Generating Source AS and Route Target Import Communities Overview

Both route target import (rt-import) and source autonomous system (src-as) communities contain two fields (following their respective keywords). In Junos OS, a provider edge (PE) router constructs the route target import community using its router ID in the first field and a per-VRF unique number in the second field. The router ID is normally set to the primary loopback IP address of the PE router. The unique number used in the second field is an internal number derived from the routing-instance table index. The combination of the two numbers creates a route target import community that is unique to the originating PE router and unique to the VPN routing and forwarding (VRF) instance from which it is created.

For example, Router PE1 creates the following route target import community: rt-import:10.1.1.1:64.

Since the route target import community is constructed using the primary loopback address and the routing-instance table index of the PE router, any event that causes either number to change triggers a change in the value of the route target import community. This in turn requires VPN-IPv4 routes to be re-advertised with the new route target import community. Under normal circumstances, the primary loopback address and the routing-instance table index numbers do not change. If they do change, Junos OS updates all related internal policies and re-advertises VPN-IPv4 routes with the new rt-import and src-as values per those policies.

To ensure that the route target import community generated by a PE router is unique across VRF tables, the Junos OS Policy module restricts the use of primary loopback addresses to next-generation multicast virtual private network (MVPN) internal policies only. You are not permitted to configure a route target for any VRF table (MVPN or otherwise) using the primary loopback address. The commit fails with an error if the system finds a user-configured route target that contains the IP address used in constructing the route target import community.

The global administrator field of the src-as community is set to the local AS number of the PE router originating the community, and the local administrator field is set to 0. This community is used for inter-AS operations but needs to be carried along with all VPN-IPv4 routes.

For example, Router PE1 creates an src-as community with a value of src-as:65000:0.