Question

why don't we design LANs so that there is no switching loops?

Answer 1

A network loop is a network configuration where there is more than one path between two computers or devices, which causes packets to be constantly repeated. This is due to the fact that a hub will blindly transmit everything it receives to all connections — other devices, such as switches and routers, might be able to reduce or eliminate this problem.

The loop creates broadcast storms as broadcasts and multicasts are forwarded by switches out every port, the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network. Since the Layer 2 header does not support a time to live (TTL) value, if a frame is sent into a looped topology, it can loop forever.

The server in the figure sends a unicast frame to Router C. Since it’s a unicast frame, Switch A forwards the frame, and Switch B provides the same service — it forwards the unicast. This is bad because it means that Router C receives that unicast frame twice, causing additional overhead on the network.one: The MAC address filter table will be totally confused about the device’s location because the switch can receive the frame from more than one link.

Unicast Segment 1 - Unicast Uncat Smith Switch Begr 2-

LOOP AVOIDANCE

Redundant links between switches is a good idea because they help prevent complete network failures in the event one link stops working. However, they often cause more problems because frames can be flooded down all redundant links simultaneously, this creates network loops.

A looped topology is often desired to provide redundancy, but looped traffic is undesirable. The Spanning-Tree protocol was originally designed for bridges. Today, it is also applied to LAN switches and routers operating as a bridge. Spanning-Tree protocol ensures that all bridged segments are reachable but any points where loops occur will be blocked.

Router C Unicast Segment 1 Unicast Unicast Switch B Switch A Segment 2