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490 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
Important
Considerations
Consider the following guidelines when you configure virtual links:
■
You must configure a virtual link for any area border router that has an
interface connected to a location outside the backbone area.
■
You can define up to 32 virtual links per router.
■
You cannot configure a virtual link through a stub area.
■
Use virtual links sparingly for the following reasons:
■
Stability of the virtual link depends on the stability of the
underlying area that it spans.
■
This dependency on underlying areas can make troubleshooting
difficult.
OSPF Routing
Policies
Routing policies are rules that define criteria to control the flow of routes
to and from the routing table. Your system supports two types of OSPF
routing policies:
import policies that dictate which routes are added to
the routing table and export polices that dictate which routes are
advertised to other routers. You can use routing policies to:
■
Increase security — For security reasons, you may not want the
router to advertise certain routes. For example, Organization A may
have defined one of its ASBRs with a direct connection to
Organization B that they use for direct communication. For security or
performance reasons, A may not want to give other groups access to
that connection. To prevent this direct connection from being known
to other organizations, A can define an export policy that prohibits its
ASBR from advertising the direct connection that it uses to
communicate with B.
■
Conserve routing table space
— The selective nature of routing
policies can minimize routing table sizes and increase network stability.
For example, you may want to limit the number of hosts and
gateways from which routing information is accepted, in which case
you can define an import policy to selectively rule out, or reject,
unnecessary routing table entries.