This
process is done using the Hello protocol. The Hello protocol establishes and maintains neighbor
142 Chapter 4: Single-Area OSPF Implementation
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. Figure 4-2 illustrates the hello packet.
Figure 4-2 OSPF Hello
Each interface that is participating in OSPF uses IP multicast address 224.0.0.5 to periodically
send hello packets. A hello packet contains the following information:
?– Router ID: The router ID is a 32-bit number that uniquely identi?¬?es the router. The highest
IP address on an active interface is chosen by default, unless a loopback interface or the router
ID is con?¬?gured; for example, IP address 172.16.12.1 would be chosen over 172.16.1.1. This
identi?¬?cation is important in establishing and troubleshooting neighbor relationships and
coordinating route exchanges.
?– Hello and dead intervals: The hello interval speci?¬?es the frequency in seconds at which a
router sends hello packets. The default hello interval on multiaccess networks is 10 seconds.
Pages:
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228