4-12
Software Configuration Guide—Release 12.2(25)SG
OL-7659-02
Chapter4 Configuring Interfaces
Configuring Optional Interface Features
Layer 3 and Layer 2 EtherChannels
With Release Cisco IOS Release 12.2(25)EW and later releases, you can configure all the interfaces in
an EtherChannel provided that they have the same MTU. Changing the MTU of an EtherChannel
changes the MTU of all member ports. If the MTU of a me mber port cannot be changed to the new value,
that port is suspended (administratively shut down). A port cannot join an EtherChannel if the port has
a different MTU. If a member port of an EtherChannel changes MTU, the member port is suspended.

VLAN Interfaces

If switch ports reside in the same VLAN, either configure all of the switch ports to handle jumbo frames
and support the same MTU size, or configure none of them. However, such uniformity of MTU size in
the same VLAN is not enforced.
When a VLAN has switch ports with different MTU size, packets received from a port with a larger MTU
might be dropped when they are forwarded to a port with a smaller MTU.
If the switch ports in a VLAN have jumbo frames enabled, the corresponding SVI can have jumbo frames
enabled. The MTU of an SVI should always be smaller than the smallest MTU among all the switch ports
in the VLAN, but this condition is not enforced.
The MTU of a packet is not checked on the ingress side for an SVI; it is checked on the egress side of
an SVI. If the MTU of a packet is larger than the MTU of the egress SVI, the packet will be sent to the
CPU for fragmentation processing. If the “do not fragment” bit is not set, the packet is fragmented.
Otherwise, the packet is dropped.
Configuring MTU Sizes
To configure the MTU size, perform this task:
Note When configuring the MTU size for VLAN interfaces and Layer 3 and Layer 2 Ethernet ports, note that
the supported MTU values are from 1500 to 9198 bytes.
This example shows how to configure the MTU size on Gigabit Ethernet por t 1/1:
switch# conf t
switch(config)# int gi1/1
switch(config-if)# mtu 9198
switch(config-if)# end
Command Purpose
Step1 Switch(config)# interface {{vlan
vlan_ID
} |
{{
type
1
slot/port
} | {port-channel
port_channel_number
}
slot/port
}}
1. type = fastethernet, gigabitethernet, or tengigabitethernet
Selects the interface to configure.
Step2 Switch(config-if)# mtu
mtu_size
Configures the MTU size.
Switch(config-if)# no mtu Reverts to the default MTU size (1500 bytes).
Step3 Switch(config-if)# end Exits configuration mode.
Step4 Switch# show running-config interface
[{fastethernet | gigabitethernet}
slot/port
]
Displays the running configuration.