Using standard metrics, what is the maximum path cost in IS-IS?

A.
15

B.
16

C.
63

D.
1023

Explanation:
When you are deploying IS-IS on a large-scale IS-IS network, you also need to consider narrow and wide metrics, LSP flooding reduction, handling LSP corruption, and a number of other issues.

Metrics

When IS-IS was originally designed, the processors that were available for use in intermediate systems tended to be much slower than those available today. Therefore, making the calculation of the SPF tree easier was a prime consideration. One of the various techniques used in IS-IS to simplify the calculation of the SPF tree was to use a single 8-bit field to indicate all the metric information about a specific link.

To do this, you set one bit aside to indicate whether a route was learned internally or through some external source (typically redistributed). You set two bits aside to indicate a class of service, or the type of service that the link supported. In common use, the types of service were never implemented, so IS-IS ended up with the following:

* Internal links limited to a cost between 0 and 63
* External destinations limited to a cost between 64 and 127
* A maximum total path cost of 1023

These numbers leave little room to maneuver in terms of assigning link costs based on link bandwidth or other link characteristics. With the introduction of Multiprotocol Label Switching (MPLS) traffic engineering using IS-IS, the 6-bit metric space was determined to be too small, and a wider metric style was specified.

IS-IS wide metrics are 24 bits wide rather than 6 bits wide. That means interface metrics can now be assigned as high as 16777215, and the total cost of a path through the network can be 4261412864. To configure an IS running Cisco IOS Software to run wide metrics, use the command metric-style wide in the router isis configuration mode.

Transitioning a running network from narrow metrics to wide metrics is a challenge. If you change some of the intermediate systems within the network, the rest of the routers no longer understand their metrics, and the network fails. You can get around this by using the metric transition mode, configured using the metric-style transition command in the router isis configuration mode.

The metric transition mode allows an intermediate system to accept both wide and narrow metrics and use them interchangeably. As long as all the interface costs are within the narrow metric range, SPF continues to operate properly, and the network continues to work. After you have configured all the routers in the network to transition mode, you can go back and configure them all into the wide metric mode and then start using the wider range of metrics available.

Although the default link cost on an IS running Cisco IOS Software is 10, you can change the default link costs on all links that are connected to an L1 or an L2 routing domain by using the command metric metric-value [level-1 | level-2].