ospf design

Unanswered Question
Aug 6th, 2007

Hi all, what factors affect in ospf when we need to consider having another area ? and why would we need another area ?

cheers

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cjnwodo Mon, 08/06/2007 - 07:02

Hi,

This is a big question, however, I will try to answer in a few number of lines:

The factors that affect OSPF that you need to consider when contemplating creating a new area include:

1) topology- what does the physical topology of you network look like [LAN & WAN links]

2) Router cpu- OSPF performs the spf [shortest path first] algorithm to determine the best route to a destination. This can be cpu intensive depending on rate of failures etc in the network and the router platforms you have deployed

3) Router memory- OSPF makes use of the LSDB [link state database] to track LSAs and neighbours. The size of the database will depend on the number of links and routes you have in the network.

You would need a new OSPF area for the following reasons:

a) An old area is getting too large [too many routers and router] hampering convergence times. Therefore you split the area into smaller areas

b) You want to seperate security/routing domains e.g. a new site.

The following link may help as well

http://www.cisco.com/en/US/tech/tk365/tk480/tsd_technology_support_sub-protocol_home.html

HTH

jwdoherty Mon, 08/06/2007 - 07:02

OSPF areas are needed for scalability. OSPF keeps track of the complete link topology within an area, so splitting an OSPF topology into multiple areas reduces the load on the routers within each area.

The 64-bit question then becomes how large and OSPF area should be. Often there is no precise answer beyond "it depends". Some of the "it depends" are the number of routers, the number of subnets, the arrangement of the topology, the power of the routers, and how clever the OSPF implementation is on the routers.

One rule of thumb is to avoid having more then 30 to 50 routers per area.

Keep in mind that when you do have multiple OSPF areas, you create an area zero with attached areas. Having discreet addressing boundaries also allows address summarization for the different areas, so a multiple OSPF area design benefits from a coordinated addressing scheme.

massimiliano.se... Mon, 08/06/2007 - 07:05

One of the most important aspect you consider in a multi-area environment is a good design and if you really need having a multi-area. Infact all area must be connected to area 0(backbone area) directly or trought virtual link.

Having many areas reduce the size of link state database of router belonging to single area; this means less cycles of CPU and less memory utilization of routers belonging to single area.

An optimal reference is:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml

Best regards.

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