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Configuring OSPF Services

4-15

Specifying the Metric Type for an ASE Advertisement

Each ASE advertisement that a boundary router injects into the AS includes a

Type 1 or Type 2 metric. The Type 1 metric is equivalent to the metric of the non-

OSPF route. The Type 2 metric is either the metric of the non-OSPF route or the

weight value calculated for that route (see “Using the Route Weight as the Type 2

Metric” on page 4-15).

By default, an OSPF boundary router generates a Type 2 metric for BGP, EGP, or

RIP routes. For routes from all other sources, the boundary router generates a

Type 1 metric. You can construct an OSPF announce policy to override the default

metric type.

Using the Route Weight as the Type 2 Metric

The network administrator has the option of generating OSPF AS external (ASE)

routes that use the route weight as the Type 2 metric. For information about route

weights, see “Route Weights” on page 1-12.

Figure 4-6, for example, shows three routers in an OSPF domain. Router A and

router B are both conﬁgured to generate ASE routes using the route weight as the

Type 2 metric.

1. Boundary router A learns a route to destination X via EGP.

2. Boundary router A advertises the route to the internal router as an OSPF ASE

route. The Type 2 metric in the advertisement contains the route weight value

calculated for the EGP route to destination X.

3. Boundary router B learns a route to destination X via BGP.

4. Boundary router B advertises the route to the internal router as an OSPF ASE

route. The Type 2 metric in the advertisement contains the route weight value

calculated for a BGP route.

5. To determine the preferable route, the internal router compares the Type 2

metrics — the EGP route weight and the BGP route weight.

6. Router C selects the BGP route — the route with the lower weight.

For details on using an announce policy to specify the metric type for an ASE route that a

boundary router injects into an OSPF domain, see the Type parameter on page 9-35.

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Avaya Configuring IP Services Manual de usuario Pagina 153