A settlement network is built around switches: one per building in a star, joined by uplinks, with every port set the same way at both ends of each cable. When no switch can be salvaged, a computer with several network cards can bridge them in software, at the cost of throughput limited by that computer. Use a switch that supports Spanning Tree, or build the network without loops, because one extra cable between two switches can bring the whole network down.
Part of Networking
Choosing, placing and configuring the multi-port bridge at the centre of each star: how many to use, how to join them, and what to do without one.
Placement And Joining Decide How A Switched Network Behaves, Not The Internal Mechanism
How a switch learns addresses, floods unknown destinations and chooses store-and-forward or cut-through is in Switch Construction, which also covers repair of failed units. Network Hardware covers testing a salvaged switch and managed versus unmanaged. Ethernet Basics explains why a switch beats a hub: each port is its own collision domain and full-duplex links remove collisions entirely. This article is about the choices that follow: how many switches, where, joined how, and what a computer can do when no switch exists.
What You Need
- Switches that passed the bench test in Network Hardware: power on, every port tested, no heat damage
- Patch cables and uplink cables made and tested (Twisted Pair Cabling)
- Spare ports — Star Topology asks for 20-30% spare
- Power that survives — a switch is a single point of failure for everything on it, so keep it on protected power (Server Deployment)
Method: Place And Join Switches
Step 1: One switch per wiring closet, one closet per building or floor Each fixed cable is no more than 90 m from the closet (Star Topology).
Step 2: Uplink faster than the traffic below Star Topology says an uplink should carry at least as much as one downstream device and preferably more, because it carries all of their traffic. Use the fastest cable or fibre you have for uplinks.
Step 3: Avoid loops, or run Spanning Tree Join switches in a tree. If you want a redundant second link, use switches that support Spanning Tree so one link is blocked until the other fails; without it a loop produces a broadcast storm (Network Topologies). Mark every deliberate loop on the diagram.
Step 4: Set speed and duplex the same at both ends Leave both on auto-negotiation or fix both identically; a mix is a frequent cause of a slow link (Ethernet Basics).
Step 5: Read the port statistics after a day of traffic Counts of errors on one port point at that cable (Star Topology).
When No Switch Survives: Bridge In Software
A computer with two or more network cards can forward frames between them as a bridge, which is the switch function on commodity hardware. On Linux this is done by creating a bridge device and attaching each wired interface to it (ip link add name br0 type bridge, then ip link set eth0 master br0 for each card). These commands are from general knowledge, not from the other articles, so try them on a bench machine first. The limits are the computer’s: every frame crosses its bus and CPU, so throughput is set by the machine, and its power supply and disk are now part of the network.
Worked Example: What A Switch Port Does To A Frame
Delay added per hop. A store-and-forward switch receives the whole frame before forwarding (Switch Construction). Network Fundamentals gives the time to send a 1,500-byte packet on a 100 Mbps link as 120 microseconds (1,500 x 8 / 100,000,000 = 0.00012 s). At 10 Mbps the same frame takes ten times longer: 1,500 x 8 / 10,000,000 = 0.0012 s = 1,200 microseconds. Every store-and-forward hop adds one such serialisation delay, so a chain of four 100 Mbps switches adds about 4 x 120 = 480 microseconds before queuing.
Aggregate capacity. An 8-port switch with full-duplex 100 Mbps ports can in principle carry 8 x 100 x 2 = 1,600 Mbps, if the internal fabric is line-rate (Network Hardware). Most small unmanaged switches are oversubscribed, so the real figure may be lower.
Common Mistakes
- Joining two ports of the same unmanaged switch, or closing a ring of switches — a broadcast storm; remove the last cable you added
- Daisy-chaining many switches without checking the uplinks — everything below shares one uplink
- Forgetting that the software bridge is a computer — a power cut or a disk failure takes the network with it
What’s Next
- Learn next: IP Addressing — what rides on the frames the switches carry
- Combine with: Data Link Layer — the layer these devices implement