SPF Algorithm

SPF Algorithm
Once a router’s link-state database has been created, the router needs to create the shortest path
tree. The shortest path tree is then used to select the routes to populate the router’s routing table.
The IS-IS metrics that can be used to create the shortest path tree are default, delay, expense, and
error. We’re going to concentrate on default, because it’s the only IS-IS metric that is supported
by Cisco devices.
The default metric can be any value between 0 and 63, inclusive; Cisco defaults it to 10.
The default metric can be set to a different value for every IS-IS interface on the router and
for the different levels in which the interface may be participating. If you do not specify the
level of routing in which the metric should be used for SPF calculation for that interface, it will
be used for both L1 and L2 calculations, if applicable.
You can specify a different metric for each level by issuing the isis metric default_metric
[level-1|level-2] command twice, once for each level. The metric for an IS-IS route is the sum
218 Chapter 7  Integrated IS-IS
of all outgoing interfaces involved in the path. Like OSPF, the route that IS-IS will choose is the
route with the lowest metric. The maximum value IS-IS supports for a route is 1023.
IS-IS classifies routes based on whether they are L1 routes or L2 routes. L1 routes are always
internal to an IS-IS routing domain. L2 routes can be further classified as internal or external. An
L2 external route is a route that is external to the IS-IS routing domain, whereas an L2 internal
route is internal to the IS-IS routing domain. An L1 route is always preferred over an L2 route.
If multiple routes are found to a destination, the route with the best metric will be selected.
If multiple routes with the same metric are found, IS-IS will use all of the routes. For load balancing,
IS-IS supports up to six paths of equal cost.
There’s one last topic we need to cover before moving on to configuring IS-IS. That topic is
the type of networks supported by IS-IS.