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

• User priority field

• Canonical format indicator (CFI)

• A 12-bit VLAN ID (VID)

For L2TPv3, an Ethernet subinterface configured to support VLAN switching may be bound to an

Xconnect service so that all Ethernet traffic, tagged with a VID specified on the subinterface, is tunneled

to another PE. The VLAN Ethernet frames are forwarded in their entirety. The receiving PE may rewrite

the VID of the tunneled traffic to another value before forwarding the traffic onto an attachment circuit.

To successfully rewrite VLANs, it may be necessary to disable the Spanning Tree Protocol (STP). This

can be done on a per-VLAN basis by using the no spanning-tree vlan command.

Note Due to the way in which L2TPv3 handles 802.1q VLAN packets, the Ethernet interface must be

configured in promiscuous mode to capture all traffic received on the Ethernet segment attached to the

router. All frames are tunneled through the L2TP pseudowire.

HDLC

L2TPv3 encapsulates an HDLC frame arriving at a PE in its entirety (including the Address, Control,

and Protocol fields, but not the Flag fields and the frame check sequence) with an L2TP data header.

PPP

PEs that support L2TPv3 forward PPP traffic using a “transparent pass-through” model, in which the

PEs play no role in the negotiation and maintenance of the PPP link. L2TPv3 encapsulates a PPP frame

arriving at a PE in its entirety (including the HDLC Address and Control fields) with an L2TP data

header.

ATM

L2TPv3 can connect two isolated ATM clouds over a packet-switched network (PSN) while maintaining

an end-to-end ATM Service Level Agreement (SLA). The ATM Single Cell Relay features forward one

ATM cell per packet. The ATM Cell Packing over L2TPv3 features allows multiple ATM frames to be

packed into a single L2TPv3 data packet. All packets are transparently forwarded over the L2TPv3

pseudowire.

Note VPI or VPI/VCI rewrite is not supported for any ATM transport mode. The peer routers must be

configured with matching VPI or VCI values.

Table 2 shows the releases that introduced support for the ATM cell relay features.

Table 2 Release Support for the ATM Cell Relay Features

Transport Type Single Cell Relay Packed Cell Relay

VC mode 12.0(28)S, 12.2(25)S 12.0(29)S

VP mode 12.0(25)S, 12.2(25)S 12.0(29)S

Port mode 12.0(29)S 12.0(29)S