Auos
An example might help -
R1 (s0/0 - 192.168.5.2 ) -> (s0/0 192.168.5.1) R0 (s0/1 - 172.16.5.1 ) -> (s0/0 172.16.5.2) R2
R1 eigrp config
===============
router eigrp 1
network 192.168.5.0
R2 eigrp config
===============
router eigrp 1
network 192.168.5.0
network 172.16.5.0 0.0.0.255
R3 eigrp config
===============
router eigrp 1
network 172.16.5.0 0.0.0.255
So with the above config all routers know about all networks.
1) Change R0 config for EIGRP to
router eigrp 1
network 192.168.5.0
What now happens is that
a) router R0 and R2 do not form a neigborship so R2 is only aware of its directly connected network
b) this is the important bit - R1 is now only aware of the 192.168.5.0 network ie. it's directly connected network. It is no longer aware of the 172.16.5.0/24 network
2) Change R0 config for EIGRP to
router eigrp 1
network 192.168.5.0
network 172.16.5.0 0.0.0.255
passive-interface s0/1
What happens here is that
a) router R0 and R2 do not form a neigborship so R2 is only aware of its directly connected network
b) unlike the previous scenario R1 is now aware of both it's directly connected network and the 172.16.5.0/24 network because this has been entered under the "router eigrp 1" config on R0.
So the network statement under "router eigrp 1" tells the router which interfaces to start running EIGRP on AND then EIGRP advertises out the network/mask found on the interface.
The passive-interface command tells the router not to form any neighborships on that interface but it does not stop EIGRP running on that interface.
Hope this makes sense.
Jon
