11.3.2 Connecting to neighboring routers
- <Structure of this section>
(1) Hello datagram
Routers running OSPF send hello packets on each interface to verify that the links between routers are active. A hello packet is the means by which a router recognizes other routers running OSPF.
(2) Connection conditions with neighboring routers
For each network that provides a direct connection between routers, the interface parameters below must be consistent among all connected routers. Routers that do not share the same parameters are not considered to be connected.
(a) Interface address
The interfaces of all routers connected to the same network must have the same IP network address and mask.
(b) Authentication methods and authentication keys
OSPF can use authentication to verify whether routing information received from a connected router was actually sent by that router. When using authentication, the authentication method and key must match on every interface of every router that connects to a given network.
(c) Area ID
To establish a direct connection between routers, the interfaces on both routers must be assigned the same area ID.
(d) Hello Interval, Dead Interval
The hello interval is the sending interval for hello packets. The dead interval is the number of seconds that a router's hello packets must not have been seen before its neighbors declare the connection down. For a router to properly judge when a connection is lost, these two parameters must have the same values on the interfaces of the directly connected routers.
(e) Area settings
The information reported to an area differs for stub areas and NSSAs and other area types. To allow OSPF to determine that two routers are directly connected, the areas to which the interfaces belong must share the same stub configuration.