GRE packets ignored by XP MCE?

Hi,

I started this discussion over at:
http://www.microsoft.com/windowsxp/e...g=en&cr=US&p=1
because I was having problems with my VPN, but its looking more like some
sort of networking problem, so I might have another try here.

So I've got two machines, side-by-side, connected to the same wireless
router, advertised as doing "VPN pass-through". One machine is XP Home
edition, SP2, and connects to my work VPN fine (so I believe the router about
the VPN pass-through). The other is XP Media Center Edition, SP2, and an
identically configured network connection to the work VPN fails to connect.

Using a packet sniffer (ethereal) I can see the PPTP control connection set
up properly. Then, using the GRE connection thus established, the two
machines try to establish a PPP connection using PPP LC. I can see the PPP
LC Configuration Requests going out, and they must be getting out because I
can see the VPN server responding to them. Likewise, I can see the PPP LC
Configuration Requests coming in from the server. The difference is, that
the machine that works responds to those incoming requests, while the machine
that doesn't (the MCE machine) seems to ignore the incoming requests
completely. Both machines have Windows firewall only, configured
identically. But for whatever reason, its as if those incoming GRE packets -
which I can see making it to the machine - are just being ignored.

I'm stumped. Any ideas greatly appreciated....

- rob.