Cisco 4500M Software Manual page 83

Chapter 4 Configuring Interfaces
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
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.
With releases earlier than Cisco IOS Release 12.1(13)EW, you can configure the MTU size only on
Gigabit Ethernet.
Layer 3 and Layer 2 EtherChannels
With Release Cisco IOS Release 12.2(25)EW and later releases, 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 will be 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.
OL-6696-01
Ethernet Port Overview, page 4-11
Layer 3 and Layer 2 EtherChannels, page 4-11
Configuring Optional Interface Features
Software Configuration Guide—Release 12.2(25)EW
4-11