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478 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
External Link State
Advertisements
Each autonomous system boundary router generates an external link
state advertisement for each network destination (known to the router)
outside the AS. AS boundary routers use these external link state
advertisements to describe routes to destinations outside the AS. For
these advertisements, the Link State ID field in the advertisement header
specifies an IP address.
In addition, OSPF also considers the following routes to be external
routes. They are advertised using external link state advertisements:
■
The default route
■
Static routes
■
Routes derived from other routing protocols, such as RIP
■
Directly connected networks that are not running OSPF
All external routes are assigned a cost metric. External route cost is
calculated based on one of these cost metric types:
■
Type 1 — Router adds the internal cost metric to the external route
metric. For example, if an ABR is advertising Type 1 external route
metrics, the cost of the route from any router within the AS is equal to
the cost associated with reaching the advertising ABR, plus the cost of
the external route.
■
Type 2 — Routers do not add the internal route metric to the external
route metric. Therefore, the router that advertises the smallest
external metric is chosen, regardless of the internal distance to the AS
boundary router. For example, if an ABR is advertising Type 2 external
route metrics, the cost of the route from any router within the AS is
equal to the cost of the external route alone. The cost of reaching the
advertising ABR is not considered in determining the cost of the
external route. The internal cost is only used as a
tie-breaker when
several equal-cost Type 2 routes exist.
When both Type 1 and Type 2 metrics are present in an AS, Type 1
external metrics take precedence.