background image
720 Appendix A: Answers to the "Do I Know This Already?" Quizzes and Q&A Sections
other interfaces (except for interfaces blocked by Spanning Tree and the interface in which
the frame was received). The bridge table is built by examining incoming frames' source
MAC addresses.
12
How fast is Fast Ethernet?
100 million bits per second (100 Mbps).
13
Describe the benefit of the Spanning-Tree Protocol as used by transparent bridges and
switches.
Physically redundant paths in the network are allowed to exist and be used when other
paths fail. Also, loops in the bridged network are avoided. Loops are particularly bad
because bridging uses LAN headers, which do not provide a mechanism to mark a frame
so that its lifetime can be limited; in other words, the frame can loop forever.
14
If a switch hears three different configuration BPDUs from three different neighbors on
three different interfaces, and if all three specify that Bridge 1 is the root, how does the
switch choose which interface is its root port?
The root port is the port in which the CBPDU with the lowest-cost value is received. The
root port is placed in forwarding state on each bridge and switch.
15
How does a transparent bridge build its address table?
The bridge listens for incoming frames and examines the source MAC address. If not in
the table, the source address is added, along with the port (interface) by which the frame
entered the bridge. The bridge also marks an entry for freshness so that entries can be
removed after a period of disuse. This reduces table size and allows for easier table
changes in case a Spanning Tree change forces more significant changes in the bridge
(address) table.
16
How many bytes long is a MAC address?
6 bytes long, or 48 bits.
17
Assume that a building has 100 devices attached to the same Ethernet. These users then
are migrated onto two separate Ethernet segments, each with 50 devices and separated by
a router. List two benefits that would be derived for a typical user.
Collisions are reduced by creating two collision domains. Broadcasts also are reduced
because the router does not forward broadcasts. Routers provide greater control and
administration as well.
18
Does a bridge/switch examine just the incoming frame's source MAC, destination MAC,
or both? Why does it examine the one(s) it examines?
The bridge/switch examines both MAC addresses. The source is examined so that entries
can be added to the bridge/address table. The destination address is examined to determine
the interface out which to forward the frame. Table lookup is required for both addresses
apA.fm Page 720 Monday, March 20, 2000 5:24 PM