Ethernet Virtual LANs

1
Ethernet Virtual LANs

• Hubs versus Switches
• Hubs broadcast bits out all ports
• Switches usually send a frame out a one port
• More fundamentally
• In unicasting, a message is only intended to go
  to one machine, as when a client sends a message
  to a server
• Switches assume unicasting it is the basis for
  sending a frame out a single port

2
Ethernet Virtual LANs

• Broadcasting
• Sometimes, station needs to send a frame to all
  other stations this is broadcasting
• For example, servers send a frame to advertise
  their presence with a broadcast message every
  minute or so

3
Ethernet Virtual LANs

• Broadcasting with Ethernet Switches
• Broadcaster sets the destination MAC address to
  all ones (48 ones)
• When switch sees this address, it broadcasts
  frame out all stations
• All stations read frames with this address

Ethernet Switch
Broadcast Frame
4
Ethernet Virtual LANs

• Broadcasting is a Problem in Large Switched
  Networks
• Server broadcasts go to all stations, creating a
  great deal of network traffic
• Create congestion

Broadcast Frame
5
Ethernet Virtual LANs

• In multicasting, messages are only intended to go
  to some stations
• For instance, from a server only to the client
  PCs it serves
• If Ethernet switches canimplement
  multicasting,traffic overload wouldbe avoided

Multicast Frame
6
Ethernet Virtual LANs

• Ethernet switches do implement multicasting
• A server and the clients it serves are treated as
  a single virtual LAN (VLAN)
• Can only communicate among themselves, as if they
  were on their own LAN

Marketing VLAN Server
Frame
Marketing VLAN Client
7
Ethernet Virtual LANs

• VLAN Benefits
• VLANs reduce traffic on the switched network
• Other benefits
• They provide weak security because clients cannot
  reach all servers (easily defeated but good first
  line of defense)
• VLANs give ease of management because if a user
  changes organizational membership, VLAN
  membership is easily changed centrally

New
8
Ethernet Virtual LANs

• VLAN Problems
• VLANs have not been standardized
• A network of switches from different vendors
  cannot implement VLANs
• Standardization is beginning
• Using tagging (Chapter 7)
• Tag Control Information field has a 12-bit VLAN
  ID (VID) number, allowing 212 VLANs to be
  identified

New
9
Ethernet Virtual LANs

• VLAN Problems
• Communication across VLANs, when necessary, can
  be difficult
• Often, routers must be used to connect members of
  different VLANs

Router
VLAN 2 (Subnet 2)
VLAN 1 (Subnet 1)
New
10
Ethernet Virtual LANs

• VLAN Problems
• For cross-VLAN communication, routers actually
  connect multiple switches

Ethernet Switch
New
11
Ethernet VLANs Perspective

• For Ethernet switches only
• Reduce congestion from Ethernet broadcast
  messages
• Good at separating stations usually require
  routers to connect stations on different VLANs
• Limited Vendor Interoperability