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Open Shortest Path First (OSPF) is a
link-state routing protocol based on open standards. Described in
several RFCs, most recently RFC 2328, the Open in Open Shortest
Path First means that OSPF is open to the public and
nonproprietary. Among nonproprietary routing protocols, such as
RIP v1 and RIP v2, OSPF is preferred because of its remarkable
scalability. Recall that both versions of RIP are very limited.
RIP cannot scale beyond 15 hops. It converges slowly, and it
chooses suboptimal routes that ignore critical factors such as
bandwidth. OSPF addresses all of these limitations and proves to
be a powerful, scalable routing protocol.
The considerable capability that
OSPF has to
scale is achieved through hierarchical design and the use of
areas. By defining areas in a properly designed network, an
administrator can reduce routing overhead and improve performance.
To achieve scalability OSPF relies on complex communications and
relationships to maintain a comprehensive link-state database.
However, as an OSPF network scales to 100, 500, or even 1000
routers, link-state databases can balloon to include thousands of
links. To help OSPF routers route more efficiently and to preserve
their CPU and memory resources for the business of switching
packets, network engineers divide OSPF networks into multiple
areas.
This module describes how to create
and configure OSPF. Specifically, this module examines the
different OSPF area types, which include stubby, totally stubby,
and not-so-stubby areas (NSSAs). Each of these different area
types uses a special advertisement to exchange routing information
with the rest of the OSPF network. Therefore, link-state
advertisements (LSAs) will be studied in detail. The Area 0
backbone rule and how virtual links can work around backbone
connectivity problems will also be reviewed. Finally, this module
surveys important show
commands that can be used to verify multiarea OSPF operation.
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