3Com CoreBuilder 3500, Implementation Manual, Page: 382 / 592

382 C
HAPTER
14: O
PEN
S
HORTEST
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ATH
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IRST
(OSPF)
Specifying Cost Metrics for Preferred Paths
In OSPF, the best path is the one that offers the least-cost metric. A cost is
associated with each router output interface and each route as follows:
■
Each output interface is assigned a default cost by the system based
on the media bandwidth to which it is attached.
■
Each route is assigned a metric that is equal to the sum of all metrics
for all the links in the route.
You can configure area routers to use preferred paths by manually setting
higher cost metrics for those paths that are not preferred.
For example, the fastest default media for OSPF is FDDI. All interfaces that
are attached to a Fiber Distributed Data Interface (FDDI) media have a
default cost metric of 1. All interfaces attached to faster media types,
such as Gigabit Ethernet, are also assigned a default cost of 1. To ensure
that a particular media interface is the preferred route, leave that link
with its metric cost of 1 and manually configure the other links with a
higher cost metric — for example, 2.
Delay The transmit delay is the estimated time (in seconds) that it takes for the
system to transmit a link state update packet on the interface. The system
increases the age of the link state advertisements (LSAs) that are
contained in the update packets by the value that you specify for the
delay.
This delay setting has more significance for interfaces that are connected
to very low speed links because, on slower speed links, it is more
probable that the router may send out back-to-back data packets more
quickly than other routers and hosts can receive them. To avoid this
situation, set the transmit delay to configure the router to wait a specified
number or seconds between transmissions.