VLAN Trunking Questions Flashcards
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
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.
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.
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
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.
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)
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 -> A is correct.
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.
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.
802_1q_frame_structure.jpg
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
You are the network administrator at Company and switch R1 is configured as shown below:
interface GigabitEthernet0/1
switchport mode trunk
switchport trunk encapsulation dot1q
switchport trunk native vlan 5
If untagged frames are arriving on interface GigabitEthernet0/1 of R1, which of the following statement are correct?
A. Untagged frames are automatically assumed to be in VLAN 5.
B. Untagged frames are defaulted to VLAN 1 traffic.
C. Untagged frames are dropped because all packets are tagged when dot1q trunked.
D. Untagged frames are determined on the other switch
E. Untagged frames are not supported on 802.1Q trunks.
Answer: A
Explanation
The “switchport trunk native vlan 5″ sets VLAN 5 to be the native VLAN so traffic to this VLAN will be untagged. Also untagged traffic are automatically assumed to be in VLAN 5 -> A is correct.
Note: The native VLAN must match on both sides of the trunk link for 802.1Q; otherwise the link will not work.
What are three results of issuing the “switchport host” command? (Choose three)
A. disables EtherChannel
B. enables port security
C. disables Cisco Discovery Protocol
D. enables PortFast
E. disables trunking
F. enables loopguard
Answer: A D E
Explanation
Catalyst 6500 switches running Cisco IOS software support the macro command switchport host. The switchport host macro command was designed to facilitate the configuration of switch ports that connect to end stations. Entering this command sets the switch port mode to access, enables spanning tree PortFast, and disables channel grouping, all at the same time. The switchport host macro command can be used as an alternative to the switchport mode access command.
(Reference: http://www.cisco.com/en/US/docs/solutions/Enterprise/Security/Baseline_Security/sec_chap7.html)
If you were to configure an ISL Ethernet trunk between two Cisco switches, named R1 and R2, what would you have to include at the end of the link for the trunk to operate correctly? (Choose two)
A. An identical VTP mode.
B. An identical speed/duplex.
C. An identical trunk negotiation parameter.
D. An identical trunk encapsulation parameter.
Answer: B D
Explanation
One of the requirements for trunking to work is for speed and duplex to be the same on both sides. -> B is correct.
Maybe answer D wants to mention about encapsulation type (ISL or 802.1q) so it is an acceptable answer.
(Reference: http://www.cisco.com/en/US/products/hw/switches/ps663/products_configuration_example09186a008014859e.shtml)
Which three statements are correct with regard to the IEEE 802.1Q standard? (Choose three)
A. The IEEE 802.1Q frame format adds a 4 byte field to a Ethernet frame
B. The packet is encapsulated with a 26 byte header and a 4 byte FCS
C. The protocol uses point-to-multipoint connectivity
D. The protocol uses point-to-point connectivity
E. The IEEE 802.1Q frame uses multicast destination of 0×01-00-0c-00-00
F. The IEEE 802.1Q frame retains the original MAC destination address
Answer: A D F
Explanation
There are two ways to implement Ethernet trunking:
* Inter-Switch Link Protocol (ISL, a Cisco proprietary protocol)
* 802.1Q (IEEE standard)
In Cisco implementation, a trunk is a point-to-point link, although it is possible to use the 802.1Q encapsulation on an Ethernet segment shared by more than two devices. Such a configuration is seldom needed but is still possible with the disablement of DTP negotiation -> D is correct.
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 -> A is correct.
802_1q_frame_structure.jpg
The SA field is the source address field of the ISL packet. It is a 48-bit value -> F is correct.
(Reference: http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008012ecf3.shtml)
Refer to the exhibit. Why are users from VLAN 100 unable to ping users on VLAN 200?
Trunking_switch_router.jpg
A. Encapsulation on the switch is wrong.
B. Trunking needs to be enabled on Fa0/1.
C. The native VLAN is wrong.
D. VLAN 1 needs the no shutdown command.
E. IP routing needs to be enabled on the switch.
Answer: B
Explanation
In this question, maybe the exhibit forgot to describe Fa0/1 is the port on the switch which is connected to the router. To allow interVLAN routing between VLAN 100 and 200, this port must be configured as trunk port.
If you wish to see the full configuration of interVLAN routing, please read my interVLAN routing tutorial (you will see the configuration at the bottom of that tutorial).
What is the effect of applying the “switchport trunk encapsulation dot1q” command to a port on a Cisco Catalyst switch?
A. By default, native VLAN packets going out this port will be tagged.
B. Without an encapsulation command, 802.1Q will be the default encapsulation if DTP fails to negotiate a trunking protocol.
C. The interface will support the reception of tagged and untagged traffic.
D. If the device connected to this port is not 802.1Q-enabled, it will not be able to handle 802.1Q packets.
Answer: C
Explanation
The “switchport trunk encapsulation dot1q” command configures trunk encapsulation as 802.1q, which supports the reception of tagged and untagged traffic -> C is correct.
Note: If your switch does not accept this command, try to enter “switchport” command first to configure the interface as a Layer 2 port.
Two Company switches are connected via a trunk link. In this network, the original frame is encapsulated and an additional header is added before the frame is carried over a trunk link. At the receiving end, the header is removed and the frame is forwarded to the assigned VLAN. This describes which technology?
A. DISL
B. ISL
C. DTP
D. IEEE 802.1Q
E. MPLS
Answer: B
Explanation
Unlike 8021.q, ISL keeps the original frame unchanged. It only adds another header to that frame before sending out over a trunk link. For more information about this difference, please read the explanations of Question 2 and Question 3 in the first VLAN Trunking Questions part.
Which of the following trunking modes are unable to request their ports to convert their links into trunk links? (Choose two)
A. Negotiate
B. Designate
C. Nonegotiate
D. Auto
E. Manual
F. Off
Answer: C D
Explanation
The mode auto (dynamic auto) causes the device not to send DTP Request but wait for DTP Request from neighboring device.
By using and switchport mode trunk and switchport nonegotiate commands, we can enable trunking to a device that does not support DTP. But notice that the switchport nonegotiate command causes the device not to send DTP Request frames.
Therefore both “auto” and “nonegotiate” modes makes the switch not to send request (which is “unable to convert their links into trunk links”) -> C and D are correct.
You administer the network shown above. You issue the show interfaces trunk command on SwitchA and receive the following output:
show_interfaces_trunk_command.jpg
Which of the following statements is true regarding VLAN 32?
A. VLAN 32 is not allowed on the trunk port.
B. VLAN 32 is not active on the switch.
C. Traffic from VLAN 32 is not being sent over the trunk port.
D. Traffic from VLAN 32 is not restricted to only the trunk ports that require it.
Second picture on answer side
Answer: C
Explanation
In the “Vlans allowed and active in management domain” VLAN 32 is not listed so we can conclude it is not active.
Which statement is true regarding the configuration of ISL trunks?
A. A Catalyst switch cannot have ISL and IEEE 802.1q trunks enabled.
B. All Catalyst switches support ISL trunking.
C. A Catalyst switch will report giants if one side is configured for ISL while the other side is not.
D. ISL trunking requires that native VLANs match.
Answer: C
Explanation
First you should know “giant” frames are frames that exceed the maximum IEEE 802.3 frame size (usually greater then 1518 bytes). As you know, ISL does not modify the original Ethernet frame it received but it adds another outer header. In particular, it uses a 26 byte header and 4 byte FCS (30 bytes in total).
ISL Header
(26 bytes) Encapsulation Frame (Original Data) FCS
(4 bytes)
But a normal Ethernet frame itself can have a maximum size of 1518 bytes. Therefore an Ethernet frame can be up to 1518 + 30 = 1548 bytes, which creates a “giant”.
That is why both ends must be configured as ISL trunks because only ISL-aware devices are able to read it.
What are two types of Trunk Encapsulation?
ISL =26byte header & 4 byte trailer encapsulated around the Frame 802.1q=4byte embedded into the Frame
Describe 802.1q tagging
16 bit = Ethernet Type
3 bit = COS
1 bit CFI
12 bit = VLAN id
Untagged frames are assumed to be the native VLAN. Modifies original Frame. Embedds tagging.
Describe ISL
Does not modify the frame
ISL encapsulates the entire frame
26 byte header & 4 byte trailer (FCS) Double tagging
addes 30bytes to each frame. High overhead.
Refer to the exhibit. On the basis of the output generated by the show commands, which two statements are true? (Choose two) show_interface_gigabitethernet_show_vlan.jpg A. Interface gigabitethernet 0/1 has been configured as Layer 3 ports. B. Interface gigabitethernet 0/1 does not appear in the show vlan output because switchport is enabled. C. Interface gigabitethernet 0/1 does not appear in the show vlan output because it is configured as a trunk interface. D. VLAN2 has been configured as the native VLAN for the 802.1q trunk on interface gigabitethernet 0/1. E. Traffic on VLAN 1 that is sent out gigabitethernet 0/1 will have an 802.1q header applied. F. Traffic on VLAN 2 that is sent out gigabitethernet 0/1 will have an 802.1q header applied.
Answer: C F Explanation From the output of show interface gigabitethernet 0/1 switchport command we can see this port is currently configured as trunked port (Operational Mode: trunk) and uses 802.1q encapsulation. So surely the “show vlan” command will not list this port -> C is correct. Also from the first output we learned the native VLAN is VLAN 1 (Trunking Native Mode VLAN:1) so only traffic from this VLAN is sent untagged -> traffic sent from VLAN 2 out this port will have an 802.1q header applied -> F is correct.
Study the following exhibit carefully, what is the reason that users from VLAN 100 can’t ping users on VLAN 200?
inter_Routing
A – IP routing needs to be enabled on the switch
B – Trunking needs to be enabled on Fa0/1
C – VLAN 1 needs the no shutdown command
D – The native VLAN is wrong
Answer: B
Explanation:
The Fa0/1 interface on the switch is not configured with trunking mode. It needs to be configured as shown below:
SA(config)#interface Fa0/1
SA(config-if)#switchport mode trunk
SA(config-if)#switchport trunk encapsulation dot1q
What is the default MTU?
1500B
How does ISL do framing on trunks?
by encapsulating each frame in a header and trailer
What types of links can be trunked?
FE, GE, and aggregated FE/GE Etherchannels
How does dot1q do framing on trunks?
embeds the tag within the frame
What is the size of ISL encapsulation?
26-byte header, 4-byte trailer with CRC
What is a baby giant frame?
frames that barely exceed 1518B or 1622B due to headers
Refer to the configuration. For which configured VLAN are untagged frames sent over trunk between SW1 and SW2?
VLAN 99
