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Default Routing Policies

You must be familiar with the default routing policies to know when you need to modify them to suit your needs. Table 1 summarizes the default routing policies for each routing protocol that imports and exports routes. The actions in the default routing policies are taken if you have not explicitly configured a routing policy. This table also shows direct and explicitly configured routes, which for the purposes of this table are considered a pseudoprotocol. Explicitly configured routes include aggregate, generated, and static routes.

OSPF and IS-IS Import Policies

You cannot change the default import policy for IS-IS. For OSPF, import policies apply to external routes only. An external route is a route that is outside the OSPF autonomous system (AS). For internal routes (routes learned from OSPF), you cannot change the default import policy for OSPF. As link-state protocols, IS-IS and OSPF exchange routes between systems within an autonomous system (AS). All routers and systems within an AS must share the same link-state database, which includes routes to reachable prefixes and the metrics associated with the prefixes. If an import policy is configured and applied to IS-IS or OSPF, some routes might not be learned or advertised or the metrics for learned routes might be altered, which would make a consistent link-state database impossible.

The default export policy for IS-IS and OSPF protocols is to reject everything. These protocols do not actually export their internally learned routes (the directly connected routes on interfaces that are running the protocol). Both IS-IS and OSPF protocols use a procedure called flooding to announce local routes and any routes learned by the protocol. The flooding procedure is internal to the protocol, and is unaffected by the policy framework. Exporting can be used only to announce information from other protocols, and the default is not to do so.

Automatic Export

For Layer 3 VPNs, the automatic export feature can be configured to overcome the limitation of local prefix leaking and automatically export routes between local VPN routing and forwarding (VRF) routing instances.

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 with 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.

The automatic export feature leaks prefixes between VRF routing instances that are locally configured on a given PE router. The automatic export feature is enabled by using the auto-export statement.

Automatic 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.

If the vrf-export policy examined by the automatic export does not have an explicit then accept action, the automatic export essentially ignores the policy and, therefore, does not leak the route targets specified within it.

For more information, see Technology Overview: Understanding the Auto Export Feature .