I'm using a 9g20 demo board with linux-2.6.30-at91-exp.3-at91sam9g20ek.bin and Angstrom-console-at91sam9-image-glibc-ipk-2009.X-stable-at91sam9g20ek.rootfs.jffs2. I have the network interface connect to a PC and I am running Wireshark on that PC. Even with the ping command on the board I can not see any traffic. Any ideas? I did this:

eth0: link up (100/Full)
root@at91sam9g20ek:~# ifup eth0
ifup: interface eth0 already configured

The ping command tells me "ping: sendto: Network is unreachable"

This is ifconfig:

eth0 Link encap:Ethernet HWaddr 02:43:1F:B7:68:3E
inet addr:192.168.1.102 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1118 errors:2 dropped:0 overruns:0 frame:0
TX packets:13 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:90472 (88.3 KiB) TX bytes:2429 (2.3 KiB)
Interrupt:21 Base address:0x4000

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:16 errors:0 dropped:0 overruns:0 frame:0
TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:1168 (1.1 KiB) TX bytes:1168 (1.1 KiB)

It's connected via a hub/switch or crossover cable?

It is directly connected to the PC with a crossover cable. I've set/unset some of the Network Properties in the PC, with same result. (I always have TCP/IP enabled, but then I tried with the other properties all on and all off.)

Why does the 9g20 board detect the connection (the LEDs come on) and give the message "eth0: link up (100/Full)" if it can't actually use the network? It must be a setting in software?

It was more than just the need for ifconfig. I also had to use 'route' to set the default gateway. I used the default gateway that the windows machine was using to get on the internet (seen via ipconfig) and then I did this:

I saw via 'route -n' that Genmask was 255.255.255.0 and so I used 'ifconfig' to give the board an IP address where the first 3 nibbles matched the first 3 nibbles of the default gateway. So if windows said the default gateway was "1.2.3.1" I gave the board and IP address of "1.2.3.2" (ifconfig eth0 1.2.3.2). Then I did 'route add default gw 1.2.3.1' and then I could reach outside network addresses!