Connecting Two Switches with an Access Uplink

A single 24-port switch cannot support an entire growing company forever. When physical ports run out or when teams sit in separate rooms, installing a secondary switch is the most practical solution. The connection between two switches is known as an uplink.

Because both switches operate at Layer 2 without VLAN segregation, they form a single transparent bridging fabric. Neither computer knows that an extra switch exists between them. Data frames enter Switch 1, travel across the copper patch cable, and arrive at Switch 2 in microseconds.

This arrangement expands physical port density while preserving simple flat network addressing. All connected devices share the same IP subnet (192.168.1.0/24) and can communicate directly without default gateways or routing hops.

Broadcast traffic, such as Address Resolution Protocol requests and DHCP discovery messages, flows across the uplink so that all computers stay in the same logical neighborhood. This single broadcast domain design makes sharing printers, file servers, and local web applications completely effortless.

In small office environments, cascading switches creates an extended star topology.

However, network engineers avoid connecting switches in a closed ring without Spanning Tree Protocol, as loops cause broadcast storms that saturate network bandwidth within seconds.

Spanning Tree Protocol continuously exchanges Bridge Protocol Data Units to detect redundant paths. In our simple two-switch lab, a single point-to-point patch cable provides safe, loop-free expansion without blocking any ports.

To establish the physical connection, connect an Ethernet cable between port FastEthernet0/24 on Switch 1 and port FastEthernet0/24 on Switch 2.

Network administrators typically reserve the highest-numbered ports on a switch for uplinks to keep front patch panels organized.

Once connected, both port LEDs turn solid green. Modern network cards and switch ports feature Auto-MDIX, automatically swapping transmit and receive pairs in silicon so either straight-through or crossover cables work reliably.

During the initial link establishment, both switch ports exchange fast link pulses to negotiate operational parameters. Within milliseconds, both devices agree on 100 Mbps transmission speed and full-duplex operation.

Parameter-by-Parameter Status Breakdown:

Both rooms share the common network range 192.168.1.0/24. Before computers can generate network packets, they must have unique IP addresses within this range.

Open the console on the Accounting PC (connected to Switch 1 port Fa0/1) and assign IP address 192.168.1.10 with subnet mask 255.255.255.0.

Next, open the console on the Marketing PC (connected to Switch 2 port Fa0/1) and assign IP address 192.168.1.20 with the same subnet mask.

Understanding Local Delivery Logic:

When you assign 192.168.1.10/24, the workstation applies a bitwise AND operation between its IP address and subnet mask.

This calculation determines that its local network ID is 192.168.1.0. In a standard /24 subnet, 192.168.1.0 represents the network identifier, while 192.168.1.255 serves as the directed broadcast address. This leaves 254 valid host addresses from .1 through .254.

When Accounting PC attempts to send data to Marketing PC (192.168.1.20), it recognizes that the destination belongs to its own subnet.

Because the target is local, Accounting PC prepares a direct Layer 2 Ethernet frame rather than forwarding the packet to a default gateway router.

Now test end-to-end connectivity across the physical uplink cable. Open the command terminal on Accounting PC and initiate an ICMP echo test: ping 192.168.1.20.

The first ping packet triggers an Address Resolution Protocol (ARP) broadcast. Watch how both switches cooperate to deliver the request across the uplink.

Tracing Frame Traversal Across the Uplink:

To see the proof of transparent switching, open the console on Switch 1 and run the privileged EXEC command show mac address-table.

Observe how Switch 1 associates the remote Marketing PC hardware address with its local uplink port.

This lookup process takes place entirely in specialized silicon hardware known as Content Addressable Memory (CAM), ensuring that forwarding decisions occur at wire speed without straining the main CPU.

Breaking Down the MAC Table Entries:

This demonstrates the core elegance of Ethernet switching: switches do not care whether a device is directly connected or multiple switches away. As long as an uplink exists, frames reach their destination transparently.