The format of an import statement varies depending on the source protocol. The format of the import statements for the exterior gateway protocols is:

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. An important difference between the two statements is that while route importation is controlled by source information, route exportation is controlled by both source and destination. Thus, export statements define where the routes will be sent and where they originated. The destination of the route advertisement is defined by the proto clause at the beginning of the export statement. The source of the routes is defined in the export list.

Only OSPF ASE routes can be exported by gated. There are two types of OSPF ASE routes, type 1 and type 2. They are described in Chapter 7, "Configuring Routing " and earlier in this appendix. The default type is specified in the ospf protocol statement, but it can be overridden here. The ospf_tag is an arbitrary 32-bit number used to filter routing information. The default tag value is specified in the ospf protocol statement, but it can be overridden here.

The source of the routes advertised by a protocol is defined by the export list. Each of the commands listed above contains an export list option. Just like those commands, the export list syntax varies depending on the source protocol of the routes. The commands described above define the protocols that are used to advertise the routes. The export lists shown below describe the protocols from which the routes are obtained. The biggest confusion caused by the export list syntax is that it is almost identical to the syntax shown above. In both cases we define protocols, autonomous systems, interfaces, gateways, and so on. In the first case we are defining the protocols, interfaces, etc., to which routes are sent, and in this case we are defining the protocols, interfaces, etc., from which routes are received.

To export routes learned from BGP and EGP, use this export list syntax:

export proto bgp | egp autonomoussystem as_number
 [restrict [noagg]] |
 [[metric metric] {
  route_filter [restrict | metric metric] ; }] ;

This defines routes learned via BGP or EGP from a specific autonomous system. Routes can be restricted, or have a metric applied, based on matching the source AS number or the route filter. noagg can be used with restrict to prevent any aggregate routes from matching the filter.

When BGP is configured, gated assigns all routes an AS path. For interior routes, the AS path specifies igp as the origin and no autonomous systems in the AS path (the current AS is added when the route is exported). For EGP routes, the AS path specifies egp as the origin and the source AS as the AS path. For BGP routes, the AS path learned from BGP is used. If you run BGP, the export of all routes may be controlled by the AS path using this syntax:

proto proto | all
 aspath aspath_regexp origin any | igp | egp | incomplete
 [restrict] |
 [[metric metric] {
  route_filter [restrict | metric metric] ; }] ;

The source of the routes can be any one protocol (proto) or all protocols (all). The importation of routes can be controlled by matching their AS paths against the AS path regular expression (aspath_regexp) or by matching their addresses against the route_filter. Route filters and AS path regular expressions were explained previously.

To export routes learned from RIP, use this export list syntax:

proto rip
 [interface interface_list | gateway gateway_list]
 [restrict] |
 [[metric metric] {
  route_filter [restrict | metric metric] ; }] ;

The export of RIP routes may be controlled by source interface, source gateway, or route filter.

To export routes learned from OSPF, use this export list syntax:

proto ospf | ospfase
 [restrict] |
 [[metric metric] {
  route_filter [restrict | metric metric] ; }] ;

The export of OSPF and OSPF ASE routes may be controlled by protocol and route filter. Exporting OSPF routes can also be controlled by tag using the following syntax:

proto proto | all tag tag
   [restrict] |
   [[metric metric] {
   route_filter [restrict | metric metric] ; }] ;

OSPF and RIP version 2 provide a tag field. For all other protocols, the tag is always 0. Routes may be selected based on the contents of the tag field.

There are other sources of routes that are not true routing protocols, and export lists can be defined for these sources. The two export lists for these sources are:

proto direct | static | kernel
 [interface interface_list]
 [restrict] |
 [[metric metric] {
  route_filter [restrict | metric metric] ; }] ;

The export of these routes can be controlled based on the source "protocol" and the source interface. The "protocols" in this case are routes to direct interfaces, static routes, or routes learned from the kernel.

proto default | aggregate
 [restrict] |
 [[metric metric] {
  route_filter [restrict | metric metric] ; }] ;

The export of these routes may only be controlled based on source "protocol." default refers to routes created by the gendefault option. aggregate refers to routes created by the aggregate statements, the topic of the next section.