Switching and VLAN Fundamentals Questions

Fundamentals of Ethernet switching and virtual local area network design and operation. Topics include media access control addresses and how switches learn and populate forwarding tables, switch forwarding behavior, spanning tree protocol basics for loop prevention, virtual local area network concepts including access ports, trunk ports, and tagging, and the distinction between second layer switching and third layer switching. Candidates should understand practical configuration and troubleshooting scenarios, common failure modes, and when to use virtual local area networks for segmentation and traffic isolation.

MediumTechnical

40 practiced

Describe how to deploy a voice VLAN for IP phones on access ports. Include how the phone and PC behind the phone typically tag traffic, how phones learn the voice VLAN (CDP/LLDP-MED/DHCP), QoS considerations (CoS/DSCP trust), and the access-port configuration best practices to prioritize voice.

Sample Answer

**Situation / overview**Deploying a voice VLAN on access ports isolates and prioritizes IP phone traffic while allowing a PC behind the phone to use the data VLAN on the same physical port.

**How phone and PC tag traffic**- Typical behavior: phone tags voice frames with 802.1Q (voice VLAN) and forwards the PC’s traffic untagged (native access VLAN). Some vendor phones may leave voice untagged—switch handles classification.- PC: untagged, placed in access VLAN (data).

**How phones learn the voice VLAN**- CDP (Cisco) or LLDP‑MED: switch advertises a voice VLAN ID; phone configures its voice interface automatically.- DHCP options: some deployments use DHCP (vendor-specific options) or option 132/128 depending on vendor to convey VLAN ID or provisioning server.- Best practice: enable LLDP‑MED (preferred multi‑vendor), keep CDP for Cisco phones if homogenous.

**QoS considerations**- Trusting markings: - If phone tags frames with CoS (802.1p) inside the 802.1Q header, use “mls qos trust cos” on that access interface. - If phone sets DSCP in IP header, use “mls qos trust dscp”. - Many modern phones set both; prefer trusting CoS on trunked/voice‑tagged links and DSCP for routed links.- Downstream queuing: ensure voice traffic mapped to a low‑latency queue (LLQ or priority queue) to minimize jitter.- Policing: apply policing/shaping for data, not for voice priority queue; avoid starving other traffic.

**Access-port configuration best practices (Cisco IOS example)**- Sample config:

ios

interface GigabitEthernet1/0/10
 description IPPhone+PC
 switchport mode access
 switchport access vlan 20            ! data VLAN for PC
 switchport voice vlan 110            ! voice VLAN for phones
 spanning-tree portfast
 mls qos trust cos                    ! or "mls qos trust dscp" per phone behavior
 lldp med tlv-select voice-vlan       ! enable LLDP-MED voice advertisement
 power inline auto                    ! PoE

- Additional: enable CDP/LLDP globally, ensure DHCP scopes exist for both VLANs, provide QoS policy maps at distribution/core to classify and queue DSCP EF/CS5 as priority.

**Operational tips**- Verify with show commands: show lldp neighbors, show cdp neighbors, show interfaces switchport, show mls qos interface.- Test with a phone: confirm it receives voice VLAN and DSCP/CoS markings; validate end‑to‑end priority and acceptable jitter/latency.- Document VLAN IDs, QoS mappings, and PoE budget.

This setup ensures phones get correct VLAN assignment and receive low‑latency forwarding while PCs remain isolated on the data VLAN.

MediumTechnical

44 practiced

Explain precisely how STP elects the root bridge and how it determines port roles (root, designated, blocked). Describe Bridge ID composition, how path cost influences port selection, and provide an example of commands or values you would change to make a specific switch the root bridge.

Sample Answer

**How STP elects the Root Bridge**

- STP elects the root bridge by comparing Bridge IDs (BIDs) carried in BPDUs. The lowest BID wins.- Bridge ID = Bridge Priority (16-bit) + System ID Extension (VLAN ID) + MAC address. Effectively compared as: lowest priority → lowest MAC as tiebreaker.- BPDUs propagate from the current root; switches select paths based on cumulative path cost to that root.

**Port role determination**

- Root Port (RP) - On each non-root switch, the port with the lowest cumulative path cost to the root becomes the Root Port. - Tie-breakers: lower received root path cost, then lower sending bridge ID, then lower sending port ID.- Designated Port (DP) - For each network segment, the port on the switch that offers the lowest path cost to the root becomes the Designated Port and forwards traffic for that segment. - If equal cost, the switch with lower BID wins; if still tied, lower port ID wins.- Blocked Port - Any port that is neither RP nor DP is put into Blocking to prevent loops; it still listens for BPDUs.

**Path cost influence**

- Path cost is cumulative: each link contributes a cost value based on link speed. STP selects ports based on the lowest total cost to the root.- You can influence selection by changing either: - Interface cost (per-port): e.g., lower the cost on desired links so they become preferred. - Bridge priority (global per VLAN): lower priority makes the switch more likely to be root.

**Commands / Example to force a switch to be root (Cisco IOS examples)**

- Make this switch the primary root for VLAN 10:

bash

configure terminal
spanning-tree vlan 10 root primary
end

- Or set priority explicitly (lower is better; default 32768; valid increments of 4096):

bash

configure terminal
spanning-tree vlan 10 priority 4096
end

- Set an interface path cost to prefer a link:

bash

configure terminal
interface GigabitEthernet1/0/1
spanning-tree cost 2000
end

**Practical notes**- Use “root primary/secondary” helpers on Cisco to automate safe priority values.- When planning changes, account for VLAN-specific BIDs (system ID) and avoid abrupt topology changes during production hours.

EasyTechnical

33 practiced

Define a Virtual Local Area Network (VLAN) and explain the primary reasons to deploy VLANs in an enterprise environment. Cover segmentation, broadcast domain reduction, security/isolation, multitenancy, operational impacts on ARP/DHCP, and how VLANs typically map to IP subnets.

MediumTechnical

41 practiced

Describe how multicast traffic is forwarded through Ethernet switches and the role of IGMP snooping within VLANs. Explain what happens when IGMP snooping is disabled, how snooping depends on an IGMP querier or multicast router, and typical problems to diagnose when multicast traffic is being flooded to all ports.

EasyTechnical

33 practiced

Describe the functional differences between an access port and a trunk port on an Ethernet switch. Explain how frames are handled on each type, how 802.1Q tagging works on a trunk, what the 'native VLAN' means, and typical configuration commands (vendor-agnostic or Cisco example). Finally, list common operational mistakes when assigning access and trunk modes and their symptoms.

Sample Answer

**Brief answer**An access port carries traffic for a single VLAN (untagged on the host side). A trunk port carries traffic for multiple VLANs between switches/routers using 802.1Q tagging.

**Frame handling**- Access port: Switch places incoming frames into the access VLAN and forwards them untagged; frames leaving toward the end host are untagged.- Trunk port: Frames for non-native VLANs are sent with an 802.1Q tag; frames for the native VLAN are sent untagged (by default).

**802.1Q tagging**- Tag inserts a 4-byte header with TPID (0x8100) and a 12-bit VLAN ID into the Ethernet frame between source MAC and Ethertype.- Carries VLAN ID 1–4094; allows multiplexing VLANs across a single link.

**Native VLAN**- VLAN whose frames traverse the trunk untagged. Mismatch between ends can cause VLAN hopping, misclassification, or MAC-table flaps.

**Typical config (Cisco)**

bash

interface GigabitEthernet0/1
 switchport mode access
 switchport access vlan 10

interface GigabitEthernet0/2
 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport trunk native vlan 99
 switchport trunk allowed vlan 10,20,30

Vendor-agnostic: set port type to access/trunk, configure native VLAN and allowed VLAN list, enable 802.1Q.

**Common mistakes & symptoms**- Access vs trunk mismatch (one side access, other trunk): connectivity loss, unexpected VLAN flooding.- Native VLAN mismatch: untagged frames land in wrong VLAN, intermittent connectivity, security exposure.- Forgetting allowed VLANs: traffic dropped for missing VLANs.- Using default VLANs for management: accidental topology loops or exposure.- Not pruning unused VLANs: unnecessary broadcast domains and CPU load.

I would verify with show commands (show interfaces trunk, show vlan, show mac address-table) and packet captures to troubleshoot.

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