Cisco Catalyst 4500 series Administration Manual page 409

Chapter 8 Configuring Interfaces
If the maximum limit of 32 is reached and an attempt is made to configure a new MTU size on a new
interface, the system only allows configuration to proceed if the new MTU size has previously been
configured on some interface. Otherwise, an error message is displayed and the default MTU size is
assigned to the interface being configured.
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 MTU size on a port or interface.
Catalyst 4500 series switch Ethernet LAN ports configured with a nondefault MTU size accept frames
containing packets with a size between 1500 and 9216 bytes (including Ethernet payload, header and
trailer). (The maximum MTU size for a Catalyst 4948 series switch is 9198 bytes (not including header
and trailer.)) 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.
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.
Jumbo frame support is only configured per interface; jumbo frame support cannot be configured
Note
globally.
Ethernet Ports
These sections describe configuring nondefault MTU sizes on Ethernet ports:
•
•
Ethernet Port Overview
Starting with Cisco IOS Release 12.2(25)EW, 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.
OL-30933-01
Ethernet Port Overview, page 8-27
Layer 3 and Layer 2 EtherChannels, page 8-28
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
Configuring Optional Interface Features
8-27