Chapter 5 - Analyzing Ethernet LAN Switching

Question 1

What name would be given to the below speeds of Ethernet interfaces?

1 - 10/100
2 - 10/100/1000

Answer 1

Cisco switches list their interfaces as the fastest possible speed that interface can obtain so:

1 - Fast Ethernet
2 - Gigabit Ethernet

Question 2

What are the three main tasks a switch completes when making a switching decision?

Answer 2

• Deciding when to forward or filter (not forward) a frame based on the destination MAC. Originally known as a forward-versus-filter decision.
• Learning MAC addresses from frames by examining the src MAC and adding an entry into its MAC address table along with the receiving interface.
• Only forwarding one copy of the frame to the destination by creating a layer 2 loop-free environment with other switches by using STP (Spanning Tree Protocol).

Question 3

What is the process that a switch uses to forward KNOWN Unicast frames?

Answer 3

The switch will check its MAC address table for this particular MAC address. Since the MAC address is known it forwards it out of the interface that the switch received traffic from this MAC address on.

Question 4

What is the process that a switch uses to learn MAC addresses not stored in its MAC address table?

Answer 4

For each frame a switch receives it adds the src address of the frame to its MAC address table along with the interface it was received on. If the MAC address is already there then this isn’t necessary.

Question 5

What is the process that a switch uses to forward UNKNOWN Unicast frames?

Answer 5

The switch floods the frame out of all interfaces apart from the one it was received on. Once the device replies to this frame, that devices MAC is added to the switches MAC address table along with the incoming interface.

Question 6

What is STP (Spanning Tree Protocol)?

Answer 6

• Used to prevent looping frames by ensuring that there is only ever one active path between any pair of LAN segments at one time.
• Part of the way it does this is by entering each interface on the switch into either a Blocking or Forwarding state.
• Blocking means that the interface cannot forward or receive frames, forwarding is the opposite.

Question 7

What is MAC aging?

Answer 7

Switches remove entries in their MAC tables that have not been used for a predefined amount of time (many switches default this to 300 seconds). For every frame that enters the switch, a lookup is done on the MAC address table. If the MAC does not have an entry then it is added, however if it does already have an entry then the inactivity timer is reset to 0.

Question 8

What form of memory does a MAC address table use?

Answer 8

• TCAM (Ternary Content-Addressable Memory) which is a physical memory that has great table lookup capabilities.
• It determines how many entries can be present in a MAC address table.
• If the MAC address table reaches its limit then the switch will purge the oldest entry even if they have not met their aging limit yet.

Question 9

True or False. Routers do not contain MAC address tables.

Answer 9

True.

Question 10

What is an Unknown Unicast Frame?

Answer 10

• A frame which the destination MAC address is not know to the forwarding switch.
• This frame will be forwarded out all interfaces.
• On its reply the switch will learn the incoming interface of this MAC and add it to its MAC address table.