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Chapter 2
Protocol Attributes
Initializing OSPF
The Hello protocol is used to establish peering sessions between routers.
Hello packets are multicast out every interface. The information that is mul-
ticast includes the router ID, timing intervals, existing neighbors, area iden-
tification, router priority, designated and backup router information,
authentication password, and stub area information. All this information is
used when establishing new peers. Descriptions of each element can be found
in Table 2.10.
Figure 2.16 displays a flow chart that depicts each step of the initialization
process. The process starts by sending out Hello packets. Every listening
router then adds the originating router to the adjacency database. The
responding routers reply with all of their Hello information so that the orig-
inating router can add them to its adjacency table.
T A B L E 2 . 1 0
OSPF Multicast Information
Information Description
Router ID
This is the highest active IP address on the router.
Time intervals
Contains intervals between Hello packets and the
dead time interval.
Existing neighbors
Contains addresses for any existing OSPF neigh-
bors.
Area identification
OSPF area number and link information must be
the same for a peering session to be established.
Router priority
This value is used when choosing the DR and BDR.
DR and BDR
If they have already been chosen, their information
is contained in the Hello packet.
Authentication
password
All peers within the same area must have the same
authentication password if authentication is
enabled.
Stub area flag
This is a special area--two routers must share the
same stub information. This is not necessary to ini-
tiate a regular peering session with another OSPF
router.
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