IS-IS metric calculation.

[Sundeep Dsouza]

Guys I would like to know the formula for calculating metric in IS-IS routing protocol. There has to be some formula used for calculating metric in IS-IS. Did Google but could not find any information. I know IS-IS uses cost as metric but whats the formula used?

Regards

[johnlloyd_13 · Accepted solution]

hi sundeep,

as per my notes with me, here's what i got:

- Unlike some other IP routing protocols, a Cisco router running IS-IS does not take into account line speed or bandwidth when it sets its link metrics

- All interfaces are configured with a metric of 10 by default. In a network with links of varying types, this assignment can result in suboptimal routing

- #Router(config-if)#isis metric

    * Configures metric for an interface; the default is 10

    * Metric value is from 1-63

- #Router(config-router)#metric

    * Alternatively, configures metric globally for all interfaces

- Note: The IS-IS specification defines 4 types of metrics. Cost is the default metric and is supported by all routers. Delay, expense and error are optional metrics. The delay metric measures transit delay, the expense metric measures the monetary cos of link utilization and the error metric measures residual error probability associate with a link

- The default Cisco implementation uses cost only

- If the keyword level-1 or level-2 is not entered, the metric is applied to both Level1 and 2 interfaces

[Giuseppe Larosa]

Hello Sundeep,

there is no formula because IS-IS metric by default is 10 on every link.

What is really important to know is that to accomodate a great variety of link speeds you should use wide metrics to go over the limitation of narrow metric.

The use of wide metric is also necessary in MPLS environments for use of Traffic Engineering extensions.

Another important difference from OSPF is that a link can have different metric values one for level-1 and one for level-2.

actually it is possible to change the value of 10 with another one to be applied at the ISIS process level to all interfaces

http://www.cisco.com/en/US/docs/ios/iproute_isis/configuration/guide/irs_netd.html#wp1055363

and also see

http://www.cisco.com/en/US/docs/ios/iproute_isis/configuration/guide/irs_ovrvw_ps6441_TSD_Products_Configuration_Guide_Chapter.html

Edit:

this was done also to make ISIS lighter.

Hope to help

Giuseppe