Manual

Layer 2 Tunnel Protocol Version 3

Information About Layer 2 Tunnel Protocol Version 3

Cisco IOS Releases 12.0(29)S and 12.2(25)S

As described in the section “L2TPv3 Header Description,” the UTI data header is identical to the

L2TPv3 header but with no sequence numbers and an 8-byte cookie. By manually configuring an

L2TPv3 session using an 8-byte cookie (see the section “Manually Configuring L2TPv3 Session

Parameters”) and by setting the IP protocol number of outgoing data packets to 120 (as described in the

section “Configuring the L2TPv3 Pseudowire”), you can ensure that a PE running L2TPv3 may

interoperate with a peer PE running UTI. However, because UTI does not define a signaling plane,

dynamically established L2TPv3 sessions cannot interoperate with UTI.

When a customer upgrades from a pre-L2TPv3 Cisco IOS release to a post-L2TPv3 release, an internal

UTI-to-Xconnect command-line interface (CLI) migration utility will automatically convert the UTI

commands to Xconnect and pseudowire class configuration commands without the need for any user

intervention. After the CLI migration, the UTI commands that were replaced will not be available. The

old-style UTI CLI will be hidden from the user.

Note The UTI keepalive feature will not be migrated. The UTI keepalive feature will no longer be supported

in post-L2TPv3 releases. You should convert to using dynamic L2TPv3 sessions in order to preserve the

functionality provided by the UTI keepalive.

L2TPv3 Operation

L2TPv3 provides similar and enhanced services to replace the current UTI implementation, including

the following features:

• Xconnect for Layer 2 tunneling via a pseudowire over an IP network

• Layer 2 VPNs for PE-to-PE router service via Xconnect that support Ethernet, 802.1q (VLAN),

Frame Relay, HDLC and PPP Layer 2 circuits, including both static (UTI-like) and dynamic (using

the new L2TPv3 signaling) forwarded sessions

The initial Cisco IOS Release 12.0(23)S features supported only the following features:

• Layer 2 tunneling (as used in an L2TP access concentrator, or LAC) to an attachment circuit, not

Layer 3 tunneling

• L2TPv3 data encapsulation directly over IP (IP protocol number 115), not using User Datagram

Protocol (UDP)

• Point-to-point sessions, not point-to-multipoint or multipoint-to-point sessions

• Sessions between the same Layer 2 protocols; for example, Ethernet-to-Ethernet, VLAN-to-VLAN,

but not VLAN-to-Ethernet or Frame Relay

The attachment circuit is the physical interface or subinterface attached to the pseudowire.

Figure 1 shows an example of how the L2TPv3 feature is used for setting up VPNs using Layer 2

tunneling over an IP network. All traffic between two customer network sites is encapsulated in IP

packets carrying L2TP data messages and sent across an IP network. The backbone routers of the IP

network treat the traffic as any other IP traffic and need not know anything about the customer networks.