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

Additional Resources

RFC4360 - BGP Extended Community Attribute


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