Figure 5-6 Ospf Area And Router Type - Raisecom ISCOM2600G-HI (A) Series Configuration Manual

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ISCOM2600G-HI (A) Series Configuration Guide
OSPF divides an AS into different areas to solve the previous problem. An area logically
contains some routers and is identified by the area ID. As shown in Figure 5-5, a route in an
area maintains routing information of the area instead of the entire AS.

Figure 5-6 OSPF area and router type

The border of each area is a router instead of a link. A router may belong to different areas,
but a network segment (link) must belong to only one area, or an interface running OSPF
must belong to a specific area. After the network is divided into different areas, aggregate
routes on border routers to reduce the number of LSAs advertised to other areas and minimize
impact from changes of network topology.
Router types
As shown in Figure 5-6, OSPF routers can be divided into four types according to location in
the AS:
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Internal router: all interfaces of an interval router belong to only one OSPF area.
Area Border Router (ABR): this router may belong to two or more areas which must
contain a backbone area. The ABR can connect a backbone area and a non-backbone
area. It can be physically or logically connected to a backbone area.
Backbone router: at least one interface of this router belongs to the backbone area, so all
ABRs and internal routers in Area 0 are backbone routers.
Autonomous System Border Router (ASBR): the router exchanges information with
other AS is called the ASBR. The ASBR is not necessarily located at the border of an AS,
and may be an internal router or ABR. When an OSPF router imports external routes, it
becomes the ASBR.
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