Configuring The System Mtu - Cisco 3750G - Catalyst Integrated Wireless LAN Controller Configuration Manual

Chapter 11 Configuring Interface Characteristics

Configuring the System MTU

The default maximum transmission unit (MTU) size for frames received and transmitted on all interfaces
on the switch stack is 1500 bytes. You can increase the MTU size for all interfaces operating at 10 or
100 Mbps by using the system mtu global configuration command. You can increase the MTU size to
support jumbo frames on all Gigabit Ethernet interfaces by using the system mtu jumbo global
configuration command. You can change the MTU size for routed ports by using the system mtu routing
global configuration command.
You cannot configure a routing MTU size that exceeds the system MTU size. If you change the system
Note
MTU size to a value smaller than the currently configured routing MTU size, the configuration change
is accepted, but not applied until the next switch reset. When the configuration change takes effect, the
routing MTU size automatically defaults to the new system MTU size.
Gigabit Ethernet ports are not affected by the system mtu command; 10/100 ports are not affected by
the system jumbo mtu command. If you do not configure the system mtu jumbo command, the setting
of the system mtu command applies to all Gigabit Ethernet interfaces.
You cannot set the MTU size for an individual interface; you set it for all 10/100 or all Gigabit Ethernet
interfaces on the switch stack. When you change the system or jumbo MTU size, you must reset the
switch before the new configuration takes effect.The system mtu routing command does not require a
switch reset to take effect.
Frames sizes that can be received by the switch CPU are limited to 1998 bytes, no matter what value was
entered with the system mtu or system mtu jumbo commands. Although frames that are forwarded or
routed are typically not received by the CPU, in some cases packets are sent to the CPU, such as traffic
sent to control traffic, SNMP, Telnet, or routing protocols.
Routed packets are subjected to MTU checks on the output ports. The MTU value used for routed ports
is derived from the applied system mtu value (not the system mtu jumbo value). That is, the routed
MTU is never greater than the system MTU for any VLAN. The routing protocols use the system MTU
value when negotiating adjacencies and the MTU of the link. For example, the Open Shortest Path First
(OSPF) protocol uses this MTU value before setting up an adjacency with a peer router. To view the
MTU value for routed packets for a specific VLAN, use the show platform port-asic mvid privileged
EXEC command.
If Layer 2 Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces,
Note
jumbo frames received on a Layer 2 Gigabit Ethernet interface and sent on a Layer 2 10/100 interface
are dropped.
Beginning in privileged EXEC mode, follow these steps to change MTU size for all 10/100 or Gigabit
Ethernet interfaces:
Command
Step 1 configure terminal
Step 2
system mtu bytes
OL-8550-02
Purpose
Enter global configuration mode.
(Optional) Change the MTU size for all interfaces on
the switch stack that are operating at 10 or 100 Mbps.
The range is 1500 to 1998 bytes; the default is 1500
bytes.
Catalyst 3750 Switch Software Configuration Guide
Configuring the System MTU
11-27