The Purpose of a Route Target Community in a Layer 3 VPN

A

In a Layer 3 VPN, a route target (RT) community is a BGP extended community attribute that is used to control the distribution of VPN routes between Provider Edge (PE) routers within a Virtual Private Network ( VPN). The main purpose of RT communities is to help PE routers determine which routes to import and export from other PE routers in the same VPN.

A Layer 3 VPN allows customers to connect their sites to a service provider's MPLS network and create a private network that spans multiple locations. Each customer's VPN is assigned a unique VPN identifier known as a route distinguisher (RD), which is used to distinguish between routes belonging to different VPNs. To ensure that the correct routes are imported and exported within a VPN, each VPN is also assigned one or more RT communities, which are used to tag the VPN's routes.

When a PE router receives a route with an RT community, it checks the community value against its own list of import and export policies to determine whether to import or export the route to other PE routers in the same VPN. If the RT community matches one of the import policies configured on the PE router, the route is imported into the VPN. If the RT community matches one of the export policies, the route is exported to other PE routers in the VPN.

Therefore, answer A is the correct option as it accurately describes the purpose of RT communities in a Layer 3 VPN. The community value is used to identify routes that should be imported by remote PEs, allowing PE routers to determine which routes should be shared within a VPN and which should not.

Answer B is incorrect because the RT community is not used to determine valid remote devices when signaling a Layer 3 VPN. Instead, the VPN identifier (RD) is used for this purpose.

Answer C is also incorrect because the RT community does not identify the source of incoming routes. Instead, the source of incoming routes is determined by the router that originated the route and the BGP path attributes associated with the route.

Answer D is also incorrect because the RT community is not used to determine transit devices when signaling a Layer 3 VPN. Instead, the path attributes of BGP routes are used to determine the best path between PE routers for VPN traffic.