Cisco Catalyst 4500 Series Configuration Manual page 310

Configuring Optional Interface Features
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.
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:
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•
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.
Layer 3 and Layer 2 EtherChannels
Starting with Release Cisco IOS Release 12.2(25)EW, 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.
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
7-26
Ethernet Port Overview, page 7-26
Layer 3 and Layer 2 EtherChannels, page 7-26
Chapter 7 Configuring Interfaces
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