Configuring IP
December 2005 © Foundry Networks, Inc. 16 - 21
• Port MTU – A port’s default MTU depends on the encapsulation type enabled on the port.
Changing the Encapsulation Type
The Layer 3 Switch encapsulates IP packets into Layer 2 packets, to send the IP packets on the network. (A Layer
2 packet is also called a MAC layer packet or an Ethernet frame.) The source address of a Layer 2 packet is the
MAC address of the Layer 3 Switch interface sending the packet. The destination address can be one of the
following:
• The MAC address of the IP packet’s destination. In this case, the destination device is directly connected to
the Layer 3 Switch.
• The MAC address of the next-hop gateway toward the packet’s destination.
• An Ethernet broadcast address.
The entire IP packet, including the source and destination address and other control information and the data, is
placed in the data portion of the Layer 2 packet. Typically, an Ethernet network uses one of two different formats
of Layer 2 packet:
• Ethernet II
• Ethernet SNAP (also called IEEE 802.3)
The control portions of these packets differ slightly. All IP devices on an Ethernet network must use the same
format. Foundry Layer 3 Switches use Ethernet II by default. You can change the IP encapsulation to Ethernet
SNAP on individual ports if needed.
NOTE: All devices connected to the Layer 3 Switch port must use the same encapsulation type.
To change the IP encapsulation type on interface 5 to Ethernet SNAP, enter the following commands:
FESX424 Router(config)# int e 5
FESX424 Router(config-if-e1000-5)# ip encapsulation snap
Syntax: ip encapsulation snap | ethernet_ii
Changing the Maximum Transmission Unit (MTU)
The Maximum Transmission Unit (MTU) is the maximum length of IP packet that a Layer 2 packet can contain. IP
packets that are longer than the MTU are fragmented and sent in multiple Layer 2 packets. You can change the
MTU globally or on individual ports.
The default MTU is 1500 bytes for Ethernet II packets and 1492 for Ethernet SNAP packets.
MTU Enhancements
Foundry devices contain the following enhancements to jumbo packet support:
• Hardware forwarding of Layer 3 jumbo packets – Layer 3 IP unicast jumbo packets received on a port that
supports the frame's MTU size and forwarded to another port that also supports the frame's MTU size are
forwarded in hardware. Previous releases support hardware forwarding of Layer 2 jumbo frames only.
• ICMP unreachable message if a frame is too large to be forwarded – If a jumbo packet has the Don't
Fragment (DF) bit set, and the outbound interface does not support the packet's MTU size, the Foundry
device sends an ICMP unreachable message to the device that sent the packet.
NOTE: These enhancements apply only to transit traffic forwarded through the Foundry device.
Configuration Considerations for Increasing the MTU
• When you increase the MTU size of a port, the increase uses system resources. Increase the MTU size only
on the ports that need it. For example, if you have one port connected to a server that uses jumbo frames
and two other ports connected to clients that can support the jumbo frames, increase the MTU only on those
three ports. Leave the MTU size on the other ports at the default value (1500 bytes). Globally increase the
MTU size only if needed.
• Use the same MTU size on all ports that will be supporting jumbo frames. If the device needs to fragment a
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