IS-IS Wide Metric

In the earlier ISO 10589, the largest metric of an interface is 63. TLV type 128 and TLV type 130 contain information about routes, and TLV type 2 contains information about IS-IS neighbors. However, on large-scale networks, the metric range cannot meet the requirements. Moreover, IS-IS TE needs to be configured. Therefore, the wide metric was introduced.

As defined in standard protocols, with IS-IS wide metric, the largest metric of an interface is extended to 16777215, and the largest metric of a route is 4261412864.

After IS-IS wide metric is enabled, TLV type 135 contains information about routes; TLV type 22 contains information about IS-IS neighbors.

The following lists the TLVs used in narrow mode:
- IP Internal Reachability TLV: carries routes within an area.
- IP External Reachability TLV: carries routes outside an area.
- IS Neighbors TLV: carries information about neighbors.
The following lists the TLVs used in wide mode:
- Extended IP Reachability TLV: replaces the earlier IP Reachability TLV and carries information about routes. This TLV expands the range of the route cost to 4 bytes and carries sub-TLVs.
- IS Extended Neighbors TLV: carries information about neighbors.

The metric style can be set to narrow, narrow-compatible, compatible, wide-compatible, or wide mode. Table 1 shows which metric styles are carried in received and sent packets. A device can calculate routes only when it can receive, send, and process corresponding TLVs. Therefore, to ensure correct data forwarding on a network, the proper metric style must be configured for each device on the network.

**Table 1** Metric style carried in received and sent under different metric style configurations
Configured Metric Style	Metric Style Carried in Received Packets	Metric Style Carried in Sent Packets
Narrow	Narrow	Narrow
Narrow-compatible	Narrow and wide	Narrow
Compatible	Narrow and wide	Narrow and wide
Wide-compatible	Narrow and wide	Wide
Wide	Wide	Wide

When the metric style is set to compatible, IS-IS sends the information both in narrow and wide modes.

Process

Once the metric style is changed, the IS-IS process restarts.

If the metric style carried in sent packets is changed from narrow to wide:

The information previously carried by TLV type 128, TLV type 130, and TLV type 2 is now carried by TLV type 135 and TLV type 22.
If the metric style carried in sent packets is changed from wide to narrow:

The information previously carried by TLV type 135 and TLV type 22 is now carried by TLV type 128, TLV type 130, and TLV type 2.
If the metric style carried in sent packets is changed from narrow or wide to narrow and wide:

The information previously carried in narrow or wide mode is now carried by TLV type 128, TLV type 130, TLV type 2, TLV type 135, and TLV type 22.

Usage Scenario

IS-IS wide metric is used to support IS-IS TE, and the metric style needs to be set to wide, compatible or wide compatible.