TCP/IP Tutorial and Technical Overview - IBM Redbooks

The routes describe an end-to-end path consisting of two portions:
► External portion: This is the portion of the path external to the OSPF network.
When these routes are distributed into OSPF, the ASBR assigns an initial
cost. This cost represents the external cost associated with traversing the
external portion of the path.
► Internal portion: This is the portion of the path internal to the OSPF network.
Costs for this portion of the network are calculated using standard OSPF
algorithms.
OSPF differentiates between two types of external routes. They differ in the way
the cost of the route is calculated. The ASBR is configured to redistribute the
route as:
► External type 1: The total cost of the route is the sum of the external cost and
any internal OSPF costs.
► External type 2: The total cost of the route is always the external cost. This
ignores any internal OSPF costs required to reach the ASBR.
Figure 5-19 illustrates an example of the types of OSPF external routes.
RIP
Network
10.99.5.0/24
ASBR
Figure 5-19 OSPF route redistribution
OSPF
Network
(10)
(15)
10.99.5.0/24 redistributed
with external cost 50
R1
R1 Routing Table
10.99.5.0/24
E1: Cost 60
or
E2: Cost 50
External Internal
(20)
R2
R2 Routing Table
10.99.5.0/24
E1: Cost 65
or
E2: Cost 50
Chapter 5. Routing protocols 209