CISCO CCNA - LAYER 2 SWITCHING

Layer 2 Switching

• Switches use Application-Specific-Integrated Circuits (ASIC's) to build and maintain filter tables.
• Switching provides:
  • Hardware-based bridging
  • Wire speed
  • Low latency
  • Low cost
• Layer 2 switching breaks up collision domains making a flatter network than an ordinary 10BaseT network.
• Cisco 80/20 rule - Make sure users spend 80% of their time on the local segment.
• Broadcasts, multicasts and slow converging spanning tree limits the size of a switched network.
• Bridges are software based (slower) compared to switches which are hardware based.
• Bridges can only have one Spanning Tree instance per bridge.
• Bridges can only have up to 16 ports.
• Trashing the MAC table - Occurs when there is a loop in a switched network and the MAC filter table is being constantly updated.
• Ethernet is half duplex - Only one station can send or receive at any one time.
• Full duplex Ethernet allows transmission and reception at the same time. Two pairs of wires, full duplex NIC cards and a switched connection are required.
• Switches segment collision domains but forward broadcast packets to all ports.
• Ethernet 5-4-3 rule - Between any 2 nodes there can only be a maximum of 5 segments, 4 repeaters and only 3 of those segments can have hosts.

Switch Memory Buffering - 2 Types (Port Based and Shared Memory)

• Port Based can cause problems holding up transmission of all other packets if there is a delay on one port.
• Shared Memory Buffering is not linked to a specific port but dynamically allocated within a shared pool of memory.

Switching Methods

• Store and Forward - Entire frame is received before forwarding is commenced. Error detection is high, latency low.
• Cut through - Comes in 2 types (Fast Forward and Fragment Free).

1. Fast Forward - Forwards frame after receiving destination address, error prone.
2. Fragment Free - Filters out collision packets (If packet data > 64bytes ok)

• Ethernet frame maximum size = 1518bytes (All of this is received before forwarding by Store and Forward)
• Ethernet frame minimum size = 64 bytes (Only this much of packet read by Fragment Free)

Repeaters - 100BASE-T LANs