Switching Logic
On a local area network (LAN), switching logic is the basic process that a network switch utilizes to decide how to respond to incoming Ethernet frames and effectively route traffic to the appropriate location. It focusses on sending frames selectively to the intended receiver rather than broadcasting them to all connected devices.
In brief:
Switching logic functions similarly to a switch’s decision brain, determining “where should this frame go?” by examining MAC addresses, updating its database, and employing the appropriate forwarding technique.
The Content-Addressable Memory (CAM) table, sometimes referred to as a forward/filter table or MAC address table, is the central component of switching logic. Device MAC addresses are mapped to the appropriate physical ports on the switch by this table, which functions as a database that is dynamically constructed and kept in RAM. For quick table lookups, switches use Ternary Content-Addressable Memory (TCAM) and Application-Specific Integrated Circuits (ASIC). These devices enable rapid MAC address matching without the need for intricate search algorithms.
Switching Logic Functions
The switching logic usually consists of three main steps or functions:
MAC Address Learning (Learning Process)
- A switch examines the source MAC address in the frame’s header after receiving a frame.
- It then adds the port on which the frame was received and this source MAC address to its MAC address table.
- This procedure enables the switch to create a network map. The option makes sure that frequently used addresses are not prematurely eliminated by updating its timestamp if an entry for a MAC address already exists. With the use of features like port security, MAC addresses can be statically predefined in the database, even though they are usually dynamic.
Forwarding/Filtering Decisions
After learning the source MAC address, the switch checks the frame header’s destination MAC address and its MAC address database to find the port.
Forwarding (Known Unicast): Only if the destination MAC address linked to a different port than the frame came from is in the table does the switch advance the frame. Thus, a logical, full-bandwidth, and efficient point-to-point link is created. This is most common and effective.
Filtering (Same Port): The switch knows that the frame has previously arrived at its destination if the destination MAC address is on the same port and can be located in the table. The frame is discarded (filtered) in this instance to avoid needless network traffic on other segments.
Flooding (Unknown Unicast, Broadcast, Multicast)
If the destination MAC address is broadcast (such as FFFF.FFFF.FFFF) or multicast, or if the switch cannot locate it in its table, this happens. In these situations, the switch emits a duplicate of the frame from every port except the one that received it. Since the switch is unaware of the destination’s position, this is required.
The intended recipient’s source MAC address is discovered and entered into the table for upcoming particular forwarding when they reply. In the absence of specific multicast handling configuration, broadcast and multicast frames are constantly flooded.
Other Important Aspects of Switching Logic
Loop Avoidance with Spanning Tree Protocol (STP): STP is used in networks with redundant physical links to inhibit the endless looping of flooded frames. In order to guarantee that there is only one active link connecting any two LAN segments, STP logically blocks certain switch ports. A variety of port states are defined by STP.
- Blocking: STP messages are processed and forwarded, but no data frames.
- Listening: When the switch is in this temporary mode, it transmits and receives Bridge Protocol Data Units (BPDUs) but does not transfer data frames.
- Learning: The switch does not forward data frames even after it starts building its MAC address table and keeps using BPDUs.
- Forwarding: Data frames are sendable and receiveable using the interface.
- Disabled: Administratively, the port is not operating.
- Rapid Spanning Tree Protocol (RSTP), which is frequently used in modern switches, uses fewer states discarding, learning, and forwarding to accelerate convergence.
Switching Methods: Depending on how a switch forwards a frame, speed and dependability are affected:
- Store-and-Forward Switching: Prior to forwarding, the switch receives the complete frame and buffers it. In order to identify corruption, it does a Cyclic Redundancy Check (CRC). Higher latency is introduced but reliability is increased if an error is detected, as the frame is deleted. On contemporary switches, this is usually the default setting.
- Cut-Through Switching: The switch drastically lowers latency by forwarding the frame as soon as it reads the destination MAC address. It does not, however, check for mistakes, therefore frames that include errors can be sent.
- Fragment-Free Switching: This compromise technique checks for collision fragments, a typical kind of network problem, in the first 64 bytes of the frame before forwarding. If there are no fragments found, the frame is sent on.
Symmetric vs. Asymmetric Switching
- Symmetric switching allows ports with the same bandwidth to be connected.
- In order to manage the speed disparities between ports with varying bandwidths (such as 10-Mbps and 100-Mbps), asymmetric switching frequently necessitates memory buffering.
Memory Buffering: Switches employ memory buffers, which are frequently connected to particular receiving ports (port-based memory), to temporarily store frames.
EtherChannel Load Distribution: When several physical links are combined into one EtherChannel, switches determine which physical link to utilise for each frame using load distribution logic based on different numeric values in frame headers (Layer 2, 3, and 4).
VLANs and Forwarding: When a frame is received in a virtual local area network (VLAN), switches process and forward it according to MAC table entries unique to that VLAN, forwarding only to ports that are part of that VLAN.
The networking device’s data plane, also known as the forwarding plane, is where all of these operations, including forwarding decisions and MAC learning, mostly take place, frequently with the help of ASICs.
You can also read CISCO Switch Configuration: A Safe and Effective Setup Guide