Supported Platforms
Auto Export Feature Overview
An MPLS Layer 3 VPN consists of provider edge (PE) routers that operate between different service provider areas, and customer edge (CE) routers that operate between the customer location and a service provider area. The traffic is carried over the MPLS backbone.
In Layer 3 VPNs, multiple CE routers can belong to a single VRF routing instance on a PE router. A PE router can have multiple VRF routing instances. In some cases, shared services might require routes to be written to multiple VRF routing tables, both at the local and remote PE router. This requires the PE router to share route information among each configured VRF routing instance. This exchange of route information is accomplished by creating and applying custom vrf-export and vrf-import policies that utilize BGP extended community attributes to create hub-and-spoke topologies. This exchange of routing information, such as route prefixes, is known as prefix leaking.
There are two kinds of prefix leaking: local and remote.
Remote prefix leaking takes place when the source VRF routing instance and destination VRF routing instance are on different PE routers. This is achieved by using BGP extended community attributes through the use of the route-target community that works on remote PE routers, but not on local PE routers. Custom policies, such as vrf-export and vrf-import, do not leak prefixes between VRF routing instances if the source VRF routing instance and the destination VRF routing instance are on the same PE router.
Local prefix leaking is when the source VRF routing instance and the destination VRF routing instance are on the same PE router. Similar to the split-horizon rule, prefixes learned on the PE router from local CE routers can be shared with other remote PE routers, but the routes cannot be shared locally.
The Auto Export feature leaks prefixes between VRF routing instances that are locally configured on a given PE router. The Auto Export feature is enabled by using the auto-export statement.
Auto Export is always applied on the local PE router, because it takes care of only local prefix leaking by evaluating the export policy of each VRF and determining which route targets can be leaked locally. The standard VRF import and export policies still affect only the remote PE prefix leaking.
