Ethernet Virtual LANs Hubs versus Switches Hubs broadcast
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 Marketing VLAN Server Frame 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
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
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)
Ethernet Virtual LANs • VLAN Problems – For cross-VLAN communication, routers actually connect multiple switches 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
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