Bridging/Switching

Bridging/Switching:

Bridge -

 A layer 2 device used to connect different networks types or networks of the same type. It maps the Ethernet addresses of the nodes residing on each segment and allows only the necessary traffic to pass through the bridge. Packet destined to the same segment is dropped. This "store-and-forward" mechanism inspects the whole Ethernet packet before making a decision. Unfortunately, it cannot filter out broadcast traffic. Also, it introduces a 20 to 30 percent latency when processing the frame. Only 2 networks can be linked with a bridge.

Switch -

 Switches are layer 2 devices that can link up four, six, eight or even more networks. Switches are the only devices that allow for microsegmentation. Cut-through switches run faster because when a packet comes in, it forwards it right after looking at the destination address only. A store-and-forward switch inspects the entire packet before forwarding. Most switches cannot stop broadcast traffic. Switches are considered dedicated data link device because they are close to a 100 % of the bandwidth. While bridging does most of its work by hardware, switches use fabric/software to handle most of its work. 

Store-and-forward - The entire frame is received before any forwarding takes place. The destination and/or the source addresses are read and filters are applied before the frame is forwarded. Latency occurs while the frame is being received; the latency is greater with larger frames because the entire frame takes longer to read. Error detection is high because of the time available to the switch to check for errors while waiting for the entire frame to be received. This method discards frames smaller than 64 bytes (runts) and frames larger than 1518 bytes (giants).

Cut-Through - The switch reads the destination address before receiving the entire frame. The frame is then forwarded before the entire frame arrives. This mode decreases the latency of the transmission and has poor error detection. This method has two forms, Fast-forward and fragment-free.

• Fast-forward switching - Fast-forward switching offers the lowest level of latency by immediately forwarding a packet after receiving the destination address. Because fast-forward switching does not check for errors, there may be times when frames are relayed with errors. Although this occurs infrequently and the destination network adapter discards the fault frame upon receipt. In networks with high collision rates, this can negatively affect available bandwidth.
• Fragment Free Switching - Use the fragment-free option to reduce the number of collisions frames forwarded with errors. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted, or first in, first out (FIFO). Fragment-free switching filters out collision fragments, which are the majority of packets errors, before forwarding begins. In a properly functioning network, collision fragments must be smaller then 64 bytes. Anything greater than 64 byes is a valid packet and is usually received without error. Fragment-free switching waits until the received packet has been determined not to be a collision fragment before forwarding the packet. In fragment-free, latency is measured as FIFO.

Spanning-Tree Protocol - Allows duplicate switched/bridged paths without incurring the latency effects of loops in the network. 

The Spanning-Tree Algorithm, implemented by the Spanning-Tree Protocol, prevents loops by calculating stable spanning-tree network topology. When creating a fault-tolerant network, a loop-free path must exist between all nodes in the network The Spanning-Tree Algorithm is used to calculate a loop-free paths. Spanning-tree frames, called bridge protocol data units (BPDUs), are sent and received by all switches in the network at regular intervals and are used to determine the spanning-tree topology. A switch uses Spanning-Tree Protocol on all Ethernet-and Fast Ethernet-based VLANs. Spanning-tree protocol detects and breaks loops by placing some connections in standby mode, which are activated in the event of an active connection failure. A separate instance Spanning-Tree Protocol runs within each configured VLAN, ensuring topologies, mainly Ethernet topologies that conform to industry standards throughout the network. These modes are as follows:

• Blocking- NO frames forwarded, BPDUs heard.
• Listening – No frames forwarded, listening for frames
• Learning- No frames forwarded, learning addresses.
• Forwarding- Frames forwarded, learning addresses.
• Disabled- No frames forwarded, no BPDUs heard.

The state for each VLAN is initially set by the configuration and later modified by the Spanning-Tree Protocol process. You can determine the status, cost and priority of ports and VLANs, by using the show spantree command. After the port-to-VLAN state is set, Spanning-Tree Protocol determines whether the port forwards or blocks frames.