Cisco Catalyst 4500 Series Software Configuration Manual page 476

For traffic that needs to be routed, the MTU of the egress port is checked. If the MTU is smaller than the packet size, the packet
is forwarded to the CPU. If the "do not fragment bit" is not set, it is fragmented. Otherwise, the packet is dropped.
Jumbo frame support does not fragment Layer 2 switched packets.
Note
The Catalyst 4500 series switch does not compare the packet size with the MTU at the egress port, but jumbo frames are
dropped in ports that do not support them. The frames can be transmitted in ports that do support jumbo frames, even though
the MTU is not configured to jumbo size.
Note Jumbo frame support is only configured per interface; jumbo frame support cannot be configured
globally.
Ethernet Ports
These sections describe configuring nondefault MTU sizes on Ethernet ports:
• Ethernet Port Overview, page 9-36
• Layer 3 and Layer 2 EtherChannels, page 9-36
Ethernet Port Overview
Starting with Cisco IOS Release 12.2(31)SGA, configuring a nondefault MTU size on certain Ethernet ports limits the size of
ingress packets. The MTU does not impact the egress packets.
Prior to Cisco IOS Release 12.1(13)EW, you could configure the MTU size only on Gigabit Ethernet.
Layer 3 and Layer 2 EtherChannels
Starting with Release Cisco IOS Release 12.2(31)SGA, you could 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 Optional Interface Features
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS
9
Chapter 9 Configuring Interfaces