Figure 10-32 Common Scenario for OSPF routing protocol
The network topology is more prone to changes in an autonomous system of larger size. The
network adjustment of any one router could destabilize the whole network and cause massive
OSPF packets to be forward repeatedly, and all the routers need to recalculate the routes,
which would waste lots of network resources. In this case, area partition would be an effective
solution. The routers only need to maintain the same link state database in their own area, and
then the ABR would collect the routing information from different areas and advertise to other
areas. For more details about area partition, please refer to the following chapters.
OSPF Principles
This section would introduce in details the working principles of OSPF protocol. First of all, let’s
get to know some basic concepts about the OSPF routing protocol.
1.
Autonomous System
Autonomous System, short for AS, is a set of routers using the same routing protocol to
exchange routing information. OSPF, working within an AS, is an internal gateway protocol.
2.
Router ID
A router running OSPF protocol identifies its uniqueness by its router ID – a 32-bit unsigned
integer, which could be manually assigned by the administrator or automatically selected by
the router itself. In case different routers might obtain the same ID in automatic selection, you
are recommended to configure router ID manually.
In RFC protocol, two means of automatically electing router ID are recommended:
If the loopback interfaces are configured, the highest IP address among them will be
selected as the router ID.
If no loopback interface is configured, the highest IP address among those of active router
interfaces will be selected as the router ID.
The good stability of loopback interfaces (always in active state as long as the router boots)
ensures that every time the router boots it would automatically elect the loopback interface IP
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