When would you want to change cost in OSPF?

[JoshGarmonCiscoCCNA]

When would you want to do things like change OSPF cost, bandwidth, maximum paths, perform summarization, and load balance?

Hope that's not a loaded question.

[Masoud Pourshabanian · Accepted solution]

Hello,

OSPF cost, bandwidth : If you have two or three paths to a destination and you prefer one path over the other, you use OSPF cost, bandwidth.  Lesser cost and higher bandwidth determine the best path. Cost is automatically calculated by bandwidth; however, you can change it manually by "ip ospf cost" command.

maximum paths: OSPF puts all equal paths to a destination(in terms of metric) in the routing table. With this command you can define how many equal paths can be placed in the routing table. default is 4.

Load balancing : OSPF puts all equal paths to a destination in the routing table. Then router load balances packets across those equall paths.

summarization. You do summarization to send less routes to OSPF neighbors. OSPF summarizes the routes and sends them to its neighbors.

Masoud

[JoshGarmonCiscoCCNA]

good help Masoud!

[Masoud Pourshabanian]

Happy to help.

[Joseph W. Doherty]

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Some additional information. . .

Concerning summarization, OSPF only supports it on just some "special" role OSPF routers, i.e. ABRs and ASBRs.

Although Cisco routers "autocost" OSPF based on bandwidth, this is not part of the OSPF standard and Cisco's reference bandwidth is 100 Mbps.  This means other vendors can work differently (and often do) and when using greater than FE bandwidths, you often need to manually adjust costs or adjust the reference bandwidth.

Besides the forgoing, a reason you might manually set OSPF costs is because you "know more" than the router does.  For example, you might adjust OSPF costs to favor a less expensive path even if it has less bandwidth, or you might want multiple paths to appear to be equal cost even though they have slightly different bandwidths.