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2. Chapter 7-Ethernet LAN Switching Concepts • LAN Switching Concepts • LAN Design Considerations

3. LAN Switching Concepts • Hubs • Hubs led to too much congestion • Only one device could send at a time • Shared bandwidth • Bridges • Usually two ports • Segment network into 2 collision domains • Reduce collisions and improve network performance • More effective bandwidth

4. Switching Logic • Based on source and dest MAC address • Unicast, Broadcast or Multicast 1. Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address 2. Learning MAC addresses by examining the source MAC address of each frame received by the bridge 3. Creating a (Layer 2) loop-free environment with other bridges by using Spanning Tree Protocol (STP)

5. How Switches Learn Addresses • Listen to incoming Frames • Examine source MAC address • If not in table-add it and associate with interface

6. Flooding Frames • If no MAC addresses match the destination-send to everyone • Unknown Unicast frames • Flooding • Interactivity Timer • Track how long since MAC address has been used • Discard oldest when full

7. Spanning Tree Protocol • Prevent Loops • Networks often built with redundant links • Good design • How to shut down the redundant links to prevent broadcast loops or flood loops • Ports are Forwarding or Blocking

8. Internal Processing • How do we process the frames • store-and-forward processing. • switch must receive the entire frame before forwarding the first bit of the frame. • cut-through • switch starts sending the frame out the output port as soon as possible. Although this might reduce latency, it also propagates errors. Because the frame check sequence (FCS) is in the Ethernet trailer, the switch cannot determine if the frame had any errors before starting to forward the frame • Fragment-free processing • works similarly to cut-through, but it tries to reduce the number of errored frames that it forwards. One interesting fact about Ethernet carrier sense multiple access with collision detection (CSMA/CD) logic is that collisions should be detected within the first 64 bytes of a frame. Fragment-free processing works like cut-through logic, but it waits to receive the first 64 bytes before forwarding a frame.

9. Switching Summary • Switch ports connected to a single device microsegment the LAN, providing dedicated bandwidth to that single device. • Switches allow multiple simultaneous conversations between devices on different ports. • Switch ports connected to a single device support full duplex, in effect doubling the amount of bandwidth available to the device. • Switches support rate adaptation, which means that devices that use different Ethernet speeds can communicate through the switch (hubs cannot).

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11. Design Considerations • Collision Domains and Broadcast Domains • A collision domain is a set of network interface cards (NIC) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain. • A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain.

12. VLANS • A LAN is all devices in the same Broadcast Domain • VLANS let you assign switch ports to different Broadcast Domain • Acts like separate switches • Need a router to connect broadcast domains

13. VLAN Benefits • To create more flexible designs that group users by department, or by groups that work together, instead of by physical location • To segment devices into smaller LANs (broadcast domains) to reduce overhead caused to each host in the VLAN • To reduce the workload for STP by limiting a VLAN to a single access switch • To enforce better security by keeping hosts that work with sensitive data on a separate VLAN • To separate traffic sent by an IP phone from traffic sent by PCs connected to the phones

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15. LAN Terminology • Access, Distribution and Core • Access: Provides a connection point (access) for end-user devices. Does not forward frames between two other access switches under normal circumstances. • Distribution: Provides an aggregation point for access switches, forwarding framesbetween switches, but not connecting directly to end-user devices. • Core: Aggregates distribution switches in very large campus LANs, providing very high forwarding rates.

17. Ethernet Standards and Cable Lengths

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