IS-IS routing

IS-IS routing domains

routing
level 0 - взаимодействие End-System - IS
level 1 - взаимодействие IS-IS внутри area
level 2 - внутри AS
level 3 - с другой AS

IS-IS routing process

The Decision Process

After the link-state databases have been synchronized, it is necessary to decide which path to take 

to reach the destination. Because the routers and hosts may have multiple connections to each 

other, there may be many paths from which to choose.

To make the best path decision, link-state protocols employ the algorithm defined by Dijkstra. This 

algorithm creates a tree that shows the shortest paths to all destinations. The tree is used in turn 

to create the routing table.

If there is more than one path to a remote destination, the criteria by which the lowest cost paths 

are selected and placed in the forwarding database are as follows:

1. If there is more than one path with the lowest value metric, Cisco equipment places some or 

all paths into the table. Older versions of IOS support as many as six load-sharing paths, 

newer versions support more.

2. Internal paths are chosen before external paths. 

3. Level 1 paths within the area are more attractive than Level 2 paths.

4. The address with the most specific address in IP is the address with the longest IP subnet 

mask.

5. If there is no path, the forwarding database sends the packet to the nearest Level 2 router, 

which is the default router. 

The metric defines the cost of the path. Integrated IS-IS has four metrics, only one of which is 

required and supported. The metrics defined in ISO 10589 are as follows:

■ Default—Every Integrated IS-IS router must support this metric. Cisco set the default for all 

interfaces to 10.

■ Delay—Cisco does not support the transit delay metric.

■ Expense—Cisco does not support the expense metric.

■ Error—Cisco does not support the error metric.

By default, six-bit metrics are configured on the outgoing interface. A 10-bit field describes the 

total path cost. These default metrics are referred to as narrow. 

Because it considered these inadequate, Cisco increased the metric size to 24 bits. This larger 

metric field provides more granularity to distinguish between paths and is referred to as wide.

To determine shortest path, the lowest metric is chosen, internal paths are chosen over external 

paths, and Level 1 routes have precedence over Level 2 routes.

The default metric is the only metric supported by Cisco, because each metric used in Integrated 

IS-IS requires a different link-state database calculation for both the Level 1 and Level 2 routes.