A network switch connects devices in a LAN and forwards Ethernet frames based on MAC addresses. Modern switches can also support VLANs, security, monitoring, and Layer 3 routing features.
How a Layer 2 Switch Works
- The switch reads the source MAC address of an incoming frame.
- It learns that address on the incoming interface and VLAN.
- It looks up the destination MAC address.
- It forwards a known unicast frame to the matching port.
- It floods broadcasts and unknown unicasts only within the relevant VLAN, except the incoming port.
Collision and Broadcast Domains
Each switched Ethernet port is a separate collision domain. A VLAN defines a Layer 2 broadcast domain. Creating additional VLANs separates broadcasts, but communication between them requires Layer 3 routing.
Switch Interfaces
- Access port: connects an end device to one access VLAN.
- Trunk port: carries multiple VLANs using 802.1Q tagging.
- Routed port: operates as a Layer 3 interface on supported switches.
- SVI: a logical VLAN interface used for management or routing.
Management Access
Console access is useful for initial setup and recovery. Remote administration should normally use SSH rather than Telnet. Standard serial console settings commonly include 9600 bits per second, but always follow the platform guide.
Useful Commands
show interfaces statusshow mac address-tableshow vlan briefshow interfaces trunkshow spanning-tree
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