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Software Configuration Guide—Release 12.2(25)SG
OL-7659-02
Chapter4 Configuring Interfaces
Configuring Optional Interface Features
Each of the last three modules has two non-blocking ports that can support jumbo frames. Other
ports are over-subscribed ports and cannot support jumbo frames.
Understanding Jumbo Frame Support
These sections describe jumbo frame support:
Jumbo Frame Support Overview, page 4-11
Ethernet Ports, page 4-11
VLAN Interfaces, page 4-12

Jumbo Frame Support Overview

A jumbo frame is a frame larger than the default Ethernet size. Enable jumbo frame support by
configuring a larger-than-default maximum transmission unit (MTU) size on a port or interface.
Catalyst 4500 series switch Ethernet LAN ports configured with a n ondefault MTU size accept frames
containing packets with a size between 1500 and 9198 bytes. With a nondefault MTU size configured,
the packet size of ingress frames is checked. If the packet is larger than the configured MTU, it is
dropped.
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.
Note Jumbo frame support does not fragment Layer 2 switched packets.
The Catalyst4500 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 fra mes can be transmitted in ports that
do support jumbo frames, even though the MTU is not configured to jumbo si ze.
Note Jumbo frame support is only configured per interface; jumbo f rame support cannot be configured
globally.

Ethernet Ports

These sections describe configuring nondefault MTU sizes on Ethernet ports:
Ethernet Port Overview, page 4-11
Layer 3 and Layer 2 EtherChannels, page 4-12
Ethernet Port Overview
With Cisco IOS Release 12.2(25)EW, configuring a nondefault MTU size on cert ain Ethernet ports
limits the size of ingress packets. The MTU does not impact the egress packets.
With releases earlier than Cisco IOS Release 12.1(13)EW, you can configure the MTU size only on
Gigabit Ethernet.