Overview of XPL

Definition

Extended routing-policy language (XPL) is a language used to filter routes and modify route attributes. By modifying route attributes (including reachability), XPL changes the path through which network traffic passes. XPL provides the same functions as routing policies do, but it uses different editing and filtering methods from routing policies. Therefore, XPL can meet different customer requirements.

Table 1 compares XPL and routing policies.

**Table 1** Comparison between XPL and routing policies
Item	Key Functions	Editing Method	Filtering Method	User Experience
XPL	Filters routes and modifies route attributes.	Line-by-line or paragraph-by-paragraph editing	Uses sets or single elements to filter routes.	Users can configure or modify policies as required in a text editor.
Routing policies	Filter routes and modify route attributes.	Line-by-line editing	Use filters or single elements to filter routes.	Users must follow strict command configuration rules.

For details about routing policies, see "Routing Policies" in HUAWEI NetEngine 8000 F SeriesRouter Feature Description — IP Routing.

Line-by-Line and Paragraph-by-Paragraph Editing

XPL supports line-by-line editing and paragraph-by-paragraph editing, whereas routing policies support line-by-line editing only. Line-by-line editing is a traditional configuration method, whereas paragraph-by-paragraph editing is an innovative configuration method. Table 2 compares the two methods.

**Table 2** Line-by-line and paragraph-by-paragraph editing comparison
Item	Applicable to	Differences	Help and Error Correction Mechanisms
Line-by-line editing	Users who are used to the traditional configuration method or unfamiliar with XPL	Each command is run in a command view, and one command is presented in one line, which is considered a configuration unit. NOTE: To modify an existing global variable set, route attribute set, or route-filter through line-by-line editing, enter the specific command view and reconfigure the set or policy.	The desired command can be suggested using the command association function. If any configuration error occurs, it is reported after the command is configured.
Paragraph-by-paragraph editing	Users who are familiar with XPL clause configuration and want to simplify the configuration process	The paragraph editing UI functions as a text editor, in which users edit XPL clauses. The XPL clauses are committed after a paragraph of them are configured, and each paragraph is considered a configuration unit.	The command association function is not supported, and complete clauses must be entered in the paragraph editing UI. If any configuration error occurs, it is reported after the configurations of the whole paragraph are committed.

Purpose

When advertising, receiving, or importing routes, the router can use XPL based on actual networking requirements to filter routes and modify route attributes. XPL serves the following purposes:

Controls route advertisement.

Only routes that match the rules specified in the XPL are advertised.
Controls route acceptance.

Only necessary and valid routes are accepted, which reduces the routing table size and improves network security.
Filters and controls imported routes.

A routing protocol may import routes discovered by other routing protocols. XPL ensures that only the routes that meet certain conditions are imported and route attributes of the imported routes are modified to meet the requirements of the protocol.
Modifies route attributes.

Attributes of the routes that match the specified route-filter can be modified as required.

Benefits

XPL offers the following benefits:

Saves system resources by controlling the routing table size.
Improves network security by controlling route advertisement and acceptance.
Improves network performance by modifying route attributes for effective traffic planning.
Simplifies routing policy configurations.