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Configuring Interfaces
Configuring the System MTU
routed packets larger than the routing MTU value
For example, if the system mtu value is 1998 bytes and the system mtu jumbo value is 5000 bytes, packets up to 5000
bytes can be received on interfaces operating at 1000 Mbps. However, although a packet larger than 1998 bytes can be
received on an interface operating at 1000 Mbps, if its destination interface is operating at 10 or 100 Mbps, the packet
is dropped.
Routed packets are subjected to MTU checks on the sending ports. The MTU value used for routed ports is derived from
the configured 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 the 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
This example shows the response when you try to set Gigabit Ethernet interfaces to an out-of-range number:
Switch(config)# system mtu jumbo 25000
^
% Invalid input detected at '^' marker.
Command Purpose
1. configure terminal Enter global configuration mode.
2. system mtu bytes (Optional) Change the MTU size for all interfaces on the
switch that are operating at 10 or 100 Mbps. The range
is 1500 to 1998 bytes; the default is 1500 bytes.
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.
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.
5. end Return to privileged EXEC mode.
6. copy running-config startup-config Save your entries in the configuration file.
7. reload Reload the operating system.
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