Cisco 6500 Series Software Configuration Manual page 91

Chapter 6 Configuring Interfaces
Jumbo Frame Support Overview
A jumbo frame is a frame larger than the default Ethernet frame size. You enable jumbo frame support
by configuring a larger-than-default maximum transmission unit (MTU) size on a port or VLAN
interface and, with Release 12.1(13)E and later releases, configuring the global LAN port MTU size.
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These sections provide an overview of jumbo frame support:
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Bridged and Routed Traffic Size Check at Ingress 10, 10/100, and 100 Mbps Ethernet and 10 Gigabit Ethernet Ports
Jumbo frame support compares ingress traffic size with the global LAN port MTU size at ingress 10,
10/100, and 100 Mbps Ethernet and 10 Gigabit Ethernet LAN ports that have a nondefault MTU size
configured. The port drops traffic that is oversized.
With Release 12.1(13)E and later releases, you can configure the global LAN port MTU size (see the
"Configuring the Global LAN Port MTU Size" section on page
LAN port MTU size is 9216 bytes and 9216 bytes is the only configurable nondefault MTU size for
Layer 2 LAN ports.
Bridged and Routed Traffic Size Check at Ingress Gigabit Ethernet Ports
Gigabit Ethernet LAN ports configured with a nondefault MTU size accept frames containing packets
of any size larger than 64 bytes. With a nondefault MTU size configured, Gigabit Ethernet LAN ports
do not check for oversize ingress frames.
Routed Traffic Size Check on the PFC
For traffic that needs to be routed Jumbo frame support on the PFC compares traffic sizes to the
configured MTU sizes and provides Layer 3 switching for jumbo traffic between interfaces configured
with MTU sizes large enough to accommodate the traffic. Between interfaces that are not configured
with large enough MTU sizes, if the "do not fragment bit" is not set, the PFC sends the traffic to the
MSFC to be fragmented and routed in software. If the "do not fragment bit" is set, the PFC drops the
traffic.
Bridged and Routed Traffic Size Check at Egress 10, 10/100, and 100 Mbps Ethernet Ports
10, 10/100, and 100 Mbps Ethernet LAN ports configured with a nondefault MTU size transmit frames
containing packets of any size larger than 64 bytes. With a nondefault MTU size configured, 10, 10/100,
and 100 Mbps Ethernet LAN ports do not check for oversize egress frames.
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Jumbo frame support fragments routed traffic in software on the MSFC.
Jumbo frame support does not fragment bridged traffic.
Bridged and Routed Traffic Size Check at Ingress 10, 10/100, and 100 Mbps Ethernet and 10 Gigabit
Ethernet Ports, page 6-11
Bridged and Routed Traffic Size Check at Ingress Gigabit Ethernet Ports, page 6-11
Routed Traffic Size Check on the PFC, page 6-11
Bridged and Routed Traffic Size Check at Egress 10, 10/100, and 100 Mbps Ethernet Ports, page
6-11
Bridged and Routed Traffic Size Check at Egress Gigabit Ethernet and 10 Gigabit Ethernet Ports,
page 6-12
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
Configuring Optional Interface Features
6-14). With earlier releases, the global
6-11