Answers to Chapter 14 Review Questions

1:

What are some of the unique requirements of an IDF switch?

A:

Cost and port density are the two most important considerations. Other considerations include redundancy options and ease of management.

2:

What are some of the unique requirements of an MDF switch?

A:

The key requirements are high availability and throughput, especially Layer 3 throughput. Routing capabilities (such as supporting a wide variety of robust routing protocols) is also important.

3:

Describe the access/distribution/core terminology.

A:

Access layer devices are used for end-station connections (through horizontal cabling). They also connect to distribution devices through vertical cabling. In a campus network, the term access device is essentially a synonym for IDF device.

Distribution devices are used to provide a central point of connectivity for an entire building (or portion of a large building). They are equivalent to MDF devices.

The core layer is used to link distribution devices.

4:

Why is routing an important part of any large network design?

A:

Routing has many advantages in a properly designed campus network:

• Scalability

• Broadcast and multicast control

• Optimal and flexible path selection

• Load balancing

• Fast convergence

• Hierarchy and summarized addressing

• Policy and access lists

• Value-added features such as DHCP relay

5:

What networks work best with the router and hub model?

A:

Networks work best with the router and hub model if they have limited bandwidth requirements and mostly use departmental servers that keep the traffic on the local segment.

6:

What are the benefits of the campus-wide VLANs model?

A:

The main advantage of the campus-wide VLAN approach to network design is that it allows a direct, Layer 2 path from end users to servers. This is an attempt to avoid the slowness of software-based routers. This design can also be useful for networks that design lots of flexibility in subnet and VLAN assignments. For example, members of the Finance group can all be assigned to the same VLAN even if they are located in different buildings or locations within the campus. This can then simplify VLAN and security assignments.

7:

What are the downsides of the campus-wide VLANs model?

A:

• Management and troubleshooting can be very difficult.

• Spanning Tree can be very difficult to optimize, manage, and control.

• Trunks allow a problem in one VLAN to starve out all VLANs.

• To achieve stability, it often requires all redundancy to be eliminated.

• It is highly dependent upon the 80/20 rule, something that no longer holds true in most networks.

• It is based on the assumption that routers are slow, something that is no longer true.

8:

Describe the concept of a distribution block.

A:

A distribution block is a self-contained unit of devices and associated VLANs, subnets, and connectivity. The MDF and IDF switches in distribution blocks form triangles of connectivity. Because routing is configured in MDF devices, a Layer 3 barrier is created between each distribution block and the campus core, increasing the network's scalability.

9:

Why is it important to have modularity in a network?

A:

There are many advantages to building modularity into the network:

• Scalability is improved because new modules can be easily added.

• The network becomes easier to understand, troubleshoot, and maintain.

• It is easier to use cookie cutter configurations.

• It is easier to handle migrations.

• It is easier to provide redundancy and load balancing.

• It is easier to provide fast failover performance.

• It is much easier to substitute different technologies at various places within the network. For example, the core can easily use Fast Ethernet, Gigabit Ethernet, ATM, Tag Switching, or Packet Over SONET.

10:

What are the concerns that arise when using a Layer 2 core versus a Layer 3 core?

A:

Layer 2 cores are not as scalable as Layer 3 cores. Tuning Spanning Tree and load balancing in a Layer 2 core can be tricky. In many cases, physical loops should be removed to improve failover performance.

11:

How should a server farm be implemented in the multilayer model?

A:

As another distribution block off of the core. Workgroup servers can attach to MDF or IDF switches (depending on what users they serve).