3Com 4007, Implementation Manual, Page: 450 / 684

450 C
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19: O
PEN
S
HORTEST
P
ATH
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IRST
(OSPF) R
OUTING
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Virtual links — All areas of an OSPF routing domain must connect to
the backbone area. In cases where an area does not have direct,
physical access to the backbone, you can configure a logical
connection to the backbone, called a
virtual link . Virtual links can also
add fault-tolerance and redundancy to the backbone. For more
information, see “Virtual Links” later in this chapter.
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OSPF routing policies — Routing policies let you control what
external routes OSPF routers store in their routing tables, as well as
what external routes they advertise. Although routing policies are not
part of the OSPF protocol itself, you can use them for increased
security, enhanced performance, and overall control of OSPF routing
data. For more information, see “OSPF Routing Policies” later in this
chapter.
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OSPF statistics — You can also display general statistics for specific
OSPF interfaces. These statistics can give you an overview of OSPF
activity on the interface. For more information, see “OSPF Statistics”
later in this chapter.
Benefits The benefits of OSPF are what set it apart from both RIP and other
Shortest Path First-based algorithms before it. While designing OSPF, the
Internet Engineering Task Force (IETF) proposed a number of
modifications which dealt with improving the existing SPF model. These
modifications ranged from improving fault-tolerance to reducing routing
traffic overhead. This focus toward improving the existing SPF model
resulted in the following OSPF capabilities:
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No hop count limitation — OSPF places no limit on hop count. This
capability is extremely important in larger networks. For example, a RIP
network that spans more than 15 hops (15 routers) is considered
unreachable. With OSPF, hop count is no longer an issue.
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Efficient use of bandwidth for router updates — OSPF uses IP
multicast to send link-state updates only when routing changes have
occurred, or once every 30 minutes. RIP, on the other hand, uses a 30
second interval. This policy ensures less processing on routers that are
not listening to OSPF packets and better use of bandwidth.