5.2.17 Lab – Viewing the Switch MAC
Address Table
Topology
Addressing Table
|
Device
|
Interface
|
IP Address
|
Subnet Mask
|
Default Gateway
|
|
S1
|
VLAN 1
|
192.168.1.11
|
255.255.255.0
|
N/A
|
|
S2
|
VLAN 1
|
192.168.1.12
|
255.255.255.0
|
N/A
|
|
PC-A
|
NIC
|
192.168.1.3
|
255.255.255.0
|
N/A
|
|
PC-B
|
NIC
|
192.168.1.2
|
255.255.255.0
|
N/A
|
Objectives
Part 1: Build and Configure the Network
Part 2: Examine the Switch MAC
Address Table
Background / Scenario
The
purpose of a Layer 2 LAN switch is to deliver Ethernet frames to host devices
on the local network. The switch records host MAC addresses that are visible on
the network, and maps those MAC addresses to its own Ethernet switch ports.
This process is called building the MAC address table. When a switch receives a
frame from a PC, it examines the frame’s source and destination MAC addresses.
The source MAC address is recorded and mapped to the switch port from which it
arrived. Then the destination MAC address is looked up in the MAC address table.
If the destination MAC address is a known address, then the frame is forwarded
out of the corresponding switch port associated with that MAC address. If the
MAC address is unknown, then the frame is broadcasted out of all switch ports,
except the one from which it came. It is important to observe and understand
the function of a switch and how it delivers data on the network. The way a
switch operates has implications for network administrators whose job it is to ensure
secure and consistent network communication.
Switches
are used to interconnect and deliver information to computers on local area
networks. Switches deliver Ethernet frames to host devices identified by
network interface card MAC addresses.
In
Part 1, you will build a multi-switch topology with a trunk linking the two
switches. In Part 2, you will ping various devices and observe how the two
switches build their MAC address tables.
Note: The switches used are Cisco Catalyst
2960s with Cisco IOS Release 15.0(2) (lanbasek9 image). Other switches and Cisco
IOS versions can be used. Depending on the model and Cisco IOS version, the
commands available and output produced might vary from what is shown in the
labs.
Note: Make sure that the switches have been erased and have no startup
configurations. If you are unsure contact your instructor.
Required Resources
·
2 Switches (Cisco 2960 with
Cisco IOS Release 15.0(2) lanbasek9 image or comparable)
·
2 PCs (Windows 7 or 8 with
terminal emulation program, such as Tera Term)
·
Console cables to configure the
Cisco IOS devices via the console ports
·
Ethernet cables as shown in the
topology
Note: The Fast Ethernet interfaces on Cisco 2960 switches are
autosensing and an Ethernet straight-through cable may be used between switches
S1 and S2. If using another model Cisco switch, it may be necessary to use an
Ethernet crossover cable.
Part 1:
Build and Configure the Network
Step 1:
Cable the network according to
the topology.
Step 2:
Configure PC hosts.
Step 3:
Initialize and reload switches
as necessary.
Step 4:
Configure basic settings for
each switch.
a.
Configure device name as shown
in the topology.
b.
Configure IP address as listed
in Addressing Table.
c.
Assign cisco as the console and vty passwords.
d.
Assign class as the privileged EXEC password.
Part 2:
Examine the Switch MAC Address
Table
A switch learns MAC addresses and builds
the MAC address table, as network devices initiate communication on the
network.
Step 1:
Record network device MAC
addresses.
a.
Open a command prompt on PC-A and
PC-B and type ipconfig /all. What
are the Ethernet adapter physical addresses?
PC-A MAC Address: 192.168.1.3
PC-B MAC Address: 192.168.1.2
b.
Console into switch S1 and S2 and
type the show interface F0/1 command
on each switch. On the second line of command output, what is the hardware
addresses (or burned-in address [bia])?
S1 Fast Ethernet 0/1 MAC Address: 0023.abf6.2981
S2 Fast Ethernet 0/1 MAC Address: 0013.c4ea.d183
Step 2:
Display the switch MAC address
table.
Console into switch S2 and view the MAC
address table, both before and after running network communication tests with ping.
a.
Establish a console connection
to S2 and enter privileged EXEC mode.
b.
In privileged EXEC mode, type
the show mac address-table command
and press Enter.
S2# show
mac address-table
Even though there has been no network
communication initiated across the network (i.e., no use of ping), it is
possible that the switch has learned MAC addresses from its connection to the
PC and the other switch.
Are there any MAC addresses recorded in
the MAC address table?
Yes
What MAC addresses are recorded in the
table? To which switch ports are they mapped and to which devices do they belong?
Ignore MAC addresses that are mapped to the CPU.
If you had not previously recorded MAC
addresses of network devices in Step 1, how could you tell which devices the
MAC addresses belong to, using only the output from the show mac address-table command? Does it work in all scenarios?
You will be able to tell because
if you follow the topology there will be one device attached to each port so
you can follow the Ethernet cable from the port back to the device.
Step 3:
Clear the S2 MAC address table
and display the MAC address table again.
a.
In privileged EXEC mode, type
the clear mac address-table dynamic
command and press Enter.
S2# clear
mac address-table dynamic
b.
Quickly type the show mac address-table command again.
Does the MAC address table have any addresses in it for VLAN 1? Are there other
MAC addresses listed?
It has one VLAN 1
MAC address but it is missing the other. The PC-B is also not shown on the MAC
address table either.
Wait 10 seconds, type the show mac address-table command, and press
Enter. Are there new addresses in the MAC address table? Yes
Step 4:
From PC-B, ping the devices on
the network and observe the switch MAC address table.
a.
From PC-B, open a command
prompt and type arp -a. Not
including multicast or broadcast addresses, how many device IP-to-MAC address
pairs have been learned by ARP?
None were learned
by ARP
b.
From the PC-B command prompt, ping
PC-A, S1, and S2. Did all devices have successful replies? If not, check your
cabling and IP configurations.
Yes
c.
From a console connection to
S2, enter the show mac address-table
command. Has the switch added additional MAC addresses to the MAC address table?
If so, which addresses and devices?
Yes it added Fa0/1
with a mac address of e840.f2c2.d019
From PC-B, open a command prompt and retype arp -a. Does the PC-B ARP cache have additional entries for all
network devices that were sent pings?
Yes
Reflection
On Ethernet networks, data is delivered
to devices by their MAC addresses. For this to happen, switches and PCs
dynamically build ARP caches and MAC address tables. With only a few computers
on the network this process seems fairly easy. What might be some of the
challenges on larger networks?
Some of the
challenges on larger networks would be there are many devices that you will
need to look through. And finding a specific devices might take some time when
looking through a network with hundreds of devices.
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