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

Cisco IOS Release 12.0(24)S introduced the ability to allow IP traffic from the CE router to be

fragmented before the data enters the pseudowire, forcing the computationally expensive reassembly to

occur in the CE network rather than in the service provider network. The number of fragments that must

be generated is determined based on the discovered pseudowire path MTU. The original Layer 2 header

is then copied to each of the generated fragments, the L2TP/IP encapsulation is added, and the frames

are then forwarded. This feature will be implicitly enabled whenever the ip pmtu command is enabled

in the pseudowire class. It will be applied to any packets received from the CE network that have a Don’t

Fragment (DF) bit set to 0 and that exceed the L2TP path MTU in size.

Support for the fragmentation of IP packets before the data enters the pseudowire was introduced on the

Cisco 7200 series and Cisco 7500 series routers in Cisco IOS Release 12.0(24)S.

L2TPv3 Type of Service Marking

When Layer 2 traffic is tunneled across an IP network, information contained in the ToS bits may be

transferred to the L2TP-encapsulated IP packets in one of the following ways:

• If the tunneled Layer 2 frames encapsulate IP packets themselves, it may be desirable to simply copy

the ToS bytes of the inner IP packets to the outer IP packet headers. This action is known as “ToS

byte reflection.”

• Static ToS byte configuration. You specify the ToS byte value used by all packets sent across the

pseudowire.

See the section “Configuring a Negotiated L2TPv3 Session for Local HDLC Switching Example” for

more information about how to configure ToS information.

Keepalive

The keepalive mechanism for L2TPv3 extends only to the endpoints of the tunneling protocol. L2TP has

a reliable control message delivery mechanism that serves as the basis for the keepalive mechanism. The

keepalive mechanism consists of an exchange of L2TP hello messages.

If a keepalive mechanism is required, the control plane is used, although it may not be used to bring up

sessions. You can manually configure sessions.

In the case of static L2TPv3 sessions, a control channel between the two L2TP peers is negotiated

through the exchange of start control channel request (SCCRQ), start control channel replay (SCCRP),

and start control channel connected (SCCCN) control messages. The control channel is responsible only

for maintaining the keepalive mechanism through the exchange of hello messages.

The interval between hello messages is configurable per control channel. If one peer detects that the

other has gone down through the keepalive mechanism, it sends a StopCCN control message and then

notifies all of the pseudowires to the peer about the event. This notification results in the teardown of

both manually configured and dynamic sessions.

MTU Handling

It is important that you configure an MTU appropriate for a each L2TPv3 tunneled link. The configured

MTU size ensures the following:

• The lengths of the tunneled Layer 2 frames fall below the MTU of the destination attachment circuit

• The tunneled packets are not fragmented, which forces the receiving PE to reassemble them

L2TPv3 handles the MTU as follows:

• The default behavior is to fragment packets that are larger than the session MTU.