VLANs and 802.1Q trunking explained for Network+

Try 3 questions on this

Pick an answer to see why each option is right or wrong.

1. A protocol analyzer capturing on a link between two switches sees frames carrying the type value 0x8100 ahead of the original type field. Which of the following interface configurations is applied to the link?

   Right. In the game that’s Renown for your Network Implementation building.

   Not that one: C is right. In the game your Network Implementation building would lose a little Renown, and this question would come back later.

   Your answerA.Native VLANThe native VLAN is the one VLAN a trunk sends without a tag. Frames belonging to it have no 0x8100 value at all, so seeing that value means the opposite of native-VLAN handling.Your answerB.Link aggregationLink aggregation combines several physical links into one logical link. It decides which member link a frame travels on; it does not insert anything into the frame.The answerC.802.1Q taggingIANA's IEEE 802 Numbers registry assigns Ethertype 0x8100 to the IEEE Std 802.1Q Customer VLAN Tag Type (C-Tag, formerly called the Q-Tag).Your answerD.Jumbo framesA jumbo frame is simply a frame with a payload larger than 1500 bytes. The frame header is unchanged, so no extra type value appears.

   Show the full explanation and doc quotes

   A frame is recognised as 802.1Q-tagged by that value sitting where the type field would otherwise be, with the four-byte tag following and the original type field pushed behind it.

   Written from www.iana.org/assignments/ieee-802-numbers

2. A desk phone with a PC behind it uses one switch port; the phone's calls need their own prioritized VLAN while the PC's frames stay untagged. Which of the following should be configured on the port?

   Right. In the game that’s Renown for your Network Implementation building.

   Not that one: B is right. In the game your Network Implementation building would lose a little Renown, and this question would come back later.

   Your answerA.Native VLANThe native VLAN is the VLAN whose frames travel untagged on a trunk. Setting it decides where the PC's untagged frames go, but it creates no separate, prioritized VLAN for the phone's calls.The answerB.Voice VLANThe switch configuration guide says 'The voice VLAN feature enables access ports to carry IP voice traffic from an IP phone', with the phone sending voice 'in the voice VLAN tagged with a Layer 2 CoS priority value' while the attached PC's traffic rides the access VLAN.Your answerC.802.1Q taggingAn 802.1Q trunk can carry many tagged VLANs, and that is how the voice frames are tagged on the wire. But turning the port into a trunk does not tell the phone which VLAN ID and priority to tag with; the voice VLAN feature does, and it does so on an access port.Your answerD.Link aggregationLink aggregation combines several physical links between two devices into one logical link for bandwidth and resilience. It does not separate one device's traffic from another's on a single port.

   Show the full explanation and doc quotes

   The port stays an access port; the switch tells the phone which VLAN and priority to use.

   Written from www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_55_se/configuration/guide/scg_2960/swvoip.html

3. Hosts in VLAN 10 and VLAN 20 connect to access ports on one Layer 3 device, which must route between them without an external router. Which of the following provides each VLAN with its default gateway?

   Right. In the game that’s Renown for your Network Implementation building.

   Not that one: B is right. In the game your Network Implementation building would lose a little Renown, and this question would come back later.

   Your answerA.SubinterfaceA subinterface divides a router's physical interface so that one 802.1Q trunk can carry several VLANs to the router. Here the routing happens on the device the hosts plug into, which represents each VLAN to its routing function as a virtual interface rather than a slice of a physical port.The answerB.Switch virtual interface (SVI)The configuration guide states that a switch virtual interface represents a VLAN of switch ports as one interface to the routing or bridging function in the system.Your answerC.First Hop Redundancy Protocol (FHRP)A first hop redundancy protocol lets two routers share one gateway address so that the gateway survives a failure. It does not create the interface that routes a VLAN; it protects an address that already exists on one.Your answerD.Virtual IP (VIP)A virtual IP is the floating address an FHRP group or a load balancer answers to. On its own it routes nothing, and the scenario asks for the interface that gives the VLAN a gateway.

   Show the full explanation and doc quotes

   Giving that interface an IP address makes it the VLAN's gateway on the Layer 3 device itself, with no trunk to a router and no per-VLAN subinterfaces.

   Written from www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_55_se/configuration/guide/scg_2960/swint.html