Fore Systems ES-2810 - Fragment; Cut-Through Forwarding; Fragment-Free Forwarding

FORE Systems ES-2810 Ethernet Switch User’s Manual
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Concepts in Switching
Concepts in Switching
A.1.5 Fragment
A fragment is a frame consisting of only part of a packet; these can be caused by collisions on
the network and are normal occurrences.
A.1.6 Cut-through Forwarding
Cut-through forwarding sends the packet to the destination as soon as the first 14 bytes of the
packet are read—an approximate latency of 30 microseconds for 10Mbps devices and 11
microseconds for 100Mbps devices. The delay is minimal and the packets reach their destina-
tion in the shortest possible time.
The packets are sent through the switch as a continuous flow of data—the transmit and
receive rates are always the same. Because of this, cut-through forwarding cannot pass packets
to higher speed networks, for example, to forward packets from a 10Mbps to a 100Mbps
Ethernet network.
Since the switch has forwarded most of the packet when the CRC is read, the switch cannot
discard packets with CRC errors. However, the CRC check is still made and, if errors are
found, the error count is updated.
Cut-through forwarding is recommended for networks intended to provide one switch port
per user, or for lightly loaded networks. It is essential for multimedia applications and ideal
for workgroup environments where minimum delays are required.
A.1.7 Fragment-free Forwarding
Fragment-free forwarding is suitable for backbone applications in a congested network, or
when connections are allocated to a number of users.
Fragment-free forwarding checks that there are no collisions within the first 64 bytes of the
packet—the minimum valid message size required by the IEEE 802.3 specification. This guar-
antees that message fragments less than 64 bytes (runts) are not forwarded to other network
segments. Runts are typically the result of collision fragments.
The packets are sent through the switch as a continuous flow of data—the transmit and
receive rates are always the same. Because of this, fragment-free forwarding cannot pass pack-
ets to higher speed networks, for example, to forward packets from a 10Mbps to a 100Mbps
Ethernet network. Therefore, if you opt for fragment-free forwarding, you cannot make direct
connections to higher speed networks (like FDDI) from that port.
Fragment-free forwarding offers a compromise between cut-through (which offers the fastest
possible forwarding at the expense of error checking) and store-and-forward (which offers
maximum error checking at the expense of forwarding speed), to provide a latency of approx-
imately 60 microseconds and sufficient error checking to eliminate the most common errors.