Network Fundamentals: Switching and Loop Prevention

Switched Networks

• A switched network is a single broadcast domain.
• Redundant links between switches can be useful, but they can cause problems like broadcast storms, endless looping, duplicate frames, and faulty CAM tables.
• The Spanning Tree Protocol (STP) is a solution to the problem of loops in a switched network.

MAC Learning

• A switch maintains an address table called the MAC table to switch frames between LAN ports efficiently.
• The MAC address of the sending network device is associated with the LAN port on which it was received.
• MAC address learning is enabled on all VLANs by default.
• The switch dynamically builds the address table by using the MAC source address of the frames received.
• When the switch receives a frame for a MAC destination address not listed in its address table, it floods the frame to all LAN ports of the same VLAN except the port that received the frame.
• Static MAC addresses can be entered into the table, which are retained across a reboot of the switch.

MAC Aging

• MAC aging time can be configured in interface configuration mode or VLAN configuration mode.
• The MAC aging time specifies the time before an entry ages out and is discarded from the MAC address table.
• The default MAC aging time is 300 seconds.

Understanding Switches

• Switches are multiport bridges that use hardware (ASICs) to build and maintain their tables, unlike bridges that use software.
• Switches provide a dedicated collision domain on each of their ports.
• Switches have lower latency, wire speed, and are low cost.
• However, switches do not break broadcast domains, making the network susceptible to broadcast storms and related problems.
• Routers are needed to break the broadcast domains.

Limitations of Switches

• Switches do not break broadcast domains.
• Redundancy in switched networks can cause loops.
• Dedicated protocols need to be run to ensure that the network remains loop-free.

Spanning Tree Protocol (STP)

• STP is used to prevent loops in a switched network.
• Non-designated ports are used to remove loops.
• Port states in STP include:
  • Blocked: Listen to BPDUs, but do not transmit or receive data.
  • Listening: Listen to frames, but do not learn or act on them.
  • Learning: Learn MAC addresses and populate the CAM table.
  • Forwarding: Forward traffic after learning MAC addresses and populating the CAM table.
  • Disabled: Receive BPDUs, but do not forward them to the switch processor.
• Port transition times are typically:
  • Initialization to blocking
  • Blocking to listening (20 secs)
  • Listening to learning (15 secs)
  • Learning to forwarding (15 secs)
  • Forwarding to disabled (if there is a failure)

This quiz covers the importance of preventing loops in switched networks and the issues associated with redundant links, including broadcast storms and duplicate frames.