VPN services have been traditionally deployed over IP core networks by configuring MPLS or through L2TPv3 tunnels using point-to-point links. This feature introduces the capability to deploy layer 3 VPN services by configuring multipoint L2TPv3 tunnels over an existing IP core network. This feature is configured on only the PE routers and requires no configuration on the core routers. The L2TPv3 multipoint tunnel network allows layer 3 VPN services to be carried through the core without the configuration of MPLS.
Border Gateway Protocol (BGP) is used to advertise the tunnel endpoints and the subaddress family indentifier (SAFI) specific attributes (which contains the tunnel type, and tunnel capabilities). This feature introduces the tunnel SAFI and the BGP SAFI-Specific Attribute (SSA) attribute. The tunnel SAFI defines the tunnel endpoint and carries the endpoint IPv4 address and next hop. The tunnel SAFI is identified by the SAFI number 64. The BGP SSA carries the BGP preference and BGP flags. It also carries the tunnel cookie, tunnel cookie length, and session ID. The BGP SSA is identified by attribute number 19.
These attributes allow BGP to distribute tunnel encapsulation information between PE routers. VPNv4 traffic is routed through these tunnels. The next hop, advertised in BGP VPNv4 updates, determines which tunnel to use for routing tunnel traffic.
(2008,MPLS VPN over L2TPv3 Tunnels, retrieved from http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/cs_l3vpn.html, February 2011)
PART 1
PART 2
you can find this topology here
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