Answers to Chapter 5 Do I Know This Already Quiz

1 What is EtherChannel? What types of switch links can it be used with?

EtherChannel is a method for aggregating multiple physical Ethernet ports into a single logical link. EtherChannel can be used with full-duplex Fast Ethernet or Gigabit Ethernet links.

2 How is traffic distributed over an EtherChannel?

Traffic is distributed according to addresses contained in frames passing through the switch—not according to port loads or equal distribution across the individual ports in a bundle. Switches use an XOR computation of source, destination, or both addresses of either MAC or IP, depending on the switch capabilities.

3 What is PAgP used for?

PAgP is a protocol that is used to dynamically and to automatically configure an EtherChannel over multiple physical ports.

4 What is a bridging loop? Why is it bad?

A bridging loop is a path through a bridged or switched network that provides connectivity in a loop. Broadcast, multicast, and unknown unicast frames introduced into the loop are propagated by each switch, causing the frames to circulate around the loop. Network bandwidth and CPU resources can be completely absorbed by the increasing amount of broadcast traffic. Bridging loops can be ended by breaking the loop connectivity.

5 Name two types of Spanning-Tree Protocol messages used to communicate between bridges.

Configuration BPDUs and Topology Change Notification BPDUs. Configuration BPDUs are used to inform bridges of global STP parameters and are used to form the Spanning Tree topology. Topology Change Notification BPDUs are used to inform bridges that a link state has changed potentially impacting the Spanning Tree topology.

6 What criteria are used to select the following:

a. Root Bridge b. Root Port c. Designated Port d. Redundant (or Secondary) Root Bridges a. Lowest Bridge ID (Bridge priority, MAC address)

b. Lowest Root Path Cost c. Lowest Root Path Cost on a shared segment d. Next-to-lowest Bridge ID

If a tie occurs, then this sequence of parameters are used to decide:

1. Lowest Bridge ID

2. Lowest Root Path Cost

3. Lowest Sender Bridge ID

4. Lowest Port ID

7 What conditions cause a STP topology change? What effect does this have on STP and the network?

A topology change occurs when a port moves to the Forwarding state or from Forwarding or Learning to the Blocking state. During a topology change, addresses are aged out in Forward Delay seconds while active stations are not aged out of the bridging table. The

STP is not recomputed; TCN BPDUs are sent throughout the network notifying other switches of the topology change. Only the port where the topology change is occurring is affected, by moving through the STP states.

8 What is the single most important design decision to be made in a network running STP? Root Bridge placement.

9 Where should the Root Bridge be located in a switched network?

It should be located as close to the center of the network as possible. For example, in a hierarchical design, the Root Bridge should be located in the Distribution layer.

10 What happens to a port that is neither a Root Port nor a Designated Port? That port is placed in the Blocking state so that no bridging loops form from it.

11 How is the Root Path Cost calculated for a switch port?

The Root Path cost is a cumulative value incremented as Configuration BPDUs are passed from switch to switch. For each instance of STP (one per VLAN), a switch adds the Port Cost of its local port to the current Root Path cost value as a BPDU is received.

12 What is the maximum number of Root Ports that a Catalyst switch can have?

Only one per instance of STP (one per VLAN). Each instance of STP works to find a single path from each switch back to the Root Bridge.

13 What mechanism is used to set the STP timer values for all switches in a network?

The timers are set on the Root Bridge, and the values are propagated to all other switches by including them in Configuration BPDUs.

14 What parameters can be tuned to influence the selection of a port as a Root or Designated Port?

Port Cost and Port Priority.

15 What technology can be useful to decrease the amount of time STP keeps an end user's workstation in the Blocking state when it powers up?

PortFast, which moves the port immediately into the Forwarding state.

16 Where should the UplinkFast feature be used in a switched network?

Only on switches that are leaf-nodes in the Spanning-Tree topology, such as the Access Layer.

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