Chapter 7

Compare and contrast the terms "physical LAN" and "broadcast domain."

Answer: The devices in a physical LAN are in the same broadcast domain. The physical LAN includes the cabling switches and NICs.

Define the term "broadcast domain."

Answer: A set of devices for which a broadcast frame sent by one device is received by all other devices in the set

What happens to a broadcast frame when a hub or a switch receives it?

Answer: Both devices forward the frame out all ports except the one in which the frame was received.

Imagine a standalone switch with no VLANs configured, with 24 physical ports, each connected to a different PC. How many broadcast domains exist?

Answer: Because they each forward broadcasts out all ports, a broadcast sent by any of the 24 devices on the first switch is forwarded to the other 23, making a single broadcast domain.

Imagine that a 12-port switch has 11 PCs connected to it, with the twelfth port connected to a hub. The hub has four PCs connected to it as well. The switch does not have VLANs configured. How many broadcast domains exist?

Answer: Only one broadcast domain exists. Because both hubs and switches forward broadcasts out all ports, a broadcast sent by any device would be received by all other PCs.

Define the term "VLAN."

Answer: A virtual LAN (VLAN) is a broadcast domain that is created by a switch using a subset of the physical ports on the switch.

What is the key difference between a physical LAN and a VLAN, according to this chapter?

Answer: A physical LAN includes all the ports on a switch. A VLAN includes a subset of the ports on a switch, as configured on the switch.

Imagine that three VLANs were created on a single switch. The switch receives a broadcast into a port in VLAN1. Will devices in VLAN2 receive the broadcast?

Answer: No. The switch will not forward the broadcast into VLAN2 or VLAN3, treating VLAN1 as a separate LAN.

Imagine that three VLANs were created on a single switch. The switch in a port in VLAN1 received a unicast frame sent to MAC address 0200.5555.555. The VLAN1 address table in the switch does not have an entry for that MAC address. What will the switch do with this frame?

Answer: The switch will do what it always does with unknown destination unicast frames: It will flood the frame out all ports. However, the switch will only flood the frames out all ports in VLAN1, because the frame was received in VLAN1; it will not forward the frame out ports in other VLANS.

Imagine that three VLANs were created on a single switch. A unicast frame, sent to MAC address 0200.5555.555, was received by the switch in a port in VLAN1. The VLAN2 MAC address table lists an entry for 0200.5555.5555, referencing port 18. What will the switch do with this frame?

Answer: Assuming that 0200.5555.5555 is not in the VLAN1 address table, which should be the case, the switch will do what it always does with unknown destination unicast frames: It will flood the frame out all ports. However, it will only flood the frame out all ports in VLAN1, because the frame was received in VLAN1.

List two reasons why a network engineer might want to use VLANs.

Answer: Four reasons were listed in this chapter:

- To reduce the size of an individual broadcast domain

- For business or political reasons, referred to as Layer 8 issues

- Due to Layer 3 design issues, because all devices in the same VLAN will be in the same IP subnet

- For cost issues; no matter why you want more LANs, using VLANs can allow you to create the LANs, without buying more switches.

Imagine one LAN with 100 devices, and another with 1000 devices. Explain the similarities and differences of the impact of broadcasts in each LAN.

Answer: The end user devices in a LAN must process all received broadcasts to know if the broadcast is intended for that device. So, the broadcasts do affect the end user devices. The more devices in the LAN, the more broadcasts heard by each device in the LAN, having an adverse effect on wasteful CPU overhead on the devices. The devices in the larger LAN will spend more CPU cycles processing broadcasts, most of which are not intended for them.

Compare and contrast physical LANs and virtual LANs.

Answer: Both define a single broadcast domain. VLANs require that a switch not consider all physical ports to be in the same LAN, but instead, via switch configuration, treat some ports as if they are in one VLAN, and some as if they are in another. With physical LANs, the switch treats all physical ports as if they are in a single broadcast domain.

Imagine that two switches are connected with an Ethernet cable. Three VLANsVLAN1, VLAN2, and VLAN3are configured on each switch. Name the two trunking protocols that could be used on the segment between switches and explain their basic operation.

Answer: VLAN trunking would be used between the two switches, using either ISL or IEEE 802.1Q. Each protocol calls for the switches to put a header in front of the Ethernet frames before sending so that the receiver can look at the header and decide in what VLAN the frame belongs.

Imagine that two switches are connected with an Ethernet cable. No VLANs are configured. Describe the meaning of the term "trunk," and tell whether a trunk is needed or used in this small network.

Answer: Depending on who uses the term, "trunk" might simply refer to an Ethernet cable or segment between switches. With that use of the term, the cable between the switches is indeed a trunk. Others only use the word "trunk" specifically when VLANs are used, and a trunking header is placed in front of frames before they pass over the cable between the switches.

Identify the two VLAN trunking protocols and state which is Cisco proprietary.

Answer: Inter-Switch Link (ISL) is Cisco proprietary, and IEEE 802.1Q, the alternative, is defined by the IEEE.