What are VRF import/export maps and why to use it?
VRFs are an excellent tool for maintaining segregated routing topologies for separate customers or services. However, what if you needed to export only a subset of routes within a VRF? Inter-VRF routing using route targets along with BGP are the mechanisms to facilitate this. This kind of design is generally seen with a hub-spoke kind of topology but with route filtering to prevent full mesh and keep specific traffic or networks separate.
In the network below we have one router with four separate VRFs. Each VRF will be its own segregated network but will receive a subset of routes from other VRFs. In this example vrf-1, vrf-1_Colo, and vrf-2_Colo will share routes. Additionally vrf-2, vrf-1_Colo, and vrf-2_Colo will also share routes. The VRFs vrf-1 and vrf-2 will not share routing information and should not be permitted to speak to one another. Similarly all traffic and networks not part of any of the VRFs would also remain segregated.
Verify the configuration
Now that the configuration is finished lets verify our VRF deployment. Using the show ip route vrf WORD, show ip vrf WORD commands on R1 you can verify the global and separate vrf routing tables as well as VPN routing/forwarding instance information. Notice VRF vrf-1_Colo has routes to all networks whereas vrf-1 only has routes to vrf-1_Colo and vrf-2_Colo.
With the appropriate import and export maps in place PC1 is able to reach vrf-1_Colo and vrf-2_Colo. Similarly PC2 is able to reach vrf-1_Colo and vrf-2_Colo and vrf-1_Colo and vrf-2_Colo are able to reach each other. However, vrf-1 and vrf-2 are unable to communicate with each other as expected.
