link, created so that the neighboring routers have a means to exchange their LSDBs.
the same LSAs.
local copy of the LSDB, calculating the best routes, and adding those to the IPv4 routing
Of everything you learn about OSPF in this chapter, OSPF neighbor concepts have the most
to do with how you will configure and troubleshoot OSPF in Cisco routers. You configure
OSPF to cause routers to run OSPF and become neighbors with other routers. Once that
happens, OSPF does the rest of the work to exchange LSAs and calculate routers in the
background, with no additional configuration required. This section discusses the fundamen-
OSPF neighbors are routers that both use OSPF and both sit on the same data link. Two
routers can become OSPF neighbors if connected to the same VLAN, or same serial link, or
Two routers need to do more than simply exist on the same link to become OSPF neighbors;
they must send OSPF messages and agree to become OSPF neighbors. To do so, the routers
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send OSPF Hello messages, introducing themselves to the potential neighbor. Assuming the
two potential neighbors have compatible OSPF parameters, the two form an OSPF neighbor
relationship, and would be displayed in the output of the show ip ospf neighbor command.
The OSPF neighbor relationship also lets OSPF know when a neighbor might not be a good
option for routing packets right now. Imagine R1 and R2 form a neighbor relationship, learn
LSAs, and calculate routes that send packets through the other router. Months later, R1
notices that the neighbor relationship with R2 fails. That failed neighbor connection to R2
makes R1 react: R1 refloods LSAs impacted by the failed link, and R1 runs SPF to recalculate
its own routes.
Finally, the OSPF neighbor model allows new routers to be dynamically discovered. That
means new routers can be added to a network without requiring every router to be reconfig-
ured. Instead, OSPF routers listen for OSPF Hello messages from new routers and react to
those messages, attempting to become neighbors and exchange LSDBs.
Meeting Neighbors and Learning Their Router ID
The OSPF Hello process, by which new neighbor relationships are formed, works somewhat
like when you move to a new house and meet your various neighbors. When you see each
other outside, you might walk over, say hello, and learn each other’s name. After talking a
bit, you form a first impression, particularly as to whether you think you’ll enjoy chatting
with this neighbor occasionally, or whether you can just wave and not take the time to talk
the next time you see him outside.
Similarly, with OSPF, the process starts with messages called OSPF Hello messages. The
Hellos in turn list each router’s router ID (RID), which serves as each router’s unique name or
identifier for OSPF. Finally, OSPF does several checks of the information in the Hello mes-
sages to ensure that the two routers should become neighbors.
OSPF RIDs are 32-bit numbers. As a result, most command output lists these as dotted-dec-
imal numbers (DDN). By default, IOS chooses one of the router’s interface IPv4 addresses to
use as its OSPF RID. However, the OSPF RID can be directly configured, as covered in the
section “Configuring the OSPF Router ID” in Chapter 20, “Implementing OSPF.”
As soon as a router has chosen its OSPF RID and some interfaces come up, the router is
ready to meet its OSPF neighbors. OSPF routers can become neighbors if they are connect-
ed to the same subnet. To discover other OSPF-speaking routers, a router sends multicast
OSPF Hello packets to each interface and hopes to receive OSPF Hello packets from other
routers connected to those interfaces. Figure 19-6 outlines the basic concept.
Hello
Interval
Hello
Hello
Hello
Interval
Hello
Hello
R1
R2
Figure 19-6
OSPF Hello Packets
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Chapter 19: Understanding OSPF Concepts 453
Routers R1 and R2 both send Hello messages onto the link. They continue to send Hellos at
a regular interval based on their Hello timer settings. The Hello messages themselves have
the following features:
■
The Hello message follows the IP packet header, with IP protocol type 89.
■
Hello packets are sent to multicast IP address 224.0.0.5, a multicast IP address intended
for all OSPF-speaking routers.
■
OSPF routers listen for packets sent to IP multicast address 224.0.0.5, in part hoping to
receive Hello packets and learn about new neighbors.
Taking a closer look, Figure 19-7 shows several of the neighbor states used by the early for-
mation of an OSPF neighbor relationship. The figure shows the Hello messages in the center
and the resulting neighbor states on the left and right edges of the figure. Each router keeps
an OSPF state variable for how it views the neighbor.
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