Brocade Communications Systems ICX 7250 series, Configuration Manual

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Configuring packet parameters
You can configure the following packet parameters on Layer 3 switches. These parameters control how the Layer 3 switch sends IP
packets to other devices on an Ethernet network. The Layer 3 switch always places IP packets into Ethernet packets to forward them on
an Ethernet port.
• Encapsulation type - The format for the Layer 2 packets within which the Layer 3 switch sends IP packets.
• Maximum Transmission Unit (MTU) - 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 an individual
ports:
–
Global MTU - The default MTU value depends on the encapsulation type on a port and is 1500 bytes for Ethernet II
encapsulation and 1492 bytes for SNAP encapsulation.
– Port MTU - A port 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 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. Brocade 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.
device(config)# interface ethernet 5
device(config-if-e1000-5)# ip encapsulation snap
Syntax: ip encapsulation { snap | ethernet_ii }
Changing the 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.
Configuring IP parameters - Layer 3 switches
Brocade FastIron Layer 3 Routing Configuration Guide
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