Encapsulation Type and MTU Packet Parameters
You can configure the following packet parameters on Layer 3 devices. These parameters control how the Layer 3 device sends IP packets to other devices on an Ethernet network. The Layer 3 device 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 device sends IP packets.
- Maximum Transmission Unit (MTU): The maximum length of IP packet that a Layer 2 packet can contain.
Encapsulation Type
The Layer 3 device 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:
The control portions of these packets differ slightly. All IP devices on an Ethernet network must use the same format. RUCKUS Layer 3 devices use Ethernet II by default. You can change the IP encapsulation to Ethernet SNAP on individual ports if needed.
MTU
The Maximum Transmission Unit (MTU) is the maximum size an IP packet can be when encapsulated in a Layer 2 packet. If an IP packet is larger than the MTU allowed by the Layer 2 packet, the Layer 3 switch fragments the IP packet into multiple parts that will fit into the Layer 2 packets, and sends the parts of the fragmented IP packet separately, in different Layer 2 packets. The device that receives the multiple fragments of the IP packet reassembles the fragments into the original packet.
When packets are larger than the default MTU, they are referred to as jumbo packets. RUCKUS 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 MTU size and forwarded to another port that also supports the frame 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 MTU size, the RUCKUS device sends an ICMP unreachable message to the device that sent the packet.
Globally Changing the MTU
You can increase the MTU size to accommodate jumbo packet sizes up to 10,200 bytes.
To globally enable jumbo support on all ports of a RUCKUS ICX device, enter commands such as the following.
device(config)# jumbo device(config)# write memory device(config)# end device# reload