Neighborship The Hello Packet
Neighbor OSPF routers must recognize each other on the network before they can share information because OSPF routing depends on the status of the link between two routers. This process is done using the Hello protocol. The Hello protocol establishes and maintains neighbor relationships by ensuring bidirectional (two-way) communication between neighbors. Bidirectional communication occurs when a router recognizes itself listed in the hello packet received from a neighbor.
Each interface participating in OSPF uses IP multicast address 224.0.0.5 to send hello packets periodically. A hello packet contains the following information:
■ Router ID: The router ID is a 32-bit number that uniquely identifies the router. The highest IP address on an active interface is chosen by default, unless a loopback interface or the router ID is configured; for example, IP address 172.16.12.1 would be chosen over 172.16.1.1. This identification is important in establishing neighbor relationships and coordinating LSU exchanges. Also, the router ID breaks ties during the designated router (DR) and backup designated router (BDR) selection processes if the OSPF priority values are equal.
■ Hello and dead intervals: The hello interval specifies the frequency in seconds at which a router sends hello packets (10 seconds is the default on multiaccess networks). The dead interval is the time in seconds that a router waits to hear from a neighbor before declaring the neighboring router out of service (four times the hello interval by default). These timers must be the same on neighboring routers; otherwise an adjacency will not be established.
3-20 Building Scalable Cisco Internetworks (BSCI) v3.0 © 2006 Cisco Systems, Inc.
■ Neighbors: The neighbors field lists the adjacent routers with established bidirectional communication. This bidirectional communication is indicated when the router recognizes itself listed in the neighbors field of the hello packet from the neighbor.
■ Area ID: To communicate, two routers must share a common segment, and their interfaces must belong to the same OSPF area on that segment (they must also share the same subnet and mask). These routers will all have the same link-state information.
■ Router priority: The router priority is an 8-bit number that indicates the priority of a router. Priority is used when selecting a DR and BDR.
■ DR and BDR IP addresses: These are the IP addresses of the DR and BDR for the specific network, if they are known.
■ Authentication password: If router authentication is enabled, two routers must exchange the same password. Authentication is not required, but if it is enabled, all peer routers must have the same password.
■ Stub area flag: A stub area is a special area. Two routers must agree on the stub area flag in the hello packets. Designating a stub area is a technique that reduces routing updates by replacing them with a default route.
Note After a DR and BDR are selected, any router added to the network will establish adjacencies with the DR and BDR only.
© 2006 Cisco Systems, Inc. Configuring OSPF 3-21
Continue reading here: Establishing Bidirectional Communication
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