Forwarding Modes

Possible forwarding modes

You can specify one of four possible forwarding modes for each port:

Forwarding policy

If two communicating ports (receive port and transmit port) have different forwarding modes, then they use the “safest” mode. For example, if one port is configured for fragment-free and the other port is configured for store-and-forward, then traffic between the two ports in either direction is always switched using store-and-forward.

CRC errors

Cyclic Redundancy Check (CRC) errors are the sum of Frame Check Sequences, longs, very longs, alignment errors and jabbers.

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.

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 destination 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.

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 guarantees 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 packets to higher speed networks.

Example

To forward packets from a 10Mbps to a 100Mbps Ethernet network. Therefore, if you choose fragment-free forwarding, you cannot make direct connections to higher speed networks (such as 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 approximately 60 microseconds and sufficient error checking to eliminate the most common errors.

Store-and-forward forwarding

Store-and-forward forwarding temporarily stores a packet and checks it against the CRC field. If the packet is error free, it is forwarded; otherwise, it is discarded. Store-and-forward forwarding is therefore the best forwarding mode to prevent errors being forwarded throughout the network. The buffering used by store-and- forward also allows the switch to dispatch packets at a different rate than it receives them; for example, to forward packets from a 10Mbps network to higher speed networks, such as a 100Mbps Ethernet.

Adaptive forwarding

Adaptive forwarding mode is a user-defined facility to maximize the efficiency of the switch. Adaptive forwarding starts in the default switch forwarding mode you have selected in the Switch & Port window (cut-through if you selected adaptive mode as the default forwarding mode). Depending on the number of runts and CRC errors at that port, the mode changes to the “best” of the other two forwarding modes. As the numbers of runts and CRC errors change, so does the forwarding mode. This is best illustrated by the following table:

Forwarding mode: Detects: Then, adaptive mode changes the forwarding mode to:
Cut-through High numbers of CRC errors Store-and-forward
High numbers of runts Fragment-free
Fragment-free High numbers of CRC errors Store-and-forward
Low numbers of runts Cut-through
Store-and-forward Low numbers of CRC errors Fragment-free
Low numbers of CRC errors and runts Cut-through

Note:

While CRC errors and runts are the most likely parameters to cause the forwarding mode to change, they are not the only ones.

Forwarding mode affect on latency

Latency is the delay measured from the time the packet first enters a network device until it leaves it. The closer a device is to zero latency, the better.

The type of network can affect latency. Over wide-area networks, latency is negligible in comparison to the time it takes the signals to travel over long distance lines. On local area networks, reducing latency normally increases performance.

Unfortunately, reducing the latency can often lead to an increase in errors on the network. The ideal situation is to change the forwarding modes to provide added reliability and flexibility.

 

Example

If you are concerned about the generation of errors on a network, you can configure the ports to store-and-forward mode to ensure safe transfer of data.

Delays

Delays depend on the forwarding mode:

Cut-through Fragment-free Store-and-forward
Min. Latency (in sec.) Low (10 Mbit is < 30 100 Mbit is < 11) Medium (< 60) High (Depends on packet size)
Amount of packet read 14 bytes

Destination address

+ Source address

+ Type/Length field)

64 bytes

(IEEE 802.3)

All

(CRC)

Error detection None Runts All
Suitable for One user per port

Light loads

Applications requiring low latency for-warding

Many users on one connection

Congested networks

Communicating with higher-speed networks

A port with many errors

 


Procedures

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