3Com 3C13636 Configuration Manual page 609

3Com Router 3000 Ethernet Family
Configuration Guide
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 redistributed 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 redistributed 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.
Perform the following configuration in OSPF view.
Table 4-8 Configure OSPF to redistribute routes of other protocols
Configure OSPF to redistribute routes of
other protocols
Cancel redistributing routing information
of other protocols
By default, OSPF does not redistribute the routing information of other protocols. For
redistributed routes, cost is 1, type is 2, and tag is 1 by default.
When the protocol argument is set to BGP, the keyword allow-ibgp is optional.
Whereas the import-route bgp command redistributes only EBGP routes, the
import-route bgp allow-ibgp command redistributes IBGP routes in addition and as
such, must be used with cautions.
The protocol specifies a source routing protocol that can be redistributed. By far, it can
be Direct, Static, RIP, IS-IS or BGP.
II. Configuring parameters for OSPF to redistribute external routes
In the case that OSPF redistributes routing information found by other routing protocols
to the local AS, it is also necessary to configure some extra parameters, such as default
cost and default tag for route redistribution. A route label can be used to identify
Operation
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4-14
Chapter 4 OSPF Configuration
Command
import-route protocol [ allow-ibgp]
[ cost value ] [ type { 1 | 2 } ] [ tag value ]
[ route-policy route-policy-name ]
undo import-route protocol