Hi Ivaylo,
It won’t prevent us from receiving the hello packets but I believe it does stop processing them. Here’s a little experiment I did with two routers, directly connected to each other:
R1#debug ip ospf hello
OSPF hello debugging is on
Now we can see we are sending and receiving hello packets:
R1#
OSPF-1 HELLO Gi0/1: Send hello to 224.0.0.5 area 0 from 192.168.12.1
OSPF-1 HELLO Gi0/1: Rcv hello from 2.2.2.2 area 0 192.168.12.2
Let’s make the interface passive:
R1(config)#router ospf 1
R1(config-router)#passive-interface GigabitEthernet 0/1
At this moment my debug is not producing any output anymore. Basically OSPF has been disabled on the interface. We are still receiving packets though:
R1#debug ip packet
IP packet debugging is on
R1#
IP: s=192.168.12.2 (GigabitEthernet0/1), d=224.0.0.5, len 80, rcvd 0
IP: s=192.168.12.2 (GigabitEthernet0/1), d=224.0.0.5, len 80, input feature, packet consumed, MCI Check(109), rtype 0, forus FALSE, sendself FALSE, mtu 0, fwdchk FALSE
Above you can see that we are receiving OSPF packets from 192.168.12.2.
About your other question:
OSPF as an IGP (Interior Gateway Protocol) is typically used on our “own” networks. If you have a connection to your ISP for Internet access then all you need is a default route to them…we don’t run OSPF with the ISP.
Hope this helps!
Rene