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- OSPF areas break up a network so that routers in one area know less topology information about
the subnets in other area and do not need to know about the routers in other area. With smaller
topology databases, routers consume less memory and processing power for SPF calculations.
Areas are considered as logical subdivisions of an autonomous system.
RT1
RT1
Area 1 Area 0 (Backbone Area)
RT2
Area
Border
Router
RT4
RT5
RT6
RT7
10.1.6.0
10.1.7.0
RT3
RT2
RT3
RT4 10.1.6.0
10.1.7.0
10.1.8.0
Figure 13-1: OSPF Scalability with Hierarchical Design
- By dividing the network into multiple areas, the routers reside in Area 1 are shielded from the
details of Area 0. RT4 is known as an OSPF Area Border Router (ABR), as it resides on the
border of 2 different areas and belongs to both areas. RT4 only advertises summary information
about the subnets in Area 0 instead of full topology information of Area 0. In such a way, RT1,
RT2, and RT3 would think the network topology as the lower part of Figure 13-1 (fewer routers).
Eventually, SPF calculations would take less time and memory due to smaller topology databases.
Note: Routers in Area 0 are also shielded from the details of Area 1.
Note: Router that connects autonomous systems together is known as Autonomous System
Boundary Router (ASBR).
- There must be an area 0 (backbone area) in OSPF, which is typically configured on the routers
that resides in the backbone of the network. All other OSPF areas must connect to area 0.
- Using areas improves all 3 of the scaling problems discussed earlier.
i) Smaller topology databases require less memory and processing time and hence faster
convergence.
ii) Confine network instability to single areas of the network – when a link in an area
changes state, the routers in other areas do not need to perform SPF calculation.
- Do not confuse this concept with route summarization, which reduces the number of subnets
advertised to other routers, as OSPF ABRs do not reduce the number of subnets when
advertising routing updates to another area. Both concepts are different!
- The ABR has the topology database for both areas and runs SPF when a link changes its state in
either area. Using areas does not actually reduce memory requirements and the number of SPF
calculations for ABRs. ABRs often have more memory and much powerful processors.
94
RT8
10.1.8.0
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