SPF Tree Calculation

SPF Tree Calculation
Shortest Path First (SPF) trees are paths through the network to any given destination. A separate
path exists for each known destination. Chapter 6 goes into complete detail about the
types of destinations and their advertisements.
Once all of the OSPF routers have synchronized link-state databases, each router is responsible
for calculating the SPF tree for each known destination. This calculation is done using the
Dijkstra algorithm. In order to do calculations, metrics for each link are required.
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OSPF Metrics
OSPF uses a metric referred to as cost. A cost is associated with every outgoing interface along
an SPF tree. The cost of the entire path is the sum of costs of the outgoing interfaces along the
path. Because cost is an arbitrary value as defined in RFC 2328, Cisco had to implement its own
method of calculating the cost for each OSPF-enabled interface. Cisco uses a simple equation of
108/bandwidth. The bandwidth is the configured bandwidth for the interface.
This value may be overridden by using the ip ospf cost command. The cost is manipulated
by changing the value to a number within the range of 1 to 65,535. Because the cost is assigned
to each link, the value must be changed on each interface.
Cisco bases link cost on bandwidth. Other vendors may use other metrics to
calculate the link’s cost. When connecting links between routers from different
vendors, you may have to adjust the cost to match the other router. If both routers
do not assign the same cost to a link, it can result in suboptimal routing.