3Com CoreBuilder 3500, Implementation Manual

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404 C
HAPTER
14: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF)
Important
Considerations
Consider the following guidelines when you work with OSPF routing
policies:
■
You can only apply OSPF policies against external routes. External
routes refer to routes that are advertised over the network using
external link state advertisements (LSAs). These routes include:
■
Directly connected non-OSPF interfaces — Physical interfaces
on the router itself that are not running OSPF and that are directly
connected to the network.
■
RIP routes — Routes to destinations outside the autonomous
system learned via the Routing Information Protocol (RIP) and
imported by autonomous system boundary routers.
■
Static routes — IP routes statically defined by the user.
■
You cannot apply export policies against directly connected OSPF
interfaces, because all routers in the area must maintain identical link
state databases.
■
With the ability to wildcard policy parameters (such as 0.0.0.0 to
indicate all routers or all routes), occasions may arise when several
policies match a route. In such cases, routers use the following
procedure to determine which policy to apply to the route:
■
If multiple policies apply to the route, the router uses the policy
that has the highest administrative weight.
■
If multiple matches still exist, the router uses the policy that
matches the specified source (excluding wildcards).
■
If multiple matches still exist, the router compares route address
bits and uses the policy that best matches the route address
(excluding wildcards).
■
If multiple matches still exist, the router uses the policy that
matches the origin protocol.
■
If multiple matches still exist, then the router uses the policy that
has the lowest index number.