GateD Configuration Guide - Control Statements

# 6.4 Control Statements Overview

Control statements control routes that are imported from routing peers and routes that are exported to these peers. These are the final statements to be included in the gated.conf file. The control statements are:

### 6.4.1 Routing Filtering

Routes are filtered by specifying configuration language that will match a certain set of routes by destination, or by destination and mask. Among other places, route filters are used on `martians`, ``and in`import` and `export` statements.

### 6.4.2 Matching AS Paths

An AS path is alist of autonomous systems that routing information has passed through to get to this router, and an indicator of the origin of the AS path. This information can be used to prefer one path to a destination network over another. The primary method for doing this with GateD is to specify a list of patterns to be applied to AS paths when importing and exporting routes.

### 6.4.3 AS Path Attributes

BGP updates carry a number of path attributes. Some of these, like the AS path, are required. Others are optional and may or may not appear in any given BGP update.

The aspath-opt option to the group clause, and its variant the mod-aspath option, can be used to generate optional path attributes. Currently only the community attribute is supported. The aspath-opt attribute may also be used on the import clause to allow optional attributes to be considered when determining Gated's preference for the routes in a particular BGP update

### 6.4.4 Route Importation

Importation of routes from routing protocols and the installation of the routes in GateD's routing database is controlled by `import` statements. The format of an `import` statement varies depending on the source protocol. In all cases, one of two keywords may be specified to control how routes compete with other protocols: restrict, and preference.

### 6.4.5 Route Exportation

While the `import` statement controls which routes received from other systems are used by GateD, the `export` statement controls which routes are advertised by GateD to other systems. Like the `import` statement, the syntax of the `export` statement varies slightly per protocol. The syntax of the `export` statement is similar to the syntax of the `import` statement, and the meanings of many of the parameters are identical. The main difference between the two is that while route importation is just controlled by source information, route exportation is controlled by both destination and source.

### 6.4.6 Route Aggregation and Generation

Route aggregation is a method of generating a more general route, given the presence of a specific route. It is used, for example, at an autonomous system border to generate a route to a network to be advertised via EGP given the presence of one or more subnets of that network learned via RIP. Route aggregation is also used by regional and national networks to reduce the amount of routing information passed around.

A slight variation of aggregation is the generation of a route based on the existence of certain conditions. This is sometimes known as the route of last resort. This route inherits the next hops and aspath from the contributor specified with the lowest (most favorable) preference. The most common usage for this is to generate a default based on the presence of a route from a peer on a neighboring backbone.

Last updated April 26, 1997