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

Multiple L2TP sessions (one for each forwarded Layer 2 circuit) can exist between a pair of PEs, and

can be maintained by a single control channel. Session IDs and cookies are dynamically generated and

exchanged as part of a dynamic session setup. Information such as sequencing configuration is also

exchanged. Circuit state changes (UP/DOWN) are conveyed using the set link info (SLI) message.

Sequencing

Although the correct sequence of received Layer 2 frames is guaranteed by some Layer 2 technologies

(by the nature of the link, such as a serial line) or the protocol itself, forwarded Layer 2 frames may be

lost, duplicated, or reordered when they traverse a network as IP packets. If the Layer 2 protocol does

not provide an explicit sequencing mechanism, you can configure L2TP to sequence its data packets

according to the data channel sequencing mechanism described in the L2TPv3 IETF l2tpext working

group draft.

A receiver of L2TP data packets mandates sequencing through the Sequencing Required AVP when the

session is being negotiated. A sender that receives this AVP (or that is manually configured to send

sequenced packets) uses the Layer 2-specific pseudowire control encapsulation defined in L2TPv3.

Currently, you can configure L2TP only to drop out-of-order packets; you cannot configure L2TP to

deliver the packets out-of-order. No reordering mechanism is available.

Cisco IOS Software Release 12.0(28)S introduces support for L2TPv3 distributed sequencing on the

Cisco 7500 series routers.

Local Switching

Local switching (from one port to another port in the same router) is supported for both static and

dynamic sessions. You must configure separate IP addresses for each Xconnect statement.

See the section “Configuration Examples for Layer 2 Tunnel Protocol Version 3” for an example of how

to configure local port switching.

Distributed Switching

Distributed CEF switching is supported for L2TP on the Cisco 7500 series routers.

Note For the Cisco 7500 series, sequencing is supported, but all L2TP packets that require sequence number

processing are sent to the RSP.

IP Packet Fragmentation

It is desirable to avoid fragmentation issues in the service provider network because reassembly is

computationally expensive. The easiest way to avoid fragmentation issues is to configure the CE routers

with an path maximum transmission unit (MTU) value that is smaller than the pseudowire path MTU.

However, in scenarios where this is not an option, fragmentation issues must be considered. L2TP

initially supported only the following options for packet fragmentation when a packet is determined to

exceed the L2TP path MTU:

• Unconditionally drop the packet

• Fragment the packet after L2TP/IP encapsulation

• Drop the packet and send an Internet Control Message Protocol (ICMP) unreachable message back

to the CE router