A LAN switch is a device that connects multiple devices on a local area network (LAN) and forwards data frames based on their destination addresses. But how does a LAN switch know where to send a frame? In this article, you will learn how a LAN switch uses different methods and protocols to determine the best path for a frame and avoid loops and collisions.
The first method that a LAN switch uses to forward a frame is to look up the destination MAC address in its MAC address table. This table stores the mapping between MAC addresses and switch ports, and it is updated dynamically as the switch learns new addresses from incoming frames. If the switch finds a match, it sends the frame to the corresponding port. If not, it broadcasts the frame to all ports except the one it came from.
In the intricate dance of frame forwarding, the LAN switch deftly consults its MAC address table to chart the course. This table, a masterful repository of MAC-to-port mappings, dynamically updates as the switch absorbs new addresses from incoming frames. When a frame knocks on the switch's door, it scans the table for a matching destination MAC address. Should fortune favor, the frame is gracefully dispatched to the corresponding port. Yet, in the absence of such fortune, the switch orchestrates a symphony of broadcast, casting the frame wide to all but the port of origin.
Also interesting is the Multicast MAC scheme. The most simple way to handle this is to just forward a Multicast MAC frame to all other ports except the receiving port. More advance Switches make use of the IGMP Snooping mechanism. IGMP messages alow the switch to update the Table with which ports want to listen to a Multicast MAC. In this case Multicast frames addressed to a specific Multicast MAC are only forwarded to ports which have registered to this MAC.
? MAC Address Table: LAN switches maintain a MAC address table, also known as a forwarding table or content addressable memory (CAM) table, which maps MAC addresses to specific switch ports. ? Broadcast and Unknown Unicast Flooding: If the destination MAC address is not found in the MAC address table, the switch forwards the frame out of all ports except the incoming port to ensure delivery to the intended recipient or to learn the MAC address. ? VLAN Membership: LAN switches also consider the VLAN membership of incoming frames. If the switch is configured with virtual LANs (VLANs), it forwards the frame only to ports belonging to the same VLAN as the incoming frame, based on VLAN tagging or port membership settings.
Sometimes, the switch needs to know the MAC address of a device that is not directly connected to it, but is on the same LAN segment. For example, if the switch receives a frame with an IP address as the destination, it needs to find out the MAC address of that IP device. To do this, the switch uses the Address Resolution Protocol (ARP), which sends a broadcast request asking who has that IP address. The device that has the IP address responds with its MAC address, and the switch updates its MAC address table accordingly. Similarly, the switch can use the Internet Control Message Protocol (ICMP) to ping a device and get its MAC address.
I disagree that a Switch needs to send ARP or ICMP messages. This is the task of a Router and basically distingishes a Switch from a Router. In general a Switch is a Layer 2 and a Router a Layer 3 device in the OSI Layer.
Another method that a LAN switch uses to forward a frame is to assign it to a virtual LAN (VLAN), which is a logical grouping of devices that share the same broadcast domain. By using VLANs, the switch can isolate traffic and improve security and performance. To forward a frame between VLANs, the switch needs to use a trunk port, which can carry multiple VLANs using tags. The switch adds or removes the tags based on the source and destination VLANs, and uses the MAC address table to find the appropriate port.
In the realm of LAN switching, VLANs and trunking offer a sophisticated approach. VLANs create logical groupings where devices share the same broadcast domain, enhancing security and performance. To traverse VLANs, switches rely on trunk ports, transporting frames adorned with tags that identify their origins and destinations. Guided by MAC address tables, switches navigate this landscape, ensuring seamless frame delivery.
One of the challenges that a LAN switch faces is to avoid loops in the network topology, which can cause broadcast storms and duplicate frames. To prevent this, the switch uses the Spanning Tree Protocol (STP), which creates a loop-free logical tree of the network by blocking some of the redundant links. The switch uses a special frame called a Bridge Protocol Data Unit (BPDU) to exchange information with other switches and elect a root bridge, which is the central point of the tree. The switch then determines its role and state for each port based on the distance to the root bridge and the port priority.
In the intricate world of LAN switches, a significant challenge arises: avoiding network loops, which can lead to broadcast storms and duplicate frames. To mitigate this risk, switches employ the Spanning Tree Protocol (STP), which constructs a loop-free logical network tree by selectively blocking redundant links. Special frames known as Bridge Protocol Data Units (BPDUs) are exchanged between switches to elect a root bridge, the central point of the tree. Each switch then determines the role and status of its ports based on their distance to the root bridge and port priority.
Finally, a LAN switch can also act as a router and forward frames between different networks or subnets. To do this, the switch needs to have a Layer 3 functionality, which means it can process IP packets and make routing decisions based on their destination addresses. The switch uses routing protocols, such as RIP, OSPF, or EIGRP, to exchange routing information with other routers and build a routing table, which stores the best path for each network. The switch then uses the routing table and the MAC address table to forward the frames to the correct port and network.
In the realm of LAN switches, the role expands beyond mere switching to include routing functionality. By operating at Layer 3, the switch gains the capability to process IP packets and make routing decisions based on their destination addresses. Employing routing protocols like RIP, OSPF, or EIGRP, the switch engages in routing information exchange with other routers, culminating in the creation of a routing table that stores optimal paths to each network. Leveraging both the routing table and MAC address table, the switch accurately forwards frames to the correct port and network.
One important thing to keep in mind is to clear entries of a specific port if it looses link, e.g. because the cable is disconnected. Otherwise if a before learned end node is connected to another port on the same switch the node would not get frames. And also adaptive learning is important which allows switching between ports.