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Answers to the Chapter 4 Q&A Section 719
6
Assume that a building has 100 devices attached to the same Ethernet. These users then
are migrated onto two separate shared Ethernet segments, each with 50 devices, with a
transparent bridge in between. List two benefits that would be derived for a typical user.
Fewer collisions and less waiting should occur because twice as much capacity exists.
Also, if the placement of devices causes a small percentage of traffic to be forwarded by
the bridge, fewer collisions and less waiting should occur.
7
What standards body owns the process of ensuring unique MAC addresses worldwide?
IEEE. The first half of the burned-in MAC address is a value assigned to the manufacturer
by the IEEE. As long as the manufacturer uses that prefix and doesn't duplicate values it
assigns in the last 3 bytes, global uniqueness is attained.
8
Assume that a building has 100 devices attached to the same Ethernet. These devices are
migrated to two different shared Ethernet segments, each with 50 devices. The two
segments are connected to a Cisco LAN switch to allow communication between the two
sets of users. List two benefits that would be derived for a typical user.
Two switch ports are used, which reduces the possibility of collisions. Also, each segment
has its own 10- or 100-Mbps capacity, allowing more throughput and reducing the
likelihood of collisions. Furthermore, although unlikely to be on the CCNA exam, some
Cisco switches can reduce the flow of multicasts using the Cisco Group Message Protocol
(CGMP).
9
Name two of the methods of internal switching on typical switches today. Which provides
less latency for an individual frame?
Store-and-forward, cut-through, and FragmentFree. Cut-through has less latency per
frame but does not check for bit errors in the frame, including errors caused by collisions.
Store-and-forward stores the entire received frame, verifies that the FCS is correct, and
then sends the frame. Cut-through sends the first bytes of the frame out before the last
bytes of the incoming frame have been received. Fragment free is similar to cut-through
in that the frame can be sent before the incoming frame is totally received; however,
fragment-free processing waits to receive the first 64 bytes, to ensure no collisions, before
beginning to forward the frame.
10
What is the distance limitation of 10BaseT? 100BaseTX?
10BaseT allows 100 meters between the device and the hub or switch, as does
100BaseTX. Table 4-5 on page 144 summarizes the lengths for all Ethernet LAN types.
11
Describe how a transparent bridge decides whether it should forward a frame, and tell
how it chooses the interface out which to forward the frame.
The bridge examines the destination MAC address of a frame and looks for the address in
its bridge (or address) table. If found, the matching entry tells the bridge which output
interface to use to forward the frame. If not found, the bridge forwards the frame out all
apA.fm Page 719 Monday, March 20, 2000 5:24 PM