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Chapter 2
Protocol Attributes
Multiple router types can exist within an OSPF area. Table 2.12 lists all
of the OSPF router types and the role that each plays within the area.
In addition to the above-listed responsibilities, a router can also be
assigned additional responsibilities. These additions are assumed when a
router is assigned the role of DR or BDR.
OSPF Packet Structure
Finally, look at the packet structure. It is important to realize that all OSPF
packets begin with the same 24-byte header. Different information can be
appended, depending on the type of OSPF packet, but all share the same first
24 bytes. Figure 2.17 gives a visual of the OSPF packet header.
F I G U R E 2 . 1 7
OSPF packet header structure
T A B L E 2 . 1 2
OSPF Router Types
Router Type
Description of Responsibility
Internal
All interfaces are defined on the same area. All inter-
nal routers have an identical link-state database.
Backbone
This type of router has at least one interface assigned
to area 0.
Area Border
Router (ABR)
Interfaces are connected to multiple OSPF areas. In-
formation specific to each area is stored on this type
of router.
Autonomous
System Border
Router (ASBR)
This type of router has an interface connected to an
external network or a different AS.
23
16
12
8
4
2
1
0
14
Authentication
Authenti-
cation
type
Check-
sum
Area ID
Router ID
Length Type Ver
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