Passive-interface command behavior in RIP, EIGRP & OSPF
from http://networkers-online.com/blog/2008/08/how-to-avoid-dns-lookup-when-mistyping-a-command/
Passive-interface command is used in all routing protocols to disable sending updates out from a specific interface. However the command behavior varies from one protocol to another.
In RIP this command will disable sending multicast updates via a specific interface but will allow listening to incoming updates from other RIP speaking neighbors.
This simply means that the router will still be able to receive updates on that passive interface and use them in the routing table.
In EIGRP the passive-interface command stops sending outgoing hello packets, hence the router can not form any neighbor relationship via the passive interface. This behavior stops both outgoing and incoming routing updates.
In OSPF the passive-interface has a similar behavior to EIGRP. The command suppresses hello packets and hence neighbor relationships.
I have only one final note here regarding sending unicast updates:
If you used the neighbor command under the RIP process, the router will send unicast updates as well as multicast updates.The passive-interface command must be used disable multicast updates and allowing only unicast.
In EIGRP the neighbor command disables multicast updates on an interface by default and allows only unicast updates.
