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

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

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 Broadcast Frame Ethernet Switch

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

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 can implement multicasting, traffic overload would be avoided Multicast Frame

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 Frame Marketing VLAN Server Marketing VLAN Client

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

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 2 12 VLANs to be identified New

Ethernet Virtual LANs VLAN Problems –Communication across VLANs, when necessary, can be difficult –Often, routers must be used to connect members of different VLANs New VLAN 1 (Subnet 1) VLAN 2 (Subnet 2) Router

Ethernet Virtual LANs VLAN Problems –For cross-VLAN communication, routers actually connect multiple switches New Ethernet Switch

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