31 Days Before Your CCNA Exam

Day 27 45Figure 27-1Switch Forwarding Based on MAC Address1 6ACMAC AddressOAPORT1OB3BDOCOD69FramePreambleDestination AddressOCSource AddressOAType Data Pad CRCFinally, LAN switches must have a method for creating a loop-free path for frames to take withinthe LAN. STP provides loop prevention in Ethernet networks where redundant physical links exist.Day 24, “Switching Technologies and VLAN Concepts,” reviews STP in more detail.Collision and Broadcast DomainsA collision domain is the set of LAN interfaces whose frames could collide with each other. Allshared media environments, such as those created by using hubs, are collision domains. When onehost is attached to a switch port, the switch creates a dedicated connection thereby eliminating thepotential for a collision. Switches reduce collisions and improve bandwidth use on network segmentsbecause they provide dedicated bandwidth to each network segment.However, out of the box, a switch cannot provide relief from broadcast traffic. A collection of connectedswitches forms one large broadcast domain. If a frame with the destination addressFFFF.FFFF.FFFF crosses a switch port, that switch must then flood the frame out all other activeports. Each attached device must then process the broadcast frame at least up to the network layer.Routers and VLANs are used to segment broadcast domains. Day 24 reviews the use of VLANs tosegment broadcast domains.Frame ForwardingSwitches operate in several ways to forward frames. They can differ in forwarding methods, portspeeds, memory buffering, and the OSI layers used to make the forwarding decision. The sectionsthat follow discuss these concepts in greater detail.Switch Forwarding MethodsIn the past, switches used one of the following forwarding methods for switching data betweennetwork ports: