3Com Switch 8800 Family Configuration Guide

314 CHAPTER 31: OSPF CONFIGURATION
Perform the following configuration in OSPF area view.
The ip-mask argument is IP address wildcard shielded text (similar to the
complement of the IP address mask).
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):
■ Intra-area route
■ Inter-area route
■ External route type 1
■ 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 router + 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.
Configuring OSPF to import external routes
Perform the following configuration in OSPF view.
Tabl e 273 Specifying an interface to run OSPF
Operation Command
Specify an interface to run OSPF network ip-address ip-mask
Disable OSPF on the interface undo network ip-address ip-mask