How OSPF Works

OSPF routers must first discover each other before they can exchange their topological databases. Once each router has the complete topological database, it can use the SPF algorithm to compute the shortest path to every network. This section focuses on neighbor discovery and the exchange of topological databases.

Let’s begin at the beginning. OSPF packets are encapsulated directly in IP with the protocol field set to 89. The destination IP address in OSPF depends on the network type. OSPF uses two IP multicast addresses on broadcast and point-to-point networks: 225.0.0.5 for all OSPF routers and 224.0.0.6 for all DR/BDR (designated router/backup designated router) routers. Using IP multicast addresses is more efficient than using broadcast addresses. If broadcast addresses are used, all attached devices must receive the broadcast packet, unwrap it, and then discard the contents if they are not running OSPF. NBMA networks and virtual links use unicast addresses because they do not support multicast addresses.

Following the IP header is the OSPF header (see Figure 6-5). The OSPF header is common to all types of OSPF packets. The following list defines the format of the OSPF header and the five types of OSPF packets:

Version

The OSPF version in use. The current version number is 2.

Type

There are five types of OSPF packets:

Type 1

Hello packets, described in the next section.

Type 2

Database description packets, described later under Section 6.4.5.

Type 3

Link state requests, ...

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