Open Shortest Path First
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OSPF Operation
OSPF operation can be divided into three categories:
Neighbor and adjacency initialization
LSA flooding
SPF tree calculation
We will discuss each in the following sections.
Neighbor and Adjacency Initialization
We begin with neighbor/adjacency formation. This is a very big part of OSPF
operation. These relationships are often easily formed over point-to-point
connections, but much more complex procedures are required when multiple
OSPF routers are connected via a broadcast multi-access media.
The Hello protocol is used to discover neighbors and establish adjacen-
cies. Hello packets contain a great deal of information regarding the origi-
nating router. Hello packets are multicast out every interface on a 10-second
interval by default. The data contained in the Hello packet can be seen in
Table 4.1. It is important to remember that the Router ID, Area ID, and
authentication information are carried in the common OSPF header. The
Hello packet uses the common OSPF header.
T A B L E 4 . 1
OSPF Hello Packet Information
Originating Router
Characteristic
Description
Router ID
The highest active IP address on the router.
(Loopback addresses are used first. If no loop-
back interfaces are configured, OSPF will choose
from physical interfaces.)
Area ID
The area to which the originating router interface
belongs.
Authentication
information
The authentication type and corresponding
information.
Network mask
The IP mask of the originating router's interface
IP address.
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