When to desing OSPF into muli-area

Unanswered Question
Apr 17th, 2007

When do we need to design ospf into multi area ? Any guildline ?

I have this problem too.
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minumathur Tue, 04/17/2007 - 11:37


Summary : purpose of designing OSPF Multiarea, is to reduce LSA Fluding in network , normal if any changes are happen in OSPF network , it will multicast to entire area, so to reduce LSA floding in network , we need to divide network into small area, due to this you can get advantage of small routing table with using summary address feature.

i hope this will clarify your doubt.

please rate this post.


mohammedmahmoud Tue, 04/17/2007 - 12:34

Hi there,

In brief, an area is a subset of the routers that make up a network. Dividing a network into areas enhances 3 aspects:

1.Reduces the memory usage (less LSAs - smaller topology database).

2.Reduces the CPU usage (smaller topology database to run Dijkstra's shortest path algorithm algorithm upon).

3.The flooding of link state packets adversely affects available bandwidth, particularly in unstable networks, configuring areas reduces this flooding and accordingly reduces any bandwidth waste.

From "CCIE Professional Development Routing TCP/IP, Volume I, Second Edition" by Jeff Doyle:

Many OSPF designers have a favorite rule of thumb concerning the maximum number of routers that an area can handle. This number might range from 30 to 200. However, the number of routers has little actual bearing on the maximum size of an area. Far more important factors include the number of links in an area, the stability of the topology, the memory and horsepower of the routers, the use of summarization, and the number of summary LSAs entering the area. Because of these factors, 25 routers might be too many for some areas, and other areas might accommodate well over 500 routers.

It is perfectly reasonable to design a small OSPF network with only a single area. Regardless of number of areas, a potential problem arises when an area is so underpopulated that no redundant links exist within it. If such an area becomes partitioned, service disruptions might occur. Partitioned areas are discussed in more detail in a later section.

HTH, please rate if it does.

Mohammed Mahmoud.


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