Network Fundamentals: Switching and Loop Prevention

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Questions and Answers

What is the primary issue that arises when a switched network has redundant links between switches?

Loops in the network, causing problems such as broadcast storms, endless looping, duplicate frames, and faulty CAM tables.

What is the purpose of the MAC address table in a switch?

To switch frames between LAN ports efficiently by associating MAC addresses with LAN ports.

What happens when a switch receives a frame for a MAC destination address not listed in its address table?

The switch floods the frame to all LAN ports of the same VLAN except the port that received the frame.

What is the purpose of MAC aging?

To specify the time before an entry ages out and is discarded from the MAC address table. Signup and view all the answers

What is the default MAC aging time in seconds?

300 Signup and view all the answers

What is the solution to the problem of loops in a switched network?

The Spanning Tree Protocol (STP). Signup and view all the answers

What is a single broadcast domain in the context of a switched network?

A switched network itself. Signup and view all the answers

What type of MAC address can be entered into the MAC address table manually?

A static MAC address. Signup and view all the answers

What is the primary difference between bridges and switches in terms of building and maintaining their switching and filtering tables?

Bridges use software, while switches use hardware (ASICs). Signup and view all the answers

What is one of the main advantages of switches over bridges in terms of processing time?

Lower latency and near wire speed functionality. Signup and view all the answers

What is a limitation of switches in terms of broadcast domains?

They do not break broadcast domains, making the network susceptible to broadcast storms. Signup and view all the answers

What is a potential problem that arises when introducing redundancy in a switched network?

The possibility of loops becomes very high, requiring dedicated protocols to ensure a loop-free network. Signup and view all the answers

What is a key consideration when designing a good switched or bridged network?

Breaking collision domains as much as possible. Signup and view all the answers

Why is it important to consider the local segment when designing a network?

Users should spend 80% of their time on the local segment. Signup and view all the answers

What type of protocols are necessary to ensure a loop-free network in a switched network with redundancy?

Dedicated protocols, such as Spanning Tree Protocol (STP). Signup and view all the answers

What is a benefit of using switches in a network in terms of cost?

They are low cost, making it affordable to connect each host. Signup and view all the answers

What is the purpose of a blocking port in the Spanning Tree Protocol?

To remove loops Signup and view all the answers

What happens to MAC addresses in the listening state of an STP port?

They are not placed into the CAM table Signup and view all the answers

What is the purpose of the learning state in STP?

To learn MAC addresses and populate the CAM table Signup and view all the answers

What happens to incoming frames in the disabled state of an STP port?

They are discarded Signup and view all the answers

What is the initial state of an STP port?

Blocking Signup and view all the answers

What is the function of a non-designated port in STP?

It has a higher port cost than the designated port Signup and view all the answers

How long does it take for an STP port to transition from blocking to listening?

20 seconds Signup and view all the answers

What is carried by a BPDU in STP?

Various pieces of information used by STP to determine port states and topology Signup and view all the answers

Study Notes

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)

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Description

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