L2Tpv3 Pseudowire Manager; Ip Packet Fragmentation; L2Tpv3 Type Of Service Marking; Keepalive - Cisco CRS Configuration Manual

L2TPv3 Features

L2TPv3 Pseudowire Manager

The pseudowire manager is a client library provided by the pseudowire signaling module that runs in the
context of the L2VPN process. This client library implements interface to pseudo-wire signaling protocol for
specific pseudowire type.

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 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. Previously, L2TP 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
Currently, the following options for packet fragmentation are supported:
• Path MTU is a configurable value which is configured on PE. If the packet size and the L2TP header
size are larger than the configured path MTU, packets are dropped.
• The PE configuration requires that a backbone facing interface's MTU is always greater or equal to the
customer facing interface's MTU and L2TP header size.
• IP fragmentation is not supported with L2TPv3.

L2TPv3 Type of Service Marking

When L2 traffic is tunneled across an IP network, information contained in the type of service (ToS) bits may
be transferred to the L2TP-encapsulated IP packets in one of the following ways:
• If the tunneled L2 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.

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.
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
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Implementing Layer 2 Tunnel Protocol Version 3