I've got a LAN light problem--they blink all the time.

Dlink DI-524:
Static WAN and LAN addy: 192.168.1.2xx
DHCP disabled
UPNP disabled

Dlink DI-624: connected to DSL modem
WAN PPPoE assigned by ISP
LAN addy: 192.168.1.x
DHCP range 192.168.1.100 - .150

Hard wire between the routers on LAN ports.
Both routers have different names on the network.
All comps have network access and internet access=it works.
BUT, the LAN port lights (that have cables connected) on both routers constantly blink.
When I unplug the Dlink DI-524 from the Dlink DI-624 the lights stop blinking.
What have I missed? What is being sent between the two routers causing the LAN to stay active all the time?