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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)
  • Default OSPF costs using the default reference bandwidth

 

Interface Bandwidth Cost
10 Gigabit Ethernet 10Gbps 1
Gigabit Ethernet 1Gbps 1
FastEthernet 100Mbps 1
Ethernet 10Mbps 10
T1 1.5Mbps 64
768Kbps 130
364Kbps 274
128Kbps 781
64Kbps 1562
56Kbps 1785

 

  • 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

 

Interface Bandwidth Cost
10 Gigabit Ethernet 10Gbps 1
Gigabit Ethernet 1Gbps 10
FastEthernet 100Mbps 100
Ethernet 10Mbps 1000
T1 1.5Mbps 6477
768Kbps 13021
364Kbps 27472
128Kbps 78125 (max is 65535)
64Kbps 156250 (max is 65535)
56Kbps 178571 (max is 65535)

 

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

 


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