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722 Appendix A: Answers to the "Do I Know This Already?" Quizzes and Q&A Sections
25
Describe the benefits of creating three VLANs of 25 ports each, versus a single VLAN of
75 ports, in each case using a single switch. Assume that all ports are switched ports (each
port is a different collision domain).
Three different broadcast domains are created with three VLANs, so the devices' CPU
utilization should decrease due to decreased broadcast traffic. Traffic between devices in
different VLANs will pass through some routing function, which can add some latency
for those packets. Better management and control are gained by including a router in the
path for those packets.
26
If two Cisco LAN switches are connected using Fast Ethernet, what VLAN trunking
protocols could be used? If only one VLAN spanned both switches, is a VLAN trunking
protocol needed?
ISL and 802.1Q are the trunking protocols used by Cisco over Fast Ethernet. If only one
VLAN spans the two switches, a trunking protocol is not needed. Trunking or tagging
protocols are used to tag a frame as being in a particular VLAN; if only one VLAN is used,
tagging is unnecessary.
27
Explain the function of the loopback and collision detection features of an Ethernet NIC
in relation to half-duplex and full-duplex operations.
The loopback feature copies the transmitted frame back onto the receive pin on the NIC
interface. The collision detection logic compares the received frame to the transmitted
frame during transmission; if the signals do not match, then a collision is occurring. This
logic implies that half duplex is being used because if collisions can occur, only one
transmitter at a time is allowed. With full duplex, collisions cannot occur, so the loopback
and collision detection features are not needed, and concurrent transmission and reception
is allowed.
28
Name the three interface states that the Spanning-Tree Protocol uses, other than
forwarding. Which of these states is transitory?
Blocking, listening, and learning. Blocking is the only stable state; the other two are
transitory between blocking and forwarding. Table 4-13 on page 170 summarizes the
states and their features.
29
What are the two reasons that a nonroot bridge/switch places a port in forwarding state?
If the port is the designated bridge on its LAN segment, the port is placed in forwarding
state. Also, if the port is the root port, it is placed in forwarding state. Otherwise, the port
is placed in blocking state.
30
Can the root bridge/switch ports be placed in blocking state?
The root bridge's ports are always in a forwarding state because they always have cost 0
to the root, which ensures that they are always the designated bridges on their respective
LAN segments.
apA.fm Page 722 Monday, March 20, 2000 5:24 PM