Examkin

Question

Difficulty: MediumLayer 2 Discovery Protocols (CDP and LLDP)

A network administrator observes the following syslog message on a Cisco Catalyst switch:

`%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet1/0/1 (10), with SW-Floor2 GigabitEthernet0/1 (20).`

Which mechanism enables the switch to detect this misconfiguration?

  1. CDP carries a Native VLAN Type-Length-Value (TLV) field in its Layer 2 advertisements, allowing the switch to compare its locally configured native VLAN against the neighbor's advertised native VLAN.Answer
  2. B
    CDP misinterprets the remote interface ID received in an LLDP-MED TLV and automatically modifies the native VLAN on GigabitEthernet1/0/1 to match the remote switch.
  3. C
    LLDP exchanges IEEE 802.1Q tagged frames across the trunk link and disables the local interface when mismatched VLAN IDs are detected in the frame header.
  4. D
    The switch analyzes Spanning Tree Protocol (STP) BPDUs to extract the remote interface native VLAN and generates a CDP error log.

Answer

CDP carries a Native VLAN Type-Length-Value (TLV) field in its advertisements, allowing switches to compare their configured native VLAN with that of directly connected Cisco neighbors.
Cisco Discovery Protocol (CDP) periodically sends Layer 2 multicast messages containing Type-Length-Value (TLV) metadata structures. One of these structures is the Native VLAN TLV. When a switch receives a CDP frame on an 802.1Q trunk interface, it compares the native VLAN ID advertised in the Native VLAN TLV with the local native VLAN configured on that port. If the two values differ (e.g., VLAN 10 vs. VLAN 20), the switch logs a %CDP-4-NATIVE_VLAN_MISMATCH message.

Step-by-Step Solution

1
Identify the protocol generating the syslog message.
The prefix %CDP indicates the message was generated by Cisco Discovery Protocol.
Syslog events starting with %CDP originate from the active CDP process.
2
Analyze how CDP exchanges configuration details across Layer 2 links.
CDP packs information elements into Type-Length-Value (TLV) fields within Layer 2 frames.
TLVs allow CDP to transmit specific device attributes, such as platform, capabilities, management addresses, and native VLAN IDs.
3
Determine how the native VLAN mismatch condition is detected.
The local switch compares its configured native VLAN (10) against the Native VLAN TLV value (20) in the neighbor's CDP frame.
When a discrepancy is detected between local interface configuration and incoming TLV payload, a Native VLAN Mismatch syslog event is triggered.

Key Concept

CDP Native VLAN TLV and Mismatch Detection
Estimated Time:1m 15s
Rate this question

Topics

Layer 2 Discovery Protocols (CDP and LLDP)

More questions from this topic