LAN Switches

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Switching also know as frame forwarding methods

  • Store-and-forward switching - switch receives the entire frame, and computes CRC. IF CRC is valid, the switch looks up the destination address which determines the outgoing interface. The frame is then forwarded
  • Cut-through switching - switch forwards the frame before it is entirely received. At the minimum the destination address of the frame must be read before the frame can be forwarded
    • Fast-forward switching - immediately forwards a packet after reading the destination address. Latency is measured from the first bit received to the first bit transmitted
    • Fragment-free switching - the switch stores the first 64 bytes of the frame before forwarding, the reason to read the first 64 bytes is most network errors and collisions occur during the first 64 bytes

Some switches are configured to perform cut-through switching on a per-port basis until a user-defined error threshold is reached and then they automatically change to store-and-forward. When the error rate falls below the threshold, the port automatically changes back to cut-through switching.

Methods of memory buffering

  • Port-based Memory Buffering - frames are stored in queues that are linked to specific incoming and outgoing ports. A frame is transmitted to the outgoing port only when all the frames ahead of it in the queue have been successfully transmitted. It is possible for a single frame to delay the transmission of all the frames in memory because of a busy destination port. This delay occurs even if the other frames could be transmitted to open destination ports.
  • Shared Memory Buffering - deposits all frames into a common memory buffer that all the ports on the switch share. The amount of buffer memory required by a port is dynamically allocated. The frames in the buffer are linked dynamically to the destination port. This allows the packet to be received on one port and then transmitted on another port, without moving it to a different queue.

Small Form-Factor Pluggable (SFP) port

Fast Ethernet SFP Modules
  • 100BASE-FX (multimode fiber-optic (MMF)) for 2 kilometers (km)
  • 100BASE-LX10 (single-mode fiber-optic (SMF)) for 2km
  • 100BASE-BX10 (SMF) for 10 km
  • 100BASE-EX (SMF) for 40 km
  • 100BASE-ZX (SMF) for 80 km
Gigabit Ethernet SFP Modules
  • 1000BASE-SX 50/62.5 μm (MMF) up to 550/220 m
  • 1000BASE-LX/LH (SMF/MMF) up to 10/0.550 k
  • 1000BASE-ZX (SMF) up to 70 km
  • 1000BASE-BX10-D&1000BASE-BX10-U (SMF) up to 10 km
  • 1000BASE-T (copper wire transceiver)
10 Gigabit Ethernet SFP Modules
  • 10G-SR (MMF) up 400 m
  • 10G-SR-X (MMF) up to 400 m (supporting extended temperature range)
  • 10G-LRM (MMF) up to 220 m
  • FET-10G (MMF) up to 100 m (for Nexus fabric uplinks)
  • 10G-LR (SMF) up to 10 km
  • 10G-LR-X (SMF) up to 10 km (supporting extended temperature range)
  • 10G-ER (SMF) up to 40 km
  • 10G-ZR (SMF) up to 80 km
  • Twinax (copper wire transceiver) up to 10 m
  • Active Optical up to 10 m (for intra/inter-rack connections)

Cisco Express Forwarding (CEF)

Two main components of CEF operation are the:

  • Forwarding Information Base (FIB) - conceptually similar to a routing table
  • Adjacency tables - maintain Layer 2 next-hop addresses for all FIB entries