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Auto-cost Reference Bandwidth

Overview:

Auto-cost Reference Bandwidth is the command used to modify the OSPF metric.

Study Notes:

• Auto-cost reference bandwidth is used to modify the OSPF metric
• The OSPF metric is cost
• Cost is calculated by dividing 100 by the bandwidth in Mbps
  • Cost = 100/(bw in Mbps)
• Any result less than 0 will use a cost of 1
• Default OSPF costs using the default reference bandwidth

• You can change these costs using the auto-cost reference-bandwidth Mbps
• Notice that anything above 100Mbps has a cost of 1 so we would like to modify the default cost on these links to tell OSPF to prefer the higher speed links over the lower speed links
• We set the new reference bandwidth to 10000
• This forces the cost calculation to be 10000/(bw in Mbps) and achieves the following results

• The max cost for a Cisco router interface is 65535
• The command to apply auto-cost reference bandwidth is:

Router(config)#router ospf 1
Router(config-router)#auto-cost reference-bandwidth 10000

   2 Comments

1. 

   Dani Tene
   

     April 24, 2019

   what is the command to apply auto-cost reference bandwidth?

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   • 

     Joe Barger (CCNP/CCDP)
     

       April 24, 2019

     Router(config)#router ospf 1
     Router(config-router)#auto-cost reference-bandwidth <value>

     Good question!