17: Configuring OSPF (Open Shortest Path First)
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© Virtual Access 2018
GW2020 Series User Manual
Issue: 2.1
Page 143 of 423
Both Area 1 and Area 2 must directly connect to Area 0. Routers A and B belong fully to
Area 1, while Routers E and F belong fully to Area 2. These are known as Internal
Routers.
Router C belongs to both Area 0 and Area 1; so it is an ABR. Because it has an interface
in Area 0, it can also be considered a Backbone Router (BR). The same can be said for
Router D, as it belongs to both Area 0 and Area 2.
Router G also belongs to Area 0 however it also has a connection to the internet, which
is outside this autonomous system. This makes Router G an Autonomous System Border
Router (ASBR).
A router can become an ASBR in one of two ways:
•
By connecting to a separate Autonomous System, such as the internet
•
By redistributing another routing protocol into the OSPF process.
ASBRs provide access to external networks. OSPF defines two types of external routes,
as shown in the table below.
Type 2 (E2)
Includes only the external cost to the destination network. External cost is the metric being
advertised from outside the OSPF domain. This is the default type assigned to external
routes.
Type 1 (E1)
Includes both the external cost, and the internal cost to reach the ASBR, to determine the
total metric to reach the destination network. Type 1 routes are always preferred over Type
2 routes to the same destination.
Table 44: Types of external routes
17.1.7
OSPF router types
The four separate OSPF router types are shown in the table below.
Route Type
Description
Internal Router
All router interfaces belong to only one area.
Area Border Router (ABR)
Have interfaces in at least two separate areas.
Backbone Router
Have at least one interface in area 0.
Autonomous System
Border Router (ABR)
Have a connection to a separate autonomous system.
17.2
Configuration package used
Package
Sections
ospfd
routing
network
interface
