Export RTs?

(This article assumes some understanding of MPLS VPNs)

The different methods to attach MPLS VPN RTs when routes are exported from a VRF table can be confusing.
This could be done in two ways with additional options.

  1. The default ‘all’ export RT could be used.
  2. Or the RTs could be attached using an export-map.

The first method is the most common and the easiest to understand. The command below will attach the configured RT to any routes exported from the router’s VRF RIB table into the MPBGP table for advertising.

route-target export {asn:xx}

But what if another RT should be attached in place of, or additionally but for only one prefix?

This is where the second method becomes necessary. By using an export-map one can selectively attach RTs to individual prefixes, separate or in conjunction with the default export if it is configured. This usually raises the question of when the ‘additive’ keyword is needed.

Allow me to explain by using the following diagram :

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