Cisco me 3400 Guia Do Programa

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.

Because the switch does not fragment packets, it drops:

switched packets larger than the packet size supported on the egress interface

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.

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. 

Enter global configuration mode.

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.

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.

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.

Return to privileged EXEC mode.

Save your entries in the configuration file.

Reload the operating system.