11-23
Catalyst 3750 Switch Software Configuration Guide
78-16180-02
Chapter 11 Configuring Interface Characteristics
Configuring the System MTU
support jumbo frames on all Gigabit Ethernet interfaces by using the system mtu jumbo global
configuration command. Gigabit Ethernet ports are not affected by the system mtu command; 10/100
ports are not affected by the system jumbo mtu command.
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 MTU size, you must reset the switch before the new
configuration takes effect.
The size of frames that can be received by the switch CPU is limited to 1992 bytes, no matter what value
was entered with the system mtu or system mtu jumbo commands. Although frames that are forwarded
or routed typically are 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 egress ports. The MTU value used for routed ports
is derived from the system mtu configured value (not the system mtu jumbo value). That is, the routed
MTU is never greater than the system MTU for any VLAN. The system MTU value is used by routing
protocols when negotiating adjacencies and the MTU of the link. For example 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.
Note If Layer 2 Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces,
jumbo frames ingressing on a Layer 2 Gigabit Ethernet interface and egressing 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:
If you enter a value that is outside the allowed range for the specific type of interface, the value is not
accepted.
Once the switch reloads, you can verify your settings by entering the show system mtu privileged EXEC
command.
This example shows how to set the maximum packet size for a Gigabit Ethernet port to 1800 bytes:
Switch(config)# system jumbo mtu 1800
Switch(config)# exit
Switch# reload
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
system mtu bytes (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 1546 bytes; the default is 1500
bytes.
Step 3
system mtu jumbo bytes (Optional) Change the MTU size for all Gigabit
Ethernet interfaces on the switch stack. The range is
1500 to 9000 bytes; the default is 1500 bytes.
Step 4
end Return to privileged EXEC mode.
Step 5
copy running-config startup-config Save your entries in the configuration file.
Step 6
reload Reload the operating system.
To Next Page
Loading...
Need help?
General