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.
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
220.127.116.11 for all OSPF
18.104.22.168 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:
The OSPF version in use. The current version number is 2.
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, ...