
Configuring IP Services
5-26 114065 Rev. A
Configuring AS External Advertisements
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.
Figure 5-4, 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. The internal router selects the BGP route -- the route with the lower weight.
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