Friday, September 10, 2010

BSCI - OSPF - Default Path Cost values

The OSPF metric is cost, which is calculated using the equation 100Mbps/Bandwidth of interface.

The 100Mbps is a reference bandwidth which is applied in order to calculate the Cost of an interface. The Cost is an indication of the overhead to send packets across that link. Lower Costs are better.

OSPF uses the following default metric costs for different types of interface:
56K dial-up - 1785
T1 (1.544Mbps serial link) - 64
E1 (2.048Mbps serial link) - 48
Ethernet - 10
100Mb Fast Ethernet - 1
1000Mb Gigabit Ethernet - 1

The default OSPF Cost is used to calculate the best path. The best path is then entered in to the routing table (assuming there isn't another protocol with a better AD with the same path).

In order to refine your traffic shaping you can change the reference bandwidth so that you can determine the best path when considering FE and GE links, as you'll note that the default cost is the same and there fore determining the best path between the two could result in sub-optimal routing.

To change the reference bandwidth do:
R1(config)#router ospf 1
R1(config-router)#auto-cost reference-bandwidth [ref-bw]
!where ref-bw can be a value of 1-4294967

To override the default cost value that would result from the values stated above, you can manually set a cost value on an interface:
R1(config)#int s0/0
R1(config)#ip ospf cost [int-cost]
!where int-cost is a value of 1 -65535

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