2.2 Interswitch Connectivity

Question 1

Define:

trunk port

Answer 1

A port that carries traffic from multiple VLANs.

Trunk ports are configured to allow traffic for multiple VLANs, commonly using VLAN tagging protocols like 802.1Q, typically connecting switches.

Question 2

What’s the difference between access ports and trunk ports?

Answer 2

• Access ports: Carry traffic for one VLAN.
• Trunk ports: Carry traffic for multiple VLANs.

Access ports are for end devices, and trunk ports are used to connect switches, carrying traffic for multiple VLANs.

Question 3

What command configures a port as a trunk port on a Cisco switch?

Answer 3

switchport mode trunk

This command enables trunking on the port, allowing it to carry traffic from multiple VLANs.

Question 4

True or False:

Trunk ports can only transmit untagged traffic.

Answer 4

False

Trunk ports transmit both tagged (VLAN-tagged) and untagged traffic. Untagged traffic is assigned to the native VLAN.

Question 5

What happens when a trunk port receives an untagged frame?

Answer 5

It assigns the frame to the native VLAN.

The native VLAN is the default VLAN for untagged traffic on a trunk port.

Question 6

What is the default native VLAN for a Cisco trunk port?

Answer 6

VLAN 1

VLAN 1 is the default native VLAN on Cisco trunk ports unless manually changed.

Question 7

Why should the native VLAN be the same on both ends of a trunk link?

Answer 7

To prevent misdirected untagged traffic.

Mismatched native VLANs between switches could cause untagged frames to be improperly forwarded, leading to communication failures.

Question 8

What protocol is used for VLAN tagging in trunk ports?

Answer 8

IEEE 802.1Q

802.1Q adds a tag to each Ethernet frame to identify its VLAN, enabling multiple VLANs to be carried on a single trunk link.

Question 9

True or False:

Trunk ports require both ends to be configured for trunking to work.

Answer 9

True

Both switches must be configured with trunking for the link to successfully carry multiple VLANs.

Question 10

What are the characteristics of trunk ports?

Answer 10

• Carries traffic for multiple VLANs
• Uses VLAN tagging (802.1Q or ISL)
• Commonly used to connect switches
• Can transmit both tagged and untagged frames

Trunk ports are essential for inter-switch communication and enable the efficient use of network links by allowing multiple VLANs to share a single connection.

Question 11

How can you verify if a switch port is configured as a trunk?

Answer 11

Use “show interfaces [interface-id] switchport”.

This command shows the port’s configuration, including whether it’s in trunk mode and the allowed VLANs.

Question 12

List the steps to configure a trunk port on a Cisco switch.

Answer 12

1. Enter interface configuration mode.
2. Set the port to trunk mode (switchport mode trunk).
3. Define the allowed VLANs (switchport trunk allowed vlan [vlan-list]).
4. Optionally configure the native VLAN (switchport trunk native vlan [vlan-id]).

Question 13

What is the command to disable trunking on a Cisco switch port?

Answer 13

“switchport mode access”

This command changes the port to an access port, allowing traffic for only one VLAN.

Question 14

What are the key features of 802.1Q?

Answer 14

• Allows multiple VLANs over a single link.
• Adds a 4-byte tag to Ethernet frames.
• Supports VLAN IDs from 1 to 4095.
• Can be used with both access and trunk ports (though primarily for trunks).

802.1Q is the industry-standard VLAN tagging protocol, widely adopted for enabling trunking between network devices while maintaining VLAN separation.

Question 15

How does 802.1Q tagging work?

Answer 15

It inserts a 4-byte tag in the Ethernet frame.

This tag is placed between the MAC address and EtherType fields, containing the VLAN ID and other control information.

Question 16

What are the advantages of using 802.1Q over ISL?

Answer 16

1. Open standard (non-proprietary).
2. Supports interoperability with multiple vendors.
3. Widely adopted and supported across different platforms.

ISL (Inter-Switch Link) is a VLAN tagging protocol developed by Cisco, while 802.1Q is an industry-standard protocol used by multiple vendors.

Question 17

What is the maximum VLAN ID for 802.1Q?

Answer 17

4095

The VLAN ID is 12 bits long, so the valid range is from 1 to 4095, with 0 being reserved.

Question 18

What is the role of the priority field in 802.1Q?

Answer 18

It provides Quality of Service.

(QoS)

The 3-bit priority field allows traffic to be prioritized, especially useful for real-time applications like VoIP.

Question 19

True or False:

802.1Q can only be used on trunk ports.

Answer 19

True

802.1Q is designed for trunking, where multiple VLANs need to be carried over a single link. Access ports do not use VLAN tagging.

Question 20

How does 802.1Q handle untagged traffic on a trunk?

Answer 20

It uses the native VLAN.

Untagged frames on a trunk are assigned to the native VLAN, which is typically VLAN 1 by default.

Question 21

How do you configure 802.1Q trunking on a Cisco switch?

Answer 21

Use “switchport mode trunk” and “switchport trunk encapsulation dot1q”.

These commands enable trunking and set the encapsulation type to 802.1Q, allowing multiple VLANs to be transmitted over the link.

Question 22

Fill in the blank:

To check 802.1Q trunk status on a Cisco switch, use ________.

Answer 22

“show interfaces trunk”

This command shows the trunk status, including the VLANs allowed and the native VLAN used on the trunk port.

Question 23

True or False:

802.1Q supports VLANs from 1 to 4096.

Answer 23

False

The valid range for VLANs in 802.1Q is 1 to 4095. VLAN 0 and VLAN 4096 are reserved.

Question 24

What is the purpose of the EtherType field in the 802.1Q header?

Answer 24

To identify the protocol type.

The EtherType field indicates the higher-layer protocol, such as IPv4 or IPv6, enabling the correct interpretation of the frame payload.

Question 25

Why is 802.1Q **considered** transparent?

Answer 25

It **does not alter** the original Ethernet frame. ## Footnote The *VLAN tag* is added without affecting the basic Ethernet structure, making it transparent to the devices while enabling VLAN segregation.

Question 26

What are the **key fields** in the 802.1Q tag?

Answer 26

1. Tag Protocol Identifier (TPID) 1. Priority field 1. VLAN ID 2. EtherType ## Footnote **Tag Protocol Identifier (TPID)**: Identifies the frame as an 802.1Q tagged frame. **Priority field**: Indicates the priority level of the frame for QoS. **VLAN ID**: Specifies the VLAN to which the frame belongs. **EtherType**: Identifies the protocol used in the frame.

Question 27

Why is it **important** to configure the same Native VLAN on both trunk ends?

Answer 27

To **ensure** proper communication. ## Footnote If the Native VLAN *differs* on either side of the trunk, it can cause communication issues, as untagged frames may be misdirected or dropped.

Question 28

How do you **configure** the Native VLAN on a Cisco switch trunk port?

Answer 28

Use "**switchport trunk native vlan [VLAN_ID]**" ## Footnote This command configures the VLAN for untagged frames on the trunk. Ensure both switches on the trunk have matching Native VLAN configurations.

Question 29

# True or False: The Native VLAN **can be used** for data traffic on access ports.

Answer 29

False ## Footnote The Native VLAN is specifically for *trunk links* and *untagged traffic*. Access ports use only one VLAN (typically the default VLAN).

Question 30

How does the Native VLAN **affect** security?

Answer 30

It can **create vulnerabilities**. ## Footnote If the Native VLAN is *not configured properly* or is left as *VLAN 1*, attackers may **exploit** it to gain unauthorized access to network traffic.

Question 31

What is a **common best practice** for configuring Native VLAN?

Answer 31

Use a VLAN other than VLAN 1. ## Footnote Changing the Native VLAN to a *non-default VLAN (e.g., VLAN 99)* helps prevent certain types of attacks that target VLAN 1.

Question 32

# Fill in the blank: To configure the Native VLAN as **VLAN 100** on a trunk port, use: "switchport trunk native vlan \_\_\_\_\_\_\_\_".

Answer 32

100 ## Footnote This command assigns VLAN 100 as the *Native VLAN* on the trunk port, ensuring untagged traffic is placed in VLAN 100.