Switching: How frames travel across a local network
On the same network, ARP finds the other device's MAC address, and a switch forwards the frame to the destination port. VLANs, loop prevention, and link aggregation let you divide a network connecting multiple devices and keep it running reliably. Finally, see how VLANs connect to one another through routing.
The introduction explains the different roles of IP addresses and MAC addresses. Then read about VLANs, loop prevention, and link aggregation in order.
The final article explains how traffic moves between VLANs. For boundaries and redundancy in larger networks, continue to the network design topic.
Choose two devices on a cabling diagram and follow the ports a frame could traverse. In the ARP and switching article, distinguish discovering an address from choosing a forwarding port. The VLAN and trunking article then explains why devices attached to the same equipment may belong to different communication domains. Keep the fact that a link is connected separate from the conclusion that the intended devices can communicate.
Compare STP with link aggregation when reading about the problems introduced by spare links. Check the purpose and operating conditions of each, then move to inter-VLAN routing when communication is needed between VLANs. The address lookup tool helps interpret assignment information for an unfamiliar MAC prefix; it does not prove the identity of a device or its owner. For a network spanning multiple switches, revisit the effects of a failure through the related articles on L2/L3 boundaries and redundancy.
Four sections in this chapter
Explore four sections covering MAC addresses and frame forwarding, VLANs, loop prevention and link aggregation, and inter-VLAN communication.
When my PC sends data, whose MAC address does it use?
See how ARP finds a local destination or gateway MAC, and how each router chooses the next recipient and builds a new Ethernet frame.
Find the other device's MAC address and forward the frame
Distinguish a destination on the same network from the gateway used to reach another network. See what the ARP table tells you and what a switch's MAC address table tells you.
Separate communication domains on the same switch
Use VLANs to separate local communication domains, and see how a trunk carries multiple VLANs to another switch.
Use redundant links safely
STP prevents loops by limiting active paths; EtherChannel distributes separate traffic flows across links. Learn why these are different functions.
Connect different VLANs
Follow the round trip: a device sends data to its gateway, which forwards the packet to another network after a route lookup and creation of a new frame. See how it reaches the other VLAN and how traffic returns.
Background reading
These related articles are outside the core reading sequence.
- How does my PC know whether a destination is on the same network? — Review the address check that distinguishes local and remote destinations
Explore further after the core articles
These related articles are outside the core reading sequence.
- How far should a VLAN extend, and where should routing begin? — Decide how far to extend a VLAN in an actual design
- How do you make switches and routers redundant? — Extend redundancy beyond links to gateways and routers