Manual

Layer 2 Tunnel Protocol Version 3
ip pmtu
102
Cisco IOS Releases 12.0(29)S and 12.2(25)S
ip pmtu
To enable the discovery of a path maximum transmission unit (PMTU) for Layer 2 Tunnel Protocol
Version 3 (L2TPv3) traffic, use the ip pmtu command in pseudowire class configuration mode. To
disable PMTU discovery, use the no form of this command.
ip pmtu
no pmtu
Syntax Description This command has no arguments or keywords.
Defaults Path MTU discovery is disabled.
Command Modes Pseudowire class configuration
Command History
Usage Guidelines The ip pmtu command enables the processing of Internet Control Message Protocol (ICMP)
unreachable messages that indicate fragmentation errors in the IP backbone network carrying the
tunneled traffic. The MTU of the L2TPv3 session is updated according to the MTU information
contained in the ICMP unreachable message.
The ip pmtu command also enables MTU checking for IP packets that are sent into an L2TPv3 session
with the Don’t Fragment (DF) bit set. If an IP packet is larger than the MTU of the tunnel, the packet is
dropped and an ICMP unreachable message is sent. If an IP packet is smaller than the MTU of the tunnel,
the DF bit in the packet header is reflected from the inner IP header to the tunnel header.
Examples The following example shows how to enable the discovery of the path MTU for pseudowires created
from the pseudowire class named ether-pw:
Router(config)# pseudowire-class ether-pw
Router(config-pw)# ip pmtu
Related Commands
Release Modification
12.0(23)S This command was introduced.
12.2(25)S This command was integrated into Cisco IOS Release 12.2(25)S.
Command Description
ip dfbit set Enables the DF bit in the outer L2TPv3 tunnel header.
pseudowire-class Specifies the name of an L2TP pseudowire class and enters pseudowire
class configuration mode.