Examkin

Question

Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

Which two statements correctly describe IEEE 802.1Q trunking operational behavior on Cisco Catalyst switches? (Select TWO.)

  1. By default, Ethernet frames belonging to the configured native VLAN are transmitted across the trunk link without an 802.1Q header tag.Answer
  2. An 802.1Q header inserts 4 bytes into the Ethernet frame, which includes a 12-bit field dedicated to identifying the VLAN ID.Answer
  3. C
    Enabling Spanning Tree PortFast on interswitch trunk connections is recommended to speed up convergence during switch initialization.
  4. D
    When opposing switches have mismatched native VLAN configurations, the switch automatically disables the trunk interface to prevent traffic leaks.

Answer

IEEE 802.1Q sends native VLAN traffic untagged across trunk links by default, and encapsulates tagged frames using a 4-byte header containing a 12-bit VLAN ID field.
IEEE 802.1Q trunking forwards native VLAN frames untagged by default. For all other VLANs, it inserts a 4-byte tag into the Ethernet header which incorporates a 12-bit VLAN Identifier.

Step-by-Step Solution

1
Identify standard 802.1Q frame structure and tagging rules.
Tagged frames receive a 4-byte tag containing a 12-bit VLAN ID, while frames on the native VLAN are forwarded untagged across the link by default.
802.1Q specifies untagged frame delivery for native VLAN traffic to ensure compatibility.
2
Examine the impact of configuring PortFast on switch-to-switch links.
PortFast skips STP listening and learning states, which can cause temporary Layer 2 loops if configured on interswitch trunk ports.
PortFast is designed strictly for access ports connected to end-user devices.
3
Analyze switch behavior when a native VLAN mismatch occurs across a trunk.
Traffic leaks across VLAN boundaries and CDP/STP generates warning logs, but the switch keeps the trunk operational rather than disabling the port.
Cisco switches flag PVID inconsistencies without automatically placing the interface into an err-disabled state.

Key Concept

802.1Q Frame Tagging and Trunk Operations
Estimated Time:1m 0s
Rate this question

Topics

Interswitch Connectivity and Trunking (802.1Q)

More questions from this topic