VLAN Trunking Questions
Here you will find answers to VLAN Trunking Questions
Question 1
Which statement is correct about 802.1Q trunking?
A. Both switches must be in the same VTP domain.
B. The encapsulation type of both ends of the trunk does not have to match.
C. The native VLAN on both ends of the trunk must be VLAN 1.
D. 802.1Q trunking can only be configured on a Layer 2 port.
E. In 802.1Q trunking, all VLAN packets are tagged on the trunk link, except the native VLAN.
Answer: E
Explanation
By default frames from the native VLAN are not tagged. To force a switch to tag the native VLAN on all its 802.1Q trunks, we can use the following command:
| Switch(config)#vlan dot1q tag native |
Question 2
Which switch command enables a trunking protocol that appends a four byte CRC to the packet?
A. CompanySwitch(config-if)#switchport trunk encapsulation dot1q
B. CompanySwitch(config-if)#switchport trunk encapsulation itef
C. CompanySwitch(config-if)#switchport trunk encapsulation fddi
D. CompanySwitch(config-if)#switchport trunk encapsulation isl
Answer: D
Explanation
The ISL frame consists of three primary fields: the encapsulation frame (original frame), which is encapsulated by the ISL header, and the FCS at the end:
| ISL Header | Encapsulation Frame (Original Data) | FCS |
In ISL, the original frame is encapsulated and an additional header is added before the frame is carried over a trunk link. Also, a FCS is generated based on some fields in the ISL Header and the Encapsulation Frame and added to the end of the frame. At the receiving end, the header and FCS are removed and the frame is forwarded to the assigned VLAN. The FCS field consists of 4 bytes and contains a 32-bit CRC value.
Note: The addition of the new FCS does not alter the original FCS that is contained within the encapsulated frame.
Question 3
While using a packet analyzer, you notice four additional bytes being added to the packets in the Company network. Which protocol inserts a four byte tag into the Ethernet frame and recalculates CRC value?
A. DTP
B. VTP
C. 802.1Q
D. ISL
Answer: C
Explanation
802.1Q is the IEEE standard for tagging frames on a trunk and supports up to 4096 VLANs. In 802.1Q, the trunking device inserts a 4-byte tag into the original frame and recomputes the frame check sequence (FCS) before the device sends the frame over the trunk link. At the receiving end, the tag is removed and the frame is forwarded to the assigned VLAN. 802.1Q does not tag frames on the native VLAN.
Note: IEEE 802.1Q uses an internal tagging mechanism which inserts a 4-byte tag field in the original Ethernet frame itself.
(Reference: http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml)
Note: From the Question 2 and Question 3 we can see a big difference in the way of using the FCS field in 802.1Q and ISL. 802.1Q modifies and FCS field inside the original Ethernet frame while ISL leaves the original FCS field inside the Ethernet frame unchanged, it just adds another FCS field outside the original Ethernet frame.
Therefore please pay attention to which question is asked. Question 2 asks “appends a four byte CRC to the packet” which means ISL while Question 3 says “inserts a four byte tag into the Ethernet frame” which means 802.1Q.
Question 5
Which command alone will disable trunking on a Layer 2 switch port?
A. no switchport trunk native vlan vlan-id
B. switchport nonegotiate
C. no switchport mode dynamic desirable
D. switchport mode access
Answer: D
Explanation
The “switchport mode access” command forces a switch port to always behave as an access port (with no capability of establishing trunks).
Note: When using the switchport nonegotiate command, Dynamic Inter-Switch Link Protocol and Dynamic Trunking Protocol (DISL/DTP)-negotiation packets are not sent on the interface. The device trunks or does not trunk according to the mode parameter given: access or trunk.
Question 6
ISL is being configured on a Company switch. Which of the following choices are true regarding the ISL protocol? (Choose two)
A. It can be used between Cisco and non-Cisco switch devices.
B. It calculates a new CRC field on top of the existing CRC field.
C. It adds 4 bytes of protocol-specific information to the original Ethernet frame.
D. It adds 30 bytes of protocol-specific information to the original Ethernet frame.
Answer: B D
Explanation
ISL encapsulates the entire Ethernet frame (Fast Ethernet or Gigabit Ethernet) with a 26-byte header and a 4-byte frame check sequence (FCS) for a total of 30 bytes of overhead.
| ISL Header (26 bytes) |
Encapsulation Frame (Original Data) | FCS (4 bytes) |
Question 7
A new Company switch was just configured using the “switchport trunk native vlan 7” command. What does this interface command accomplish?
A. Causes the interface to apply ISL framing for traffic on VLAN 7
B. Configures the trunking interface to forward traffic from VLAN 7
C. Configures the interface to be a trunking port and causes traffic on VLAN 7 to be 802.1q tagged
D. Configures the trunking interface to send traffic from VLAN 7 untagged
Answer: D
Explanation
The “switchport trunk native vlan 7” sets VLAN 7 to be the native VLAN so traffic to this VLAN will be untagged. Also untagged traffic are automatically assumed to be in VLAN 7 -> D is correct.
Question 8
If you needed to transport traffic coming from multiple VLANs (connected between switches), and your CTO was insistent on using an open standard, which protocol would you use?
A. 802.11B
B. spanning-tree
C. 802.1Q
D. ISL
E. VTP
F. Q.921
Answer: C
Explanation
IEEE’s 802.1Q VLAN tagging is the industry standard to carry traffic for multiple VLANs on a single trunking interface between two Ethernet switches while Inter-Switch Link (ISL) is a Cisco proprietary VLAN tagging protocol.
Question 9
The Company core switches use 802.1Q trunks to connect to each other. How does 802.1Q trunking keep track of multiple VLANs?
A. It tags the data frame with VLAN information and recalculates the CRC value
B. It encapsulates the data frame with a new header and frame check sequence
C. It modifies the port index of a data frame to indicate the VLAN
D. It adds a new header containing the VLAN ID to the data frame
Answer: A
Explanation
IEEE 802.1Q uses an internal tagging mechanism which inserts a 4-byte tag field in the original Ethernet frame itself between the Source Address and Type/Length fields. This tag includes VLAN information (12 bits) to distinguish between VLANs on the link.
Question 10
You are the network administrator tasked with designing a switching solution for the Company network. Which of the following statements describing trunk links are INCORRECT? (Choose four)
A. The trunk link belongs to a specific VLAN.
B. Multiple trunk links are used to connect multiple end user devices.
C. A trunk link only supports native VLAN.
D. Trunk links use 802.10 to identify a VLAN.
E. The native VLAN of the trunk link is the VLAN that the trunk uses for untagged packets.
Answer: A B C D
