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100

101

Configuring OSPF Services

4-19

OSPF recognizes that a simple hop count takes no account of reliability,

bandwidth, delay, or the actual dollar cost of using a path. Passing through an

extra hop to get to a 1.54-Mb T1 channel, for instance, may be more efﬁcient than

traversing a shorter, but slower route. For OSPF, the best path is the one that offers

the least-cost metric delay. With the Bay Networks implementation of OSPF,

every path automatically takes on a cost metric value of 1. You must conﬁgure cost

metrics if you want to specify a preferred path. To specify a preferred path, you

would allow the preferred path to retain the cost metric value of 1, and then assign

higher-cost metric values to the less-preferred paths.

Figure 4-8 shows the beneﬁt of using conﬁgurable cost metrics. Assigning the

56-Kb line a cost metric value of 10 forces OSPF to choose the faster T1 line path

as the best path, despite the extra hop, when transmitting a packet from Host A to

Host B.

Figure 4-8. Conﬁgurable Cost Metrics Usage Example

For instructions on using Site Manager to specify a cost metric for an OSPF interface, see

the Metric Cost parameter on page 4-33.

Router 1

Router 2

Router 3

Host

56 Kb

Assigned Cost

Metric = 10

1.544Mb

Assigned Cost

Metric = 1

1.544Mb

Assigned Cost

Metric = 1

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