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Difficulty: HardLayer 2 Discovery Protocols (CDP and LLDP)

A network administrator is troubleshooting neighbor visibility on a Cisco Catalyst switch (SW-1) connected to a non-Cisco switch. The administrator executes the `show lldp entry Core-SW2` command on SW-1 and receives the following CLI output:

text
Local Interface: Gi1/0/24
Chassis id: 0014.f2ca.8800
Port id: Eth1/1
Port Description: Trunk-to-Core1
System Name: Core-SW2

System Description:
Cisco Nexus Operating System (NX-OS) Software, Version 9.3(5)

Time remaining: 92 seconds
System Capabilities: B, R
Enabled Capabilities: B, R

Management Addresses:
IP: 10.1.100.2

Auto Negotiation: supported, enabled
Physical media capabilities:
1000baseT(FD)

Media Attachment Unit type: 30
VLAN Advertised: 10, 20, 99
Port VLAN ID (PVID): 99

Based on this LLDP detailed output, which conclusion correctly interprets the neighbor properties and link configuration?

  1. The local switch interface receiving the LLDP frames is GigabitEthernet1/0/24, while the remote neighbor's egress port is Ethernet1/1 with a native VLAN (PVID) of 99.Answer
  2. B
    The local switch interface connected to the neighbor is Ethernet1/1, and GigabitEthernet1/0/24 represents the remote switch interface.
  3. C
    The Port VLAN ID (PVID) of 99 indicates the native VLAN currently configured on the local switch interface GigabitEthernet1/0/24.
  4. D
    The neighbor device has an active LLDP holdtime set to 92 seconds, which is the default holdtime value for LLDP on Cisco switches.

Answer

The local switch interface receiving the LLDP frames is GigabitEthernet1/0/24, while the remote neighbor's egress port is Ethernet1/1 with a native VLAN (PVID) of 99.
In Cisco IOS LLDP detailed entries (`show lldp entry` or `show lldp neighbors detail`), 'Local Interface' indicates the interface on the local device where the frame arrived (Gi1/0/24). The 'Port id' field identifies the remote neighboring device's egress interface (Eth1/1). Furthermore, the 'Port VLAN ID (PVID)' field reflects the native VLAN advertised by the adjacent device for that link.

Step-by-Step Solution

1
Analyze the Local Interface and Port ID fields in the LLDP output
'Local Interface: Gi1/0/24' represents the local switch's port receiving the LLDP frame. 'Port id: Eth1/1' specifies the interface on the remote neighboring device (Core-SW2).
LLDP output explicitly distinguishes between the local port where traffic is received and the remote port ID advertised by the neighbor.
2
Examine the Port VLAN ID (PVID) TLV information
'Port VLAN ID (PVID): 99' is the native VLAN ID advertised by the neighbor device (Core-SW2) for its port Eth1/1.
IEEE 802.1AB LLDP includes Organizationally Specific TLVs that advertise port attributes of the remote sender, including its configured native VLAN (PVID).
3
Evaluate the remaining timer information
'Time remaining: 92 seconds' is the dynamic countdown timer before the cached entry expires, not the configured holdtime constant.
The default LLDP holdtime multiplier (4) with a default message frequency (30s) yields a default holdtime of 120s; 92 seconds is merely the current countdown state.

Key Concept

Interpretation of LLDP Detailed CLI Neighbor Data and IEEE 802.1AB TLVs
Estimated Time:2m 0s
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Topics

Layer 2 Discovery Protocols (CDP and LLDP)

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