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Difficulty: HardLayer 2 Discovery Protocols (CDP and LLDP)

An engineer executes the command `show lldp neighbors detail` on a Cisco Catalyst switch to troubleshoot connectivity with an adjacent multi-vendor switch. Below is an excerpt of the command output:

text
Local Intf: Gi0/1
Chassis id: 0025.9611.a480
Port id: Fa0/24
Port Description: Uplink to SW-1
System Name: SW-Edge-02
System Description: Non-Cisco Enterprise Switch OS v12.1
Time remaining: 105 seconds
System Capabilities: Bridge, Router
Enabled Capabilities: Bridge
Management Addresses:
IP: 10.1.100.2
Port VLAN ID: 30

Which statement accurately interprets the neighbor configuration and operational status based on this output?

  1. The local switch interface GigabitEthernet0/1 is connected to port FastEthernet0/24 on the neighbor device SW-Edge-02, which is operating as a Layer 2 bridge.Answer
  2. B
    The local switch interface is FastEthernet0/24, which connects to remote port GigabitEthernet0/1 on the neighbor device SW-Edge-02.
  3. C
    The neighbor device SW-Edge-02 is actively routing traffic on interface FastEthernet0/24 because System Capabilities includes Router.
  4. D
    The output indicates an active Native VLAN mismatch error because Port VLAN ID 30 is advertised by LLDP while the local port is untagged.

Answer

The local switch interface GigabitEthernet0/1 is connected to port FastEthernet0/24 on the neighbor device SW-Edge-02, which is operating as a Layer 2 bridge.
In Cisco IOS `show lldp neighbors detail` output, 'Local Intf' specifies the local interface receiving LLDP messages (GigabitEthernet0/1), while 'Port id' indicates the connected port on the neighbor device (FastEthernet0/24). Additionally, 'Enabled Capabilities' reflects the active operational feature set (Bridge), confirming that the neighbor is currently functioning as a Layer 2 switch on this link.

Step-by-Step Solution

1
Analyze the Local Intf and Port id fields in the LLDP detailed output.
Local Intf is Gi0/1 (the local switch port) and Port id is Fa0/24 (the neighbor device's port).
LLDP explicitly differentiates between the receiving interface on the local switch and the transmitting port advertised by the adjacent neighbor.
2
Evaluate the capabilities fields reported for the neighbor device.
System Capabilities displays Bridge and Router, but Enabled Capabilities lists only Bridge.
Enabled Capabilities represents the active operational role of the neighboring device on the connection, whereas System Capabilities reflects all available hardware capabilities.
3
Examine the Port VLAN ID (PVID) field.
Port VLAN ID 30 is an informational TLV advertised by the neighbor.
Unlike CDP which generates native VLAN mismatch syslog errors when PVIDs mismatch on trunk links, LLDP merely reports the PVID TLV value.

Key Concept

Interpretation of detailed LLDP neighbor outputs, distinguishing local vs remote interfaces and system vs enabled capabilities.
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Topics

Layer 2 Discovery Protocols (CDP and LLDP)

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