Ospf Areas; Classification Of Routers - Avaya 8600 Engineering

8. OSPF Areas

In large networks with many routers and networks, the link state database (LSDB) and routing table can
become very large. Large route tables and LSDBs consume memory. Processing of link state
advertisements results in more CPU cycles being needed to make forwarding decisions. To help reduce
these undesired effects, an OSPF network can be divided into sub-domains called areas. An area is
made up of a number of OSPF routers with the same area identification.
By dividing a network into multiple areas, a separate LSDB consisting of router LSA‟s and network LSA‟s
are maintained for each area. Each router within an area will maintain a LSDB only for the area to which it
belongs. In other words, the area‟s router-LSA‟s and network-LSA‟s are not flooded beyond the area‟s
borders. Hence, the impact of a topology change is localized to the area that it occurs. The only exception
to this is for the area border routers, which must maintain a LSDB for each area to which they belong.
Changes in topology are advertised to the rest of the network by the area border routers by advertising
Summary-LSAs.
Area‟s are identified by a 32-bit Area ID and is expressed in IP address format such as 0.0.0.0 for 0. Area
0 is also known as the backbone area and is responsible for distributing routing information to all other
areas. If multiple areas are used, they should all be attached to the backbone via an Area Border Router
(ABR), which connects area 0.0.0.0 to the non-backbone area(s). If an area cannot be physically directly
connected via an ABR to area 0, you will need to configure a Virtual Link to logically connect the area to
the backbone area.
8.1

Classification of Routers

When an OSPF Autonomous System is broken up into multiple OSPF areas, the routers are further
divided into specific functions:

Internal Routers
A router that has all its networks and interfaces belonging to one area. This router will have a one
LSDB belonging to this area.

Area Border Routers (ABR)
An ABR router connects to two or more OSPF areas. It will have a separate LSDB for each area
it is attached to. It will in turn condense the routing topology from its attached area for distribution
to the OSPF backbone area. The backbone in turn will distribute this information to all other
areas. ABR‟s use Summary-LSA‟s to advertise IP subnets into the backbone by using Type 3
Summary-LSAs and Type 4 ASBR-summary-LSAs. Type 4 ASBR-summary-LSAs advertises the
location of the ASBRs from area to area.

Backbone Routers
A backbone router is simply a router that has an interface connected to the backbone area. This
includes all routers including the ABR.

Autonomous System Boundary Router (ASBR)
An ASBR router is a router that has a connection to another Autonomous System. To distribute
into OSPF any external routes that originated from a protocol such as RIP, BGP-4, IS-IS or Static
Routes, the router must be configured as an ASBR. An ASBR router will import external routes
into the OSPF domain by using AS-external-LSAs (LSA Type 5) originated by the ASBR. AS-
external-LSAs are flooded across area borders. When an ASBR imports external routes, it may
import routes info OSPF using external Type 1 or Type 2 metrics. This will give a four-level
routing hierarchy as show in the table below according to routing preference.
November 2010
ERS 8600 / 8800 OSPF Technical Configuration Guide
avaya.com
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