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

A network administrator is replacing legacy Cisco access switches with multi-vendor infrastructure. On a Cisco Catalyst switch (SW-Access1), the administrator attempts to selectively enable Link Layer Discovery Protocol (LLDP) on interface GigabitEthernet1/0/5 to discover an adjacent non-Cisco VoIP gateway while disabling Cisco Discovery Protocol (CDP) on that specific link.

The administrator enters interface configuration mode and attempts the following command sequence:

SW-Access1(config)# interface gigabitethernet1/0/5
SW-Access1(config-if)# no cdp enable
SW-Access1(config-if)# lldp enable

The switch returns an error: `% Invalid input detected at '^' marker` pointing to the `enable` keyword.

Which combination of commands must the administrator issue to globally activate LLDP and correctly enable both transmission and reception of LLDP packets on interface GigabitEthernet1/0/5?

  1. Execute 'lldp run' in global configuration mode, and execute both 'lldp transmit' and 'lldp receive' in interface configuration mode.Answer
  2. B
    Execute 'lldp enable' in global configuration mode, and execute 'lldp run' in interface configuration mode.
  3. C
    Execute 'cdp run' in global configuration mode, and execute 'lldp tlv-select med-tlv' in interface configuration mode.
  4. D
    Execute 'lldp run' in global configuration mode, and execute 'lldp enable' in interface configuration mode.

Answer

Execute 'lldp run' in global configuration mode, and execute both 'lldp transmit' and 'lldp receive' in interface configuration mode.
The correct answer specifies 'lldp run' in global configuration mode and both 'lldp transmit' and 'lldp receive' in interface configuration mode. In Cisco IOS, CDP and LLDP differ significantly in CLI command syntax and defaults. CDP is enabled globally by default and uses 'cdp enable' per interface. Conversely, LLDP is disabled globally by default (requiring 'lldp run') and controls interface communication via directional parameters ('lldp transmit' and 'lldp receive').

Step-by-Step Solution

1
Identify the global configuration command required to enable LLDP across the Cisco Catalyst switch.
Unlike CDP which is enabled globally by default ('cdp run'), LLDP is disabled globally by default and must be enabled using the global configuration command 'lldp run'.
Global activation of the LLDP process is a prerequisite before interface-level behavior can be configured.
2
Analyze interface-level configuration syntax differences between CDP and LLDP in Cisco IOS.
CDP uses a simple binary interface toggle command ('cdp enable' / 'no cdp enable'). LLDP does not use the 'enable' keyword at the interface level; instead, it provides directional control using 'lldp transmit' and 'lldp receive'.
Attempting to enter 'lldp enable' on a Cisco IOS interface causes an invalid input error because 'enable' is not a valid sub-command under 'lldp'.
3
Synthesize the required global and interface commands to achieve the administrator's goal.
Issue 'lldp run' globally, and under interface GigabitEthernet1/0/5 issue 'no cdp enable', 'lldp transmit', and 'lldp receive'.
This setup disables CDP on the specific interface while enabling bidirectional LLDP communication with adjacent multi-vendor devices.

Key Concept

Cisco IOS CLI Syntax and Directional Operational Differences Between CDP and LLDP
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Topics

Layer 2 Discovery Protocols (CDP and LLDP)

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