I. Ethernet Switch Forwarding Methods and Advantages
1. Cut-through switching
2. Store-and-forward switching
3. Segment-free switching
Cut-through switching means that after receiving a frame, the switch immediately forwards it to the appropriate port based on the destination MAC address, using its MAC address table. The advantage of this method is its speed and short forwarding time. However, it may also forward erroneous or useless frames to the destination simultaneously. Store-and-forward switching allocates a buffer (memory space, typically 64KB) to each port of the switch. After data enters the switch, the destination MAC address is read, and the forwarding relationship is determined using the MAC address table. The data is then stored in the buffer on that port until it is full, at which point all data is forwarded to the destination at once. During data storage in the buffer, the switch performs a simple check. If erroneous data is found, it is discarded instead of forwarded to the destination. This method provides better data forwarding quality, but the forwarding time is slightly longer than cut-through switching. Fragment isolation switching, also known as improved cut-through switching, leverages the advantages of cut-through switching, namely low forwarding latency, while checking the length of each data frame. This is because, in principle, each Ethernet frame cannot be less than 64 bytes or more than 1518 bytes. If a switch detects frames smaller than 64 bytes or larger than 1518 bytes, it considers these frames to be "incomplete frames" or "oversized frames" and discards them before forwarding. This method combines the advantages of cut-through switching and store-and-forward, and is used by many high-speed switches, but it is not as widespread as store-and-forward.
Both pass-through forwarding and store-and-forward are Layer 2 forwarding methods, and their forwarding strategies are based on the destination MAC address (DMAC). In this respect, there is no difference between the two forwarding methods. The third method is mainly a variation of the first method, "pass-through forwarding".
The biggest difference between them lies in when they handle forwarding, that is, how switches handle the relationship between the packet receiving process and the forwarding process.
Advantages of Ethernet switches:
High-speed transmission: Ethernet switches have the advantage of high-speed data transmission, enabling data transmission speeds of gigabit or even 10 gigabit.
Flexibility: Ethernet switches can automatically switch and configure different network ports according to the needs of different network devices;
Security: Ethernet switches can ensure network security through MAC address filtering and VLAN isolation, preventing unauthorized access and data leakage;
Scalability: Ethernet switches can be stacked or cascaded to expand the network size and support more network device connections.
II. Functions and Applications of Ethernet Switches
The main functions of a switch include physical addressing, network topology management, error checking, frame sequencing, and flow control. Switches also offer newer features such as support for VLANs (Virtual Local Area Networks), link aggregation, and some even include firewall functionality.
Learning: Ethernet switches know the MAC address of each device connected to each port and map the address to the corresponding port in the MAC address table in the switch's cache.
Forwarding/Filtering: When the destination address of a data frame is mapped in the MAC address table, it is forwarded to the port connected to the destination node instead of all ports (if the data frame is a broadcast/multicast frame, it is forwarded to all ports).
Loop Elimination: When a switch includes a redundant loop, the Ethernet switch avoids loop creation through the Spanning Tree Protocol while allowing for backup paths.
In addition to connecting networks of the same type, switches can also interconnect different types of networks (such as Ethernet and Fast Ethernet). Many modern switches offer high-speed ports supporting Fast Ethernet or FDDI, allowing connection to other switches in the network or providing additional bandwidth to bandwidth-intensive critical servers.
Generally, each port of a switch is used to connect an independent network segment. However, sometimes, to provide faster access speeds, we can connect some important network computers directly to the switch ports. In this way, critical servers and important users on the network have faster access speeds and can support larger data traffic.
Finally, let's briefly summarize the basic functions of a switch:
1. Like hubs, switches provide a large number of ports for cable connections, allowing for star topology cabling.
2. Like repeaters, hubs, and bridges, when a switch forwards a frame, it regenerates a distortion-free square electrical signal.
3. Like a bridge, a switch uses the same forwarding or filtering logic on each port.
4. Like a bridge, a switch divides a local area network (LAN) into multiple collision domains, each with its own independent bandwidth, thus greatly increasing the LAN's bandwidth.
5. In addition to the functions of a bridge, hub, and repeater, switches offer more advanced features such as Virtual LAN (VLAN) and higher performance.
