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Zorluk: Çok zorLayer 2 Discovery Protocols (CDP and LLDP)

A network engineer is troubleshooting Layer 2 protocol operations between a Cisco Catalyst switch (SW-1) and an adjacent third-party switch (SW-2). The engineer executes the `show lldp neighbors detail` command on SW-1 and reviews the following output:

SW-1# show lldp neighbors detail
------------------------------------------------
Local Interface: Gi1/0/12
Chassis id: 0050.56a1.b2c3
Port id: Eth101/1/5
Port Description: Ethernet101/1/5
System Name: SW-2-AGG
System Description: Vendor-X Enterprise Operating System v8.4
Time remaining: 114 seconds
System Capabilities: B, R
Enabled Capabilities: B
Management Addresses:
IP: 192.168.40.254
Auto Negotiation - supported, enabled
Physical media capabilities:
1000baseT(FD)
VLAN Information:
Native VLAN: 10

Assuming SW-1 port GigabitEthernet1/0/12 is configured as an 802.1Q trunk using default native VLAN 1 and default LLDP timer settings, which diagnosis and remediation step accurately reflect this state?

  1. A native VLAN mismatch exists because the remote device advertises native VLAN 10 over IEEE 802.1AB TLVs while SW-1 uses native VLAN 1; the native VLAN settings must be synchronized on both switch ports to prevent traffic leakage.Cevap
  2. B
    The local interface Eth101/1/5 on SW-1 is failing to process CDP frames from port Gi1/0/12 because the default CDP holdtime timer of 180 seconds has dropped down to 114 seconds.
  3. C
    The neighbor entry is timing out because SW-1 local interface Eth101/1/5 requires the LLDP reinitialization timer to be increased from 30 seconds to 120 seconds.
  4. D
    LLDP advertisement is inhibited because PortFast must be enabled on trunk port Gi1/0/12 to allow LLDP Type-Length-Value (TLV) exchange during STP forwarding transitions.

Cevap

A native VLAN mismatch exists because the remote device advertises native VLAN 10 over IEEE 802.1AB TLVs while SW-1 uses native VLAN 1; the native VLAN settings must be synchronized on both switch ports to prevent traffic leakage.
The detailed LLDP output explicitly indicates that the remote switch (SW-2-AGG) is sending LLDP Type-Length-Value (TLV) data showing its native VLAN as 10. Since SW-1 is configured with the default native VLAN 1 on its trunk port (Gi1/0/12), a native VLAN mismatch exists across the 802.1Q trunk. Aligning the native VLAN configurations on both connected switch interfaces fixes the mismatch.

Adım Adım Çözüm

1
Analyze the local vs remote interface fields in the LLDP output.
Local Interface is GigabitEthernet1/0/12 (on SW-1). Port id/description is Eth101/1/5 (on remote neighbor SW-2-AGG).
Correctly identifying local versus remote identifiers prevents interface role misinterpretation.
2
Evaluate the protocol timers displayed in the output.
'Time remaining: 114 seconds' reflects LLDP's standard holdtime (default 120 seconds = 30s hello × 4 holdtime multiplier).
This confirms that LLDP discovery is operating normally and is not timing out or relying on CDP 180s timers.
3
Examine the Organizationally Specific VLAN TLV output.
The neighbor advertises Native VLAN: 10 via LLDP TLVs, whereas SW-1 uses default native VLAN 1 on trunk Gi1/0/12.
An unmanaged difference in native VLAN configuration across an 802.1Q trunk leads to VLAN hopping vulnerabilities, STP errors, and traffic leakage.

Anahtar Kavram

LLDP CLI Output Analysis and Native VLAN Mismatch Detection
Tahmini Süre:2m 0s
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Konular

Layer 2 Discovery Protocols (CDP and LLDP)

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