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

Bit 1 indicates whether the Sequence Number field, bits 8 to 31, contains a valid sequence number and

is to be updated.

L2TPv3 Features

L2TPv3 provides Xconnect support for Ethernet, 802.1q (VLAN), Frame Relay, HDLC, and PPP, using

the sessions described in the following sections:

• Static L2TPv3 Sessions (nonnegotiated, PVC-like forwarded sessions)

• Dynamic L2TPv3 Sessions (negotiated, forwarded sessions using the L2TPv3 control plane for

session negotiation)

L2TPv3 also includes support for the features described in the following sections:

• Sequencing

• Local Switching

• Distributed Switching

• IP Packet Fragmentation

• L2TPv3 Type of Service Marking

• Keepalive

• MTU Handling

• L2TPv3 Control Connection Hashing

• L2TPv3 Control Connection Rate Limiting

Static L2TPv3 Sessions

Typically, the L2TP control plane is responsible for negotiating session parameters, such as the session

ID or the cookie, in order to set up the session. However, some IP networks require sessions to be

configured so that no signaling is required for session establishment. You can, therefore, set up static

L2TPv3 sessions for a PE router by configuring fixed values for the fields in the L2TP data header. A

static L2TPv3 session allows the PE to tunnel Layer 2 traffic as soon as the attachment circuit to which

the session is bound comes up.

Note In an L2TPv3 static session, you can still run the L2TP control channel to perform peer authentication

and dead-peer detection. If the L2TP control channel cannot be established or is torn down because of a

hello failure, the static session is also torn down.

When you use a static L2TPv3 session, you cannot perform circuit interworking, such as LMI, because

there is no facility to exchange control messages. To perform circuit interworking, you must use a

dynamic session.

Dynamic L2TPv3 Sessions

A dynamic L2TP session is established through the exchange of control messages containing

attribute-value pairs (AVPs). Each AVP contains information about the nature of the Layer 2 link being

forwarded: the payload type, virtual circuit (VC) ID, and so on.