- Which of the following statements describes part of the process of how a switch
decides to forward a frame destined for a known unicast MAC address?
a. It compares the unicast destination address to the bridging, or MAC address,
table.
b. It compares the unicast source address to the bridging, or MAC address, table.
c. It forwards the frame out all interfaces in the same VLAN except for the incoming
interface.
d. It compares the destination IP address to the destination MAC address.
e. It compares the frame’s incoming interface to the source MAC entry in the MAC
address table.
a. It compares the unicast destination address to the bridging, or MAC address,
table.
- Which of the following statements describes part of the process of how a LAN
switch decides to forward a frame destined for a broadcast MAC address?
a. It compares the unicast destination address to the bridging, or MAC address,
table.
b. It compares the unicast source address to the bridging, or MAC address, table.
c. It forwards the frame out all interfaces in the same VLAN except for the incoming
interface.
d. It compares the destination IP address to the destination MAC address.
e. It compares the frame’s incoming interface to the source MAC entry in the MAC
address table.
c. It forwards the frame out all interfaces in the same VLAN except for the incoming
interface.
- Which of the following statements best describes what a switch does with a frame
destined for an unknown unicast address?
a. It forwards out all interfaces in the same VLAN except for the incoming interface.
b. It forwards the frame out the one interface identified by the matching entry in
the MAC address table.
c. It compares the destination IP address to the destination MAC address.
d. It compares the frame’s incoming interface to the source MAC entry in the MAC
address table.
a. It forwards out all interfaces in the same VLAN except for the incoming interface.
- Which of the following comparisons does a switch make when deciding whether a
new MAC address should be added to its MAC address table?
a. It compares the unicast destination address to the bridging, or MAC address,
table.
b. It compares the unicast source address to the bridging, or MAC address, table.
c. It compares the VLAN ID to the bridging, or MAC address, table.
d. It compares the destination IP address’s ARP cache entry to the bridging, or
MAC address, table.
b. It compares the unicast source address to the bridging, or MAC address, table.
- A Cisco Catalyst switch has 24 10/100 ports, numbered 0/1 through 0/24. Ten PCs
connect to the ten lowest numbered port, with those PCs working and sending data
over the network. The other ports are not connected to any device. Which of the following
answers lists facts displayed by the show interfaces status command?
a. Port Ethernet 0/1 is in a connected state.
b. Port Fast Ethernet 0/11 is in a connected state.
c. Port Fast Ethernet 0/5 is in a connected state.
d. Port Ethernet 0/15 is in a notconnected state
c. Port Fast Ethernet 0/5 is in a connected state.
- Consider the following output from a Cisco Catalyst switch:
SW1# show mac address-table dynamic
Mac Address Table
——————————————-
Vlan Mac Address Type Ports
—- ———– ——– —–
1 02AA.AAAA.AAAA DYNAMIC Gi0/1
1 02BB.BBBB.BBBB DYNAMIC Gi0/2
1 02CC.CCCC.CCCC DYNAMIC Gi0/3
Total Mac Addresses for this criterion: 3
Which of the following answers are true about this switch?
a. The output proves that port Gi0/2 connects directly to a device that uses address
02BB.BBBB.BBBB.
b. The switch has learned three MAC addresses since the switch powered on.
c. The three listed MAC addresses were learned based on the destination MAC
address of frames forwarded by the switch.
d. 02CC.CCCC.CCCC was learned from the source MAC address of a frame that
entered port Gi0/3.
d. 02CC.CCCC.CCCC was learned from the source MAC address of a frame that
entered port Gi0/3.
LAN switches receive Ethernet frames and then make a switching decision: either forward
the frame out some other ports or ignore the frame. To accomplish this primary mission,
switches perform three actions:
- Deciding when to forward a frame or when to filter (not forward) a frame, based on
the destination MAC address - Preparing to forward frames by learning MAC addresses by examining the source
MAC address of each frame received by the switch - Preparing to forward only one copy of the frame to the destination by creating
a (Layer 2) loop-free environment with other switches by using Spanning Tree
Protocol (STP)
Learning MAC Addresses
Switches build the address table by listening to incoming frames and examining the source
MAC address in the frame. If a frame enters the switch and the source MAC address is not
in the MAC address table, the switch creates an entry in the table. That table entry lists the
interface from which the frame arrived. Switch learning logic is that simple.
LAN Switching Summary
Switches use Layer 2 logic, examining the Ethernet data-link header to choose how to process
frames. In particular, switches make decisions to forward and filter frames, learn MAC
addresses, and use STP to avoid loops, as follows:
Step 1. Switches forward frames based on the destination MAC address:
A. If the destination MAC address is a broadcast, multicast, or unknown destination
unicast (a unicast not listed in the MAC table), the switch floods the
frame.
B. If the destination MAC address is a known unicast address (a unicast
address found in the MAC table):
i. If the outgoing interface listed in the MAC address table is different
from the interface in which the frame was received, the switch forwards
the frame out the outgoing interface.
ii. If the outgoing interface is the same as the interface in which the
frame was received, the switch filters the frame, meaning that the
switch simply ignores the frame and does not forward it.
Step 2. Switches use the following logic to learn MAC address table entries:
A. For each received frame, examine the source MAC address and note the
interface from which the frame was received.
B. If it is not already in the table, add the MAC address and interface it was
learned on.
Step 3. Switches use STP to prevent loops by causing some interfaces to block, meaning
that they do not send or receive frames.
Verifying and Analyzing Ethernet Switching:
Cisco Catalyst switches come ready to get busy switching frames because of settings like
these:
■ The interfaces are enabled by default, ready to start working once a cable is connected.
■ All interfaces are assigned to VLAN 1.
■ 10/100 and 10/100/1000 interfaces use autonegotiation by default.
■ The MAC learning, forwarding, flooding logic all works by default.
■ STP is enabled by default.
To see a switches MAC address table
use the show mac address-table command. With
no additional parameters, this command lists all known MAC addresses in the MAC table,
including some overhead static MAC addresses that you can ignore. To see all the dynamically
learned MAC addresses only, instead use the show mac address-table dynamic command.
Cisco Catalyst switches name
their ports based on the
fastest specification supported
The show interfaces command has a large number of options. One particular option, the
_________option, lists statistics about incoming and outgoing frames on the interfaces
counters
Shows all MAC table entries of all types
show mac address-table
Shows all dynamically learned MAC table entries
show mac address-table
dynamic
Shows all dynamically learned MAC table entries in that
VLAN
show mac address-table
dynamic vlan vlan-id
Shows the dynamically learned MAC table entries with that
MAC address
show mac address-table
dynamic address MAC-address
Shows all dynamically learned MAC table entries
associated with that interface
show mac address-table
dynamic interface interface-id
Shows the number of entries in the MAC table, and the
total number of remaining empty slots in the MAC table
show mac address-table count
Shows the global and per-VLAN aging timeout for inactive
MAC table entries
show mac address-table
aging-time
Empties the MAC table of all dynamic entries
clear mac address-table
dynamic(from enable mode only)
Lists one line per interface on the switch, with basic status
and operating information for each
show interfaces status
To accomplish this primary mission,
switches perform three actions:
- Deciding when to forward a frame or when to filter (not forward) a frame, based on
the destination MAC address - Preparing to forward frames by learning MAC addresses by examining the source
MAC address of each frame received by the switch - Preparing to forward only one copy of the frame to the destination by creating
a (Layer 2) loop-free environment with other switches by using Spanning Tree
Protocol (STP)
switches make decisions to forward and filter frames, learn MAC
addresses, and use STP to avoid loops, as follows
Step 1. Switches forward frames based on the destination MAC address:
A. If the destination MAC address is a broadcast, multicast, or unknown destination
unicast (a unicast not listed in the MAC table), the switch floods the
frame.
B. If the destination MAC address is a known unicast address (a unicast
address found in the MAC table):
i. If the outgoing interface listed in the MAC address table is different
from the interface in which the frame was received, the switch forwards
the frame out the outgoing interface.
ii. If the outgoing interface is the same as the interface in which the
frame was received, the switch filters the frame, meaning that the
switch simply ignores the frame and does not forward it.
Step 2. Switches use the following logic to learn MAC address table entries:
A. For each received frame, examine the source MAC address and note the
interface from which the frame was received.
B. If it is not already in the table, add the MAC address and interface it was
learned on.
Step 3. Switches use STP to prevent loops by causing some interfaces to block, meaning
that they do not send or receive frames.
