GateD Configuration Guide - Import Statement

# The Import Statement

Importation of routes from routing protocols and 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.

## Specifying preferences

In all cases, one of two keywords may be specified to control how routes compete with other protocols:

        restrict
        preference preference

restrict
  Specifies that the routes are not desired in the routing table. In some cases this means that the routes are not installed in the routing table. In others it means that they are installed with a negative preference; this prevents them from becoming active so they will not be installed in the forwarding table, or exported to other protocols.
preference preference
  Specifies the preference value used when comparing this route to other routes from other protocols. The route with the lowest preference available at any given route becomes the active route, is installed in the forwarding table, and is eligible to be exported to other protocols. The default preferences are configured by the individual protocols.

## Route Filters

All the formats allow route filters as shown below. See the section on route filters for a detailed explaination of how they work. When no route filtering is specified (i.e. when `restrict` is specified on the first line of a statement), all routes from the specfied source will match that statement. If any filters are specified, only routes that match the specified filters will be imported. Put differently, if any filters are specified, an `all restrict ;` is assumed at the end of the list.

        network [ exact | refines ]
        network mask mask [exact | refines ]
        network masklen number [ exact | refines ]
        default
        host host

## Importing routes from BGP and EGP

    import proto bgp | egp autonomoussystem autonomous_system
        restrict ;
    import proto bgp | egp autonomoussystem autonomous_system
        [ preference preference ] {
        route_filter [ restrict | ( preference preference ) ] ;
    } ;

    import proto bgp aspath aspath_regexp
        origin any | ( [ igp ] [egp ] [ incomplete ] )
        restrict ;
    import proto bgp aspath aspath_regexp
        origin any | ( [ igp ] [egp ] [ incomplete ] )
        [ preference preference ] {
        route_filter [ restrict | ( preference preference ) ] ;
    } ;

EGP importation may be controlled by autonomous system. BGP also supports controlling propagation by the use of an AS path regular expressions, which are documented in the section on Matching AS paths. Note that EGP and BGP versions 2 and 3 only support the propagation of natural networks, so the `host` and `default` route filters are meaningless. BGP version 4 supports the propagation of any destination along with a contiguous network mask.

EGP and BGP both store any routes that were rejected implicitly by not being metioned in a route filter, or explicitly with the `restrict` keyword in the routing table with a negative preference. A negative preference prevents a route from becoming active, which prevents it from being installed in the forwarding table, or exported to other protocols. This aleviates the need to break and re-establish a session upon reconfiguration if importation policy is changed.

## Importing routes from an RIP, HELLO and Redirects

    import proto rip | hello | redirect
        [ ( interface interface_list ) | (gateway gateway_list ) ]
        restrict ;
    import proto rip | hello | redirect
        [ ( interface interface_list ) | (gateway gateway_list ) ]
        [ preference preference ] {
        route_filter [ restrict | ( preference preference ) ] ;
    } ;

The importation of RIP, HELLO and Redirect routes may be controlled by any of protocol, source interface and source gateway. If more than one is specified, they are processed from most general (protocol) to most specific (gateway).

RIP and HELLO don't support the use of preference to choose between routes of the same protocol. That is left to the protocol metrics. These protocols do not save routes that were rejected since they have short update intervals.

## Importing routes from OSPF

    import proto ospfase [ tag ospf_tag ] restrict ;
    import proto ospfase [ tag ospf_tag ]
        [ preference preference ] {
        route_filter [ restrict | ( preference preference ) ] ;
    } ;

Due to the nature of OSPF, only the importation of ASE routes may be controlled. OSPF intra- and inter-area routes are always imported into the gated routing table with a preference of 10. If a tag is specified, the import clause will only apply to routes with the specified tag.

It is only possible to restrict the importation of OSPF ASE routes when functioning as an AS border router. This is accomplished by specifying an export ospfase clause. Specification of an empty export clause may be used to restrict importation of ASEs when no ASEs are being exported.

Like the other interior protocols, preference can not be used to choose between OSPF ASE routes, that is done by the OSPF costs. Routes that are rejected by policy are stored in the table with a negative preference.

Last updated 1994/09/07 00:50:28.