Introduction

Route Target is an extended BGP community attribute that is assigned to VPNv4 prefixes to control VPN distribution (import/export).

Route Target

Each VRF has at least one import and export RT value.
Import RT set decides which VPNv4 routes to accept into a vrf.
Export RT set marks outgoing vpnv4 prefixes to the rest of the network.
Regular PEs should not install VPNv4 prefixes if they don't have an attached vrf that import an RT value of the prefix. There are exceptions like Route Reflectors, or Inter-AS ASBR.
VPN Join is an action of adding an RT to a VRF and import associated new VPNv4 routes.
VPN Prune is an action of removing an RT from a VRF and deleting associated VPNv4 routes.
Both VPN join and prune should be non-disruptive.
RT format is encoded in the BGP Ext Community (see format below).
The Type High can be value 0x00, 0x01, or 0x02, while the Type Low can be 0x02.
- Type High 0x00 and 0x02 will include the AS number followed by a locally significant nn 2 byte value.
  - as-number:nn
  - Example: 4522:150
  - Example 1.233:150
- Type High 0x01 will include an IPv4 address followed by a locally significant nn 2 byte value.
  - ipv4:n
  - Example 172.16.31.1:150