Ethernet, VLANs and Token Ring: logical topologies explained

Logical topology

A logical topology describes how data actually flows, regardless of how cables are laid. For example, Token Ring was wired as a physical star but behaved as a logical ring. Today's main logical technologies are Ethernet and VLANs.

Ethernet

Ethernet is by far the most widely used LAN technology. It was developed at Xerox PARC in the 1970s by Robert Metcalfe and David Boggs and standardised by the IEEE as 802.3 in 1983. Every Ethernet network card has a unique 48-bit MAC address that identifies it on the LAN.

Sharing the medium: CSMA/CD

Early Ethernet ran on shared cable, and later on hubs, so two devices could transmit at the same moment and their signals would collide. Ethernet handled this with Carrier Sense Multiple Access with Collision Detection (CSMA/CD):

Everything connected to a hub was one collision domain. Switches changed this: each switch port is its own collision domain, and with full-duplex links collisions disappear completely. Wireless LANs use a related method, CSMA/CA (collision avoidance), because radios can't detect collisions while transmitting.

Ethernet speeds and names

Names like 10BASE-T mean: 10 Mb/s, BASEband signalling, Twisted-pair cable.

Copper Ethernet segments are limited to 100 metres. Modern Ethernet is wired as a star, with every device connected to a switch.

Virtual LANs (VLANs)

A switch separates collision domains, but by default all its ports are still one broadcast domain: a broadcast from one device reaches every other device. In a big flat network that wastes bandwidth and lets any device reach any other, which is a security problem.

A VLAN splits one physical switched network into several logical networks. Devices in different VLANs can't talk directly, even when they are on the same switch. You can group users by department or purpose (staff, guests, cameras, servers) without re-cabling.

• Access port: belongs to one VLAN and connects an end device.
• Trunk port: carries many VLANs between switches or to a router, adding an 802.1Q tag to each frame to show its VLAN.
• Inter-VLAN routing: a router (a "router on a stick" using sub-interfaces) or a layer 3 switch forwards traffic between VLANs, and firewall rules can control exactly what is allowed.

Example configuration (Cisco IOS)

! On the switch: create VLANs and assign ports
vlan 10
 name STAFF
vlan 20
 name GUEST
interface range FastEthernet0/1 - 2
 switchport mode access
 switchport access vlan 10
interface range FastEthernet0/3 - 4
 switchport mode access
 switchport access vlan 20
interface GigabitEthernet0/1
 switchport mode trunk

! On the router: one sub-interface per VLAN
interface GigabitEthernet0/0.10
 encapsulation dot1Q 10
 ip address 192.168.10.1 255.255.255.0
interface GigabitEthernet0/0.20
 encapsulation dot1Q 20
 ip address 192.168.20.1 255.255.255.0

You can practise this for free in a simulator such as Cisco Packet Tracer. For common mistakes, read VLANs and trunks.

Token Ring (historical)

Token Ring was IBM's LAN technology from the mid-1980s, standardised as IEEE 802.5. Physically it was a star, with devices connected to a central Multistation Access Unit (MAU), but logically it was a ring. A special frame called the token travelled around the ring, and a device could transmit only while holding it, so collisions never happened. It ran at 4 or 16 Mb/s, faster than early Ethernet, but needed IBM-specific cables and equipment and was soon overtaken by cheaper, faster Ethernet. It is no longer used.

Inside a frame and inside a switch

An Ethernet frame carries a destination MAC, a source MAC, an EtherType saying what is inside, the payload (up to 1500 bytes) and a frame check sequence that catches damage. Switches learn which port each source MAC lives on, forward frames for known destinations out of one port, and flood unknown destinations and broadcasts to every port in the VLAN. For the detail, see Ethernet frames and MAC addresses and how switches learn and forward.