Understand Open Shortest Path First (ospf) Design Guide

ip ospf message-digest-key keyid md5 key 
(used under the interface)
●
area area-id authentication message-digest
(used under 
router ospf
)
●
Here is an example:
interface Ethernet0
ip address 10.0.0.1 255.255.255.0
ip ospf message-digest-key 10 md5 mypassword
router ospf 10
network 10.0.0.0 0.0.255.255 area 0
area 0 authentication message-digest

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The Backbone and Area 0
OSPF has special restrictions when multiple areas are involved. If more than one area is
configured, one of these areas has be to be area 0. This is called the backbone.
It is good network design practice to start with area 0 and then expand into other areas later on.
The backbone has to be at the center of all other areas, that is, all areas have to be physically
connected to the backbone.
The reason is that OSPF expects all areas to inject routing information into the backbone and in
turn the backbone disseminates that information into other areas.
This diagram illustrates the flow of information in an OSPF network:
In this diagram, all areas are directly connected to the backbone. In the rare situations where a
new area is introduced that cannot have a direct physical access to the backbone, a virtual link
has to be configured.
Virtual links are discussed in the next section. Note the different types of routing information.
Routes that are generated from within an area (the destination belongs to the area) are called
intra-area routes.
These routes are normally represented by the letter O in the IP routing table. Routes that originate
from other areas are called 
inter-area
or 
Summary routes
.
The notation for these routes is O IA in the IP routing table. Routes that originate from other
routing protocols (or different OSPF processes) and that are injected into OSPF via redistribution

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are called