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Difficulty: Very hardInterswitch Connectivity and Trunking (802.1Q)

A network administrator configures an interswitch link between Switch-A and Switch-B using interface GigabitEthernet1/0/24 on both switches.

Switch-A interface configuration:
text
interface GigabitEthernet1/0/24
switchport trunk encapsulation dot1q
switchport mode dynamic auto
switchport trunk native vlan 20
switchport trunk allowed vlan 10,20,30

Switch-B interface configuration:
text
interface GigabitEthernet1/0/24
switchport trunk encapsulation dot1q
switchport mode dynamic desirable
switchport trunk native vlan 1
switchport trunk allowed vlan 10,30,40

Assuming both switches support DTP and IEEE 802.1Q encapsulation, what is the operational behavior of this interswitch link?

  1. A
    The trunk link forms successfully, and untagged VLAN 20 frames sent by Switch-A are accepted by Switch-B and forwarded into VLAN 1 regardless of Switch-B's allowed VLAN list configuration.
  2. The trunk link forms successfully; tagged traffic for VLANs 10 and 30 is forwarded normally, but untagged VLAN 20 frames sent by Switch-A are dropped upon ingress at Switch-B because Switch-B's native VLAN (VLAN 1) is not in its allowed VLAN list.Answer
  3. C
    CDP detects the native VLAN mismatch between VLAN 20 and VLAN 1 and automatically forces both interfaces into an err-disabled state, blocking all interswitch traffic.
  4. D
    The link fails to negotiate a trunk because dynamic DTP mode negotiation requires spanning-tree PortFast to be enabled on interswitch trunk interfaces before trunking can complete.

Answer

The trunk link forms successfully; tagged traffic for VLANs 10 and 30 is forwarded normally, but untagged VLAN 20 frames sent by Switch-A are dropped upon ingress at Switch-B because Switch-B's native VLAN (VLAN 1) is not in its allowed VLAN list.
Dynamic Trunking Protocol (DTP) successfully negotiates an operational 802.1Q trunk because Switch-A is set to dynamic auto and Switch-B is set to dynamic desirable. Tagged VLANs present in both allowed lists (VLAN 10 and VLAN 30) traverse the link normally. However, Switch-A transmits VLAN 20 frames untagged because VLAN 20 is Switch-A's native VLAN. Upon receiving these untagged frames, Switch-B assigns them to its local native VLAN (VLAN 1). Since VLAN 1 is omitted from Switch-B's explicit allowed list (10, 30, 40), Switch-B drops the frames upon ingress.

Step-by-Step Solution

1
Evaluate DTP negotiation outcome between Switch-A and Switch-B.
Switch-A is in 'dynamic auto' mode (listens for trunking requests) and Switch-B is in 'dynamic desirable' mode (actively solicits trunking). Combining 'auto' and 'desirable' successfully negotiates an operational 802.1Q trunk.
DTP operational state matrix specifies that auto + desirable results in a trunking interface.
2
Determine tagged VLAN forwarding behavior for matched allowed VLANs.
VLAN 10 and VLAN 30 are explicitly included in Switch-A's allowed list (10, 20, 30) and Switch-B's allowed list (10, 30, 40). Frames for VLAN 10 and 30 carry standard 802.1Q tags and pass bidirectionally.
Traffic for non-native allowed VLANs is tagged with 802.1Q headers and permitted across the link.
3
Analyze 802.1Q native VLAN egress tagging and ingress classification.
Switch-A has native VLAN 20, so frames originating from VLAN 20 are transmitted across the trunk link without an 802.1Q tag. When Switch-B receives an untagged frame on its trunk port, it implicitly maps the frame to its own configured native VLAN (VLAN 1).
802.1Q trunks send native VLAN frames untagged, and receiving switches associate untagged ingress frames with their local native VLAN configuration.
4
Evaluate allowed VLAN filtering logic on Switch-B for the ingress untagged traffic.
Switch-B's trunk allowed list is restricted to '10, 30, 40'. Because native VLAN 1 is not included in Switch-B's allowed VLAN list, Switch-B drops the untagged frames received from Switch-A.
Before accepting traffic into a VLAN on a trunk, the switch verifies that the destination VLAN ID is explicitly permitted in the switchport trunk allowed vlan list.

Key Concept

802.1Q Trunking Mechanics, DTP Negotiation, Native VLAN Ingress Processing, and Allowed VLAN List Enforcement
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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