Why does my Cisco 3662,3841, catalyst 3750s send EIGRP information to address 184.108.40.206?
The passive-interface command stops hello packets being exchanged between EIGRP neighbours. This means a neighborship si not formed and so no routing updates are sent or received.
You configure it under your router eigrp section eg
router eigrp 1
This means any other EIGRP routers on the subnet that fa0/1 connects to will not exchange hellos with this router and hence no neighborship is formed.
The impact of using this command is that you have just stopped the router receiving routing updates and sending routing updates on it's fa0/1 interface. This may well result in an incomplete routing table.
Attached is a link to a doc about use of passive-interface in EIGRP.
No it won't cause congestion on a LAN as it really doesn't account for much of the traffic you will see on a LAN.
What should be said is that even though it is a multicast address all devices, routers, PC's, servers, on that subnet will see the update.