The L3 Tunnel

Layer 3 tunneling uses network layer tunneling protocols to connect the PE with each of its VPN peers in the public network with matching VPN requirements, resulting in the destination address of the Layer 3 tunnel (The PE neighbor) functioning as a logical next hop.

Tunneling involves the use of a tunnel protocol that encapsulates the payload of the packet entering the tunnel within another (outer) header based upon tunnel parameters. Thus a tunneled packet has an inner and an outer header. The inner header contains the original packet header. The IP type (IPv4 or IPv6) of the original header is determined by the original packet source and destination address type. The outer delivery header is the tunnel header. The IP type of the tunnel header is determined by the route lookup source and destination IP address type configured for the tunnel. The L3 tunnel prevents and P router in the public core from having any knowledge of the VPN labels. P routers use the destination address of the outer IP header to forward packets. Only the PE router defines the tunnel interfaces and support VPN‘s MPLS label.

The S-Series platforms support Layer 3 tunneling modes:

• Generic Routing Encapsulation (GRE) which provides generic support for all supported IPv4 and IPv6 tunnel IP type combinations, as defined in RFC 2784, along with the keyword extensions defined in RFC 2890. The GRE mode should be used if you do not want to limit the tunnel to a specific IP header combination. This implementation does not support RFC 1701.
• IP-IP tunneling which provides support for IPv4 over an IPv4 Layer 3 tunnel interface as defined in RFC 2003.
• IPv6 tunneling which provides support for IPv6 over an IPv6 Layer 3 tunnel interface as defined in RFC 2473.
• IPv4 to IPv6 tunneling which supports IPv4 over an IPv6 Layer 3 tunnel interface as defined in RFC 2473.
• IPv6 to IPv4 tunneling which supports IPv6 over an IPv4 Layer 3 tunnel interface as defined in RFC 2473.

A Layer 3 tunnel interface can be assigned to a static route using the ip route or ipv6 route command, depending upon the route IP type. The Layer 3 tunnel source and destination must be reachable either by a configured static route or a supported routing protocol such as RIP, BGP, or OSPF.

See the interface command entry, in the S-, K-, and 7100 Series CLI Reference Guide, for create, enable, and disable Layer 3 tunnel command details. Refer to Tunneling Configuration for L3 tunnel configuration details.