Conditional Advertisement and Import Policy (Routing Table) with certain match conditions

 

BGP accepts all non-looped routes learned from neighbors and imports them into the RIB-In table. If these routes are accepted by the BGP import policy, they are then imported into the inet.0 routing table. In cases where only certain routes are required to be imported, provisions can be made such that the peer routing device exports routes based on a condition or a set of conditions.

The condition for exporting a route can be based on:

  • The peer the route was learned from

  • The interface the route was learned on

  • Some other required attribute

For example:

This is known as conditional installation of prefixes and is described in Example: Configuring a Routing Policy for Conditional Advertisement Enabling Conditional Installation of Prefixes in a Routing Table.

The Juniper Networks® Junos® Operating System (Junos OS) supports conditional export of routes based on the existence of another route in the routing table. Junos OS does not, however, support policy conditions for import policy.

Figure 1 illustrates where BGP import and export policies are applied. An import policy is applied to inbound routes that are visible in the output of the show route receive-protocol bgp neighbor-address command. An export policy is applied to outbound routes that are visible in the output of the show route advertising-protocol bgp neighbor-address command.

Figure 1: BGP Import and Export Policies
BGP Import and Export Policies

To enable conditional installation of prefixes, an export policy must be configured on the device where the prefix export has to take place. The export policy evaluates each route to verify that it satisfies all the match conditions under the from statement. It also searches for the existence of the route defined under the condition statement (also configured under the from statement).

If the route does not match the entire set of required conditions defined in the policy, or if the route defined under the condition statement does not exist in the routing table, the route is not exported to its BGP peers. Thus, a conditional export policy matches the routes for the desired route or prefix you want installed in the peers’ routing table.

To configure the conditional installation of prefixes with the help of an export policy:

  1. Create a condition statement to check prefixes.

  2. Create an export policy with the newly created condition using the condition statement.

  3. Apply the export policy to the device that requires only selected prefixes to be exported from the routing table.