Computer Networking Lecture Notes

2014 October 27 • Local Area Networks

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Outline

• Shared wire media.
• Ethernet switches.
• Combined LANs.
  • At layer 2 or 3.

Data-Link Networks

• The data-link layer groups hosts into two types of networks:
  • Point-to-point networks.

    

  • Shared-channel networks.

    

Local-Area Networks

• A shared-channel network is commonly called Local-Area Network (LAN).
• “Local area”:
  • Limited geographical coverage (up to a square mile).
  • Limited host population (up to a few hundred hosts).

Shared Channel

• Most modern high-capacity LANs don’t use shared-channel media.
  • Too expensive: sharing coordination overhead.
  • Too inefficient: conflict resolution overhead.

Switched Ethernet

• The most common wired LAN is switched Ethernet.

  A star architecture. Hosts around the outside. A LAN switch in the middle. Point-to-point links connecting them.

LAN Switches

• A LAN (or layer-2) switch forwards PDUs among connected hosts.
• There are no collisions.
  • Incoming PDUs are buffered at the switch.
  • Links are full duplex.
• There is no direct connection (no circuit) between hosts.

LAN Switch Forwarding

• A LAN switch forwarding table is a list of (destination, port) pairs.
  • Entries are aged.
• A PDU arrives at a switch port.
  • If the destination MAC address is not in the forwarding table, flood the PDU.
  • Otherwise send the PDU out the designated port.
• The star architecture means flooding is trivial.

Forwarding Example

PDU Snooping

• Switch forwarding tables are self-discovered.
  • A PDU from MAC_s shows up on port i.
  • Put (MAC_s, i) in the forwarding table.
    • Or reset the aging timer if it’s already there.
• No set-up, no routing algorithms.
  • But non-sending hosts don’t get discovered.

Connecting LANs

• A LAN per department is a convenient network layout.
  • Traffic and fault isolation.
• On the other hand, communication between departments is also nice.
• LANs can be connected at the data-link layer, or at the network layer.

Network-Connected LANs

Comments

• Flexible and scalable interconnect.
  • Can grow to connect 100s of LANs.
• Allows communication, but preserves LAN structure.
  • Each component LAN remains a separate entity.
• But network management is difficult and labor intensive.

Data-Link-Connected LANs

• Forget the network layer and connect at the data-link layer.

  

Comments

• Much simpler than network connections, but not as scalable.
  • Connect 10s of LANs.
  • Large forwarding tables.
• One big LAN doesn’t preserve component LAN structure.
• Large drop in aggregate resource availability.
  • Layer-2 switches help with this.

The Deadly Cycle

• Cycles provide redundant connectivity for reliability.

  Cycles can occur accidentally, particularly in large interconnected LANs.

Flooding and Cycles

• Switches flood when destinations aren’t known.
  • Floods and cycles are a bad combination.
• Routers deal with this by not flooding.
• Switches avoid flooding problems using a different approach.

Spanning Trees

• A tree is an acyclic graph.
• Being acyclic, trees can flood PDUs without problems.
  • No excessive PDU copying.
  • The flood naturally dies down.
• A spanning tree is a tree containing every switch in the LAN.
  • It spans the hosts in the LAN.
• The switches build a spanning tree.

Growing a Spanning Tree

• To build a spanning tree:
  1. Use flooding to select the root switch, the switch with the lowest MAC address.
  2. The root switch floods a path PDU.
  3. Switch S receiving the first path PDU marks the incoming port as the root port and floods the path PDU.
  4. S discards further path PDUs.
• The set of root ports forms a spanning tree.

Summary

• Shared wire media isn’t much shared anymore.
• Ethernet switches are layer 2 store and forward devices.
• LANs can be combined into larger structures.
  • At layer 3 with routers.
  • At layer 2 with switches.

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This page last modified on 2014 October 26.