Cisco Cisco IOS Software Release 12.2(27)SBC
L2TP Security
ip pmtu
7
ip pmtu
To enable the discovery of the path maximum transmission unit (MTU) for Layer 2 traffic, use the ip
pmtu command in VPDN group, VPDN template, or pseudowire class configuration mode. To disable
path MTU discovery, use the no form of this command.
pmtu command in VPDN group, VPDN template, or pseudowire class configuration mode. To disable
path MTU discovery, use the no form of this command.
ip pmtu
no ip pmtu
Syntax Description
This command has no arguments or keywords.
Command Default
Path MTU discovery is disabled.
Command Modes
VPDN group configuration
VPDN template configuration
Pseudowire class configuration
VPDN template configuration
Pseudowire class configuration
Command History
Usage Guidelines
When issued in VPDN group configuration mode, the ip pmtu command enables any tunnel associated
with the specified virtual private dialup network (VPDN) group to participate in path MTU discovery.
with the specified virtual private dialup network (VPDN) group to participate in path MTU discovery.
When issued in pseudowire class configuration mode, the ip pmtu command enables any Layer 2 Tunnel
Protocol Version 3 (L2TPv3) session derived from the specified pseudowire class configuration to
participate in path MTU discovery.
Protocol Version 3 (L2TPv3) session derived from the specified pseudowire class configuration to
participate in path MTU discovery.
Because path MTU checks decrease switching performance, this option is disabled by default.
When the ip pmtu command is enabled, the Don’t Fragment (DF) bit in the Layer 2 encapsulation header
is copied from the inner IP header to the encapsulation header.
is copied from the inner IP header to the encapsulation header.
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. If an IP packet is larger than the MTU of any interface it must pass through and the DF
bit is set, the packet is dropped and an ICMP unreachable message is returned. The ICMP unreachable
message indicates the MTU of the interface that was unable to forward the packet without fragmentation.
This information allows the source host to reduce the size of the packet before retransmission to allow
it to fit through that interface.
unreachable messages that indicate fragmentation errors in the IP backbone network carrying the
tunneled traffic. If an IP packet is larger than the MTU of any interface it must pass through and the DF
bit is set, the packet is dropped and an ICMP unreachable message is returned. The ICMP unreachable
message indicates the MTU of the interface that was unable to forward the packet without fragmentation.
This information allows the source host to reduce the size of the packet before retransmission to allow
it to fit through that interface.
Release
Modification
12.2(4)T
This command was introduced.
12.2(11)T
This command was implemented on the Cisco 1760, Cisco AS5300,
Cisco AS5400, and Cisco AS5800 platforms.
Cisco AS5400, and Cisco AS5800 platforms.
12.3(2)T
Support was added for using this command in pseudowire class configuration
mode.
mode.
12.2(28)SB
This command was integrated into Cisco IOS Release 12.2(28)SB.