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Cisco IE 3010 Switch Software Configuration Guide
OL-23145-01
Chapter 12 Configuring Interface Characteristics
Configuring the System MTU
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.
Note If Layer 2 Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces,
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:
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 mtu jumbo 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 that are operating at 10 or 100 Mb/s.
The range is 1500 to 1998 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.
The range is 1500 to 9000 bytes; the default is 1500
bytes.
Step 4 system mtu routing bytes (Optional) Change the system MTU for routed ports.
The range is 1500 to the system MTU value, the
maximum MTU that can be routed for all ports.
Although larger packets can be accepted, they cannot be
routed.
Step 5 end Return to privileged EXEC mode.
Step 6 copy running-config startup-config Save your entries in the configuration file.
Step 7 reload Reload the operating system.