I have connected to the Airport via ethernet has a reserved IP of 192.168.1.3, but now is configured statically to that address because the Airport (seemingly) does not support it contacting the DHCP server.  The Airport consistently had been drawing the first available address from the DHCP pool, 192.168.1.10, but now I have it statically configured to .100 because my brother doesn't use DHCP reservations on his network, and I'm replicating how he will configure it to leave nothing to chance.  Having a static address saves him grief having to figure out what its address has become should the Airport utility not be able to find it (which seems to be its norm) and he has to connect manually through the Airport utility.
I've spent almost every waking minute tinkering with this thing since midday yesterday and I've not yet seen evidence that the Airport has passed so much as a single bit of data.  And it isn't just the WAN port, it doesn't want to pass anything wirelessly either.  This thing is about to get my goat.  Remind me never again to offer my brother a favor.
Any thoughts?  Humorous anecdotes?  Naughty limericks?
EDIT:
I also have a network detector on my cell phone than can identify every device, wireless or otherwise, on the network its connected to.  But it does not "see" either the Airport or, naturally, the XP box beyond it.