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Difficulty: Very hardLayer 2 Discovery Protocols (CDP and LLDP)

An engineer is evaluating Layer 2 discovery protocol operations on a Cisco Catalyst switch connected to multi-vendor infrastructure. The switch is running the following active configuration:

text lldp run ! interface GigabitEthernet1/0/12 description Link to Non-Cisco Switch no cdp enable lldp transmit lldp receive ! interface GigabitEthernet1/0/24 description Link to VoIP Phone cdp enable no lldp transmit

During a network audit, the engineer issues the command `show lldp neighbors detail` and receives the following partial CLI output:

text ------------------------------------------------ Local Interface: Gi1/0/12 Chassis id: 0021.a012.3456 Port id: Eth1/1 Port Description: Ethernet1/1 System Name: SW-CORE-NEXUS System Description: Cisco Nexus Operating System (NX-OS) Software Time remaining: 105 seconds System Capabilities: Bridge, Router Enabled Capabilities: Bridge Management Addresses: IP: 10.1.100.2 Auto Negotiation: supported, enabled Physical media capabilities: 1000baseT(FD) Media Attachment Unit Type: 30 VLAN ID: 10

Which two conclusions accurately describe the protocol behavior and neighbor state for this switch based on the configuration and command output?

  1. The local interface GigabitEthernet1/0/12 is successfully processing IEEE 802.1AB Type-Length-Value (TLV) data frames from the remote neighbor SW-CORE-NEXUS.Answer
  2. B
    The neighbor device SW-CORE-NEXUS will fail to discover the local switch because CDP is globally disabled when LLDP is enabled.
  3. Interface GigabitEthernet1/0/24 will fail to transmit LLDP Data Units (LLDPDUs) to the connected VoIP phone, but it will continue sending CDP advertisements.Answer
  4. D
    The default LLDP holdtime of 180 seconds is currently decrementing for the neighbor entry on GigabitEthernet1/0/12.

Answer

The local interface GigabitEthernet1/0/12 is processing IEEE 802.1AB Type-Length-Value (TLV) frames, and interface GigabitEthernet1/0/24 will refrain from transmitting outbound LLDP frames while retaining active CDP transmission.
LLDP relies on IEEE 802.1AB standard TLVs to exchange information between multi-vendor devices, which is demonstrated by the detailed attributes shown for SW-CORE-NEXUS on GigabitEthernet1/0/12. Furthermore, interface GigabitEthernet1/0/24 explicitly disables LLDP transmissions via 'no lldp transmit' while keeping CDP active via 'cdp enable'.

Step-by-Step Solution

1
Analyze interface GigabitEthernet1/0/12 configuration and protocol standard.
Interface Gi1/0/12 has CDP disabled and LLDP enabled ('lldp transmit' and 'lldp receive'). LLDP operates under IEEE 802.1AB using TLVs.
Verify standard protocol characteristics for LLDP neighbor discovery.
2
Evaluate interface GigabitEthernet1/0/24 interface commands.
'no lldp transmit' suppresses outgoing LLDP messages on Gi1/0/24, while 'cdp enable' maintains active CDP outbound frames.
Determine granular protocol control per interface.
3
Compare default CDP vs. LLDP holdtime timers against the CLI output.
CDP default holdtime is 180 seconds, while LLDP default holdtime is 120 seconds. Output shows 'Time remaining: 105 seconds', confirming a 120-second starting value for LLDP.
Differentiate default timer parameters between CDP and LLDP.

Key Concept

Layer 2 Discovery Protocol (CDP and LLDP) Operation, IEEE 802.1AB TLV Structure, and Per-Interface Control
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Topics

Layer 2 Discovery Protocols (CDP and LLDP)

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