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    Understanding Route Distribution for a VRF using Maps

    The combination of the route-target export list of VRF A and the route-target import list of VRF B determines whether routes from VRF A are distributed to VRF B. You can provide finer-grained control of route distribution by associating any combination of export, import, global export, and global import maps with VRFs. As shown in Figure 1, a route is distributed (leaked) between RIBs and its attributes are changed as specified in the route map when the map returns an accept message. If the map returns a deny message, then the route is not distributed.

    Figure 1: Import and Export Maps

    Import and Export Maps

    Both IPv4 and IPv6 VPNs are supported. You can specify that only IPv4 or only IPv6 routes are imported or exported. By default, the import or export map applies to both kinds of routes. You can configure some maps to apply to IPv4 routes and different maps to apply to IPv6 routes.

    When the name or the contents of a route map change, BGP automatically waits for a nonconfigurable hold-down interval of 30 seconds and then re-imports or re-exports the appropriate routes using the modified route map.

    Even when suppressed by an aggregate or auto-summary route, the more specific routes are distributed. Aggregation and auto-summarization take place in each VRF independently. For example, a route that is imported into a VRF is only aggregated in that VRF if an aggregate address has been configured in the context of the BGP address family for that VRF.

    Routes maintain their type when exported. Private prefixes are exported without being converted into public prefixes. Consequently the prefix of an exported route is the same as the original route. Global export maps are therefore not useful when NAT is enabled.

    Subsequent Distribution of Routes

    Routes that are imported from the global BGP non-VPN RIB (with a global import map) into a VRF RIB are never exported again. Because these routes are not exported to the global VPN RIB, they are not advertised to other PE routers. These imported routes are never exported to the VRF RIBs of overlapping VPNs.

    Routes that are exported from a VRF RIB to the global BGP non-VPN RIB with the global export map are never imported back in to any VRF.

    Routes that are imported from the global BGP VPN RIB (with an import map) into a VRF RIB are never exported again.

    Routes that are exported from a VRF RIB to the global VPN RIB can be imported into the RIB of other VRFs. This behavior might be seen with overlapping VPNs.

    Published: 2014-08-18