Cisco Systems 4500 Configuring MTU Sizes

6-25
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 6 Configuring Interfaces Configuring Optional Interface Features
Layer 3 and Layer 2 EtherChannels
Starting with Release Cisco IOS Release 12.2(25)EW, 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 member 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 is 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 you remove a line card, and then reinsert the card, some or all of the MTU values configured on
the ports of that line card may be unconfigured. This occurs if the systemwide limit of 32 different MTUs
is reached while the card is removed. Upon reinserting the line card, the system attempts to reapply the
MTU configuration on the ports. If this attempt fails, the MTU values are set to the default.
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.
Command Purpose
Step 1 Switch(config)# interface {{vlan vlan_ID} |
{{type1 slot/port} | {port-channel
port_channel_number} slot/port}}
1. type = fastethernet, gigabitethernet, or tengigabitethernet
Selects the interface to configure.
Step 2 Switch(config-if)# mtu mtu_size Configures the MTU size.
Switch(config-if)# no mtu Reverts to the default MTU size (1500 bytes).
Step 3 Switch(config-if)# end Exits configuration interface mode.
Step 4 Switch(config)# end Exits configuration mode.
Step 5 Switch# show running-config interface
[{fastethernet | gigabitethernet} slot/port] Verifies the running configuration.