3Com Switch 8800 Configuration Guide
Chapter 19 OSPF Configuration
19-9
19.2.5 Configuring OSPF to Import Routes of Other Protocols
The dynamic routing protocols on the router can share the routing information. As far as
OSPF is concerned, the routes discovered by other routing protocols are always
processed as the external routes of AS. In the
import-route
commands, you can
specify the route cost type, cost value and tag to overwrite the default route receipt
parameters (refer to “Configuring parameters for OSPF to import external routes”).
The OSPF uses the following four types of routes (ordered by priority):
z
Intra-area route
z
Inter-area route
z
External route type 1
z
External route type 2
Intra-area and inter-area routes describe the internal AS topology whereas the external
routes describe how to select the route to the destinations beyond the AS.
The external routes type-1 refers to the imported IGP routes (such as static route and
RIP). Since these routes are more reliable, the calculated cost of the external routes is
the same as the cost of routes within the AS. Also, such route cost and the route cost of
the OSPF itself are comparable. That is, cost to reach the external route type 1 = cost to
reach the corresponding ASBR from the local cost to reach the destination
address of the route from the ASBR.
The external routes type-2 refers to the imported EGP routes. Since these routes have
lower credibility, OSPF assumes that the cost spent from the ASBR to reach the
destinations beyond the AS is greatly higher than that spent from within the AS to the
ASBR. So in route cost calculation, the former is mainly considered, that is, the cost
spent to reach the external route type 2 = cost spent to the destination address of the
route from the ASBR. If the two values are equal, then the cost of the router to the
corresponding ASBR will be considered.
I. Configuring OSPF to import external routes
Perform the following configuration in OSPF view.
Table 19-5
Configure OSPF to import external routes
Operation
Command
Configure OSPF to import
routes of other protocols
import-route protocol
[
cost
value
|
type
value
|
tag
value
|
route-policy route
-
policy
-
name
]*
Cancel importing routing
information of other protocols
undo import-route protocol
By default, OSPF will not import the routing information of other protocols. For a
imported route, type is 2, cost is 1, and tag is 1 by default.
