Switch Speeds and Forwarding Methods

Describe switch forwarding methods and port settings available on Layer 2 switch ports.

Frame Forwarding Methods on Cisco Switches

• Switches use one of the following forwarding methods for switching data between network ports:
• • Store-and-forward switching – This frame forwarding method receives the entire frame and computes the CRC. If the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. Then the frame is forwarded out of the correct port.
  • Cut-through switching – This frame forwarding method forwards the frame before it is entirely received. At a minimum, the destination address of the frame must be read before the frame can be forwarded.
• A big advantage of store-and-forward switching is that it determines if a frame has errors before propagating the frame.
• When an error is detected in a frame, the switch discards the frame.
• Discarding frames with errors reduces the amount of bandwidth consumed by corrupt data.
• Store-and-forward switching is required for quality of service (QoS) analysis on converged networks where frame classification for traffic prioritization is necessary.

For example, voice over IP (VoIP) data streams need to have priority over web-browsing traffic.

Cut-Through Switching

• In cut-through switching, the switch acts upon the data as soon as it is received, even if the transmission is not complete.
• The switch buffers just enough of the frame to read the destination MAC address so that it can determine to which port it should forward out the data.
• The switch does not perform any error checking on the frame.
• There are two variants of cut-through switching:
• • Fast-forward switching – Offers the lowest level of latency by immediately forwarding a packet after reading the destination address. Because fast-forward switching starts forwarding before the entire packet has been received, there may be times when packets are relayed with errors. The destination NIC discards the faulty packet upon receipt. Fast-forward switching is the typical cut-through method of switching.
  • Fragment-free switching – A compromise between the high latency and high integrity of store-and-forward switching and the low latency and reduced integrity of fast-forward switching, the switch stores and performs an error check on the first 64 bytes of the frame before forwarding. Because most network errors and collisions occur during the first 64 bytes, this ensures that a collision has not occurred before forwarding the frame.

Memory Buffering on Switches

• An Ethernet switch may use a buffering technique to store frames before forwarding them or when the destination port is busy because of congestion.

• Shared memory buffering also results in larger frames that can be transmitted with fewer dropped frames. This is important with asymmetric switching which allows for different data rates on different ports. Therefore, more bandwidth can be dedicated to certain ports (e.g., server port).

Duplex and Speed Settings

Two of the most basic settings on a switch are the bandwidth (“speed”) and duplex settings for each individual switch port. It is critical that the duplex and bandwidth settings match between the switch port and the connected devices. There are two types of duplex settings used for communications on an Ethernet network:

• Full-duplex – Both ends of the connection can send and receive simultaneously.
• Half-duplex – Only one end of the connection can send at a time.

Autonegotiation is an optional function found on most Ethernet switches and NICs. It enables two devices to automatically negotiate the best speed and duplex capabilities.

Note: Gigabit Ethernet ports only operate in full-duplex.

• Duplex mismatch is one of the most common causes of performance issues on 10/100 Mbps Ethernet links. It occurs when one port on the link operates at half-duplex while the other port operates at full-duplex.
• This can occur when one or both ports on a link are reset, and the autonegotiation process does not result in both link partners having the same configuration.
• It also can occur when users reconfigure one side of a link and forget to reconfigure the other. Both sides of a link should have autonegotiation on, or both sides should have it off. Best practice is to configure both Ethernet switch ports as full-duplex.

Auto-MDIX

Connections between devices once required the use of either a crossover or straight-through cable. The type of cable required depended on the type of interconnecting devices.

Note: A direct connection between a router and a host requires a cross-over connection.

• Most switch devices now support the automatic medium-dependent interface crossover (auto-MDIX) feature. When enabled, the switch automatically detects the type of cable attached to the port and configures the interfaces accordingly.
• The auto-MDIX feature is enabled by default on switches running Cisco IOS Release 12.2(18)SE or later. However, the feature could be disabled. For this reason, you should always use the correct cable type and not rely on the auto-MDIX feature.
• Auto-MDIX can be re-enabled using the mdix auto interface configuration command.

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