Quick Reference Guide

582 PowerConnect B-Series TI24X Configuration Guide
53-1002269-02
Configuring IP parameters – Layer 3 Switches
21
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 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 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. 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.
PowerConnect(config)#int e 5
PowerConnect(config-if-e10000-5)#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
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
Do not Fragment (DF) bit set, and the outbound interface does not support the packet's MTU
size, the device sends an ICMP unreachable message to the device that sent the packet.
NOTE
These enhancements apply only to transit traffic forwarded through the device.