At Layer 3 of the OSI model, Hello packets are addressed to the multicast address 224.0.0.5. This address effectively means all OSPF routers. OSPF routers use Hello packets to initiate new adjacencies and to ensure that adjacent neighbors have not disappeared. Hellos are sent every ten seconds by default on multiaccess and point-to-point networks. On interfaces that connect to NBMA networks, such as Frame Relay, Hellos are sent every 30 seconds.

Although the Hello packet is small, often less than 50 bytes, Hellos contain plenty of vital information. Like other OSPF packet types, Hello packets include an OSPF packet header, which has the form shown in Figure .

All five types of OSPF packets use the OSPF packet header, which consists of eight fields. The purpose of each of these fields is described as follows:

• Version, Type, and Packet Length – The first three fields of the OSPF packet let the recipients know the version of OSPF that is being used by the sender, the OSPF packet type, and length. The OSPF version could be either version 1 or 2. OSPF version 2 was first introduced in 1991 (RFC 1247) and is not compatible with version 1, which is obsolete. The Cisco IOS uses OSPF version 2 and cannot be configured to use OSPF version 1.
• Router ID – The function of the Hello packet is to establish and maintain adjacencies. So the sending router signs the fourth field with its router ID, which is a 32-bit number used to identify the router to the OSPF protocol. A router uses its IP address as its ID because both the router ID and the IP address must be unique within a network. Because routers support multiple IP addresses, a loopback IP address is used as the router ID. In the absence of a loopback IP address, the highest value address interface IP is used as the router ID. This is regardless of whether that interface is involved in the OSPF process.

If the interface associated with that IP address goes down, the router can no longer use that IP address as its router ID. When the ID of a router changes for any reason, the router must reintroduce itself to its neighbors on all links. To avoid the unnecessary overhead caused by reestablishing adjacency and readvertising link states, an administrator typically assigns an IP address to a loopback interface. Unless an administrator shuts down a loopback interface, it always stays up, so loopback interfaces make ideal router IDs.

Note: If a loopback interface is configured with an IP address, the Cisco IOS will use that IP address as the router ID. This happens even if the other interfaces have higher addresses.

• Area ID – Multiple areas can be defined within an OSPF network to reduce and summarize route information. This allows large and complex networks to continue to grow. When configuring a single area OSPF network, always use Area 0 because it is defined as the "backbone" area. There must be a backbone area to scale or add other OSPF areas.
• Checksum – A 2-byte checksum field is used to check the message for errors as seen with other protocols. Good packets are retained and damaged packets are discarded.
• Authentication Type and Authentication Data – OSPF supports different methods of authentication so that OSPF routers will not believe just anyone sending Hellos to 224.0.0.5. Routers with unequal authentication fields will not accept OSPF information from each other.

The Hello header, which is found only in Type1 Hello packets, carries essential information. The following are the fields in the Hello header:

• Network Mask – This 32-bit field carries subnet mask information for the network.
• Hello Interval and Dead Interval – The Hello interval is the number of seconds that an OSPF router waits to send the next Hello packet. The default for multiaccess broadcast and point-to-point networks is ten seconds. The dead interval is the number of seconds that a router waits before it declares a neighbor down. That is if the neighbor's Hello packets are no longer being received. The dead interval is four times the Hello interval by default, or 40 seconds. Both of these intervals are configurable, which is the reason they are advertised. If two routers have different Hello intervals or if they have different dead intervals, they will not accept OSPF information from each other.
• Options – The router can use this field to indicate optional configurations, including the stub area flag, which is discussed in Module 5.
• Router Priority – This field contains a value that indicates the priority of this router when selecting a designated router (DR) and backup designated router (BDR). The default priority is one (1) and can be configured to a higher number to ensure that a specified router becomes the DR.
• Designated Router and Backup Designated Router – The router IDs of the DR and BDR are listed here, if known by the source of the Hello packet.
• Neighbor Address – If the source of the Hello packet has received a valid Hello from any neighbor within the dead interval, its router ID is included here.

Interactive Media Activity Drag and Drop: OSPF Packet Header Upon completion of this activity, the student will be able to identify the different fields in an OSPF packet header.