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:
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.
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.
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
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.
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.
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