CDP & LLDP

Cisco Discovery Protocol (CDP)

CDP is a Layer 2, media- and protocol-independent protocol used by Cisco devices to advertise and discover directly connected neighbors. It operates over interfaces that support SNAP headers and is enabled by default on most Cisco platforms.

Key Points

• Devices send CDP advertisements every 60 seconds to the multicast MAC address 01:00:0C:CC:CC:CC.
• Each advertisement includes a Time-To-Live (TTL) value, indicating how long the information should be retained.
• CDP never forwards packets beyond the local segment.
• CDP supports TLVs (Type-Length-Value structures) to carry various device information.
• CDP is Cisco proprietary. Use LLDP for multi-vendor environments.

CDP Configuration

! Enable CDP globally
cdp run

! Disable CDP on a specific interface
interface Ethernet1/1
  no cdp enable

! Adjust advertisement interval and hold time (in seconds)
cdp timer 5
cdp holdtime 10

! Disable version 2 advertisement extensions
no cdp advertise-v2

! Define a TLV filter group (use '?' to view available TLVs)
cdp tlv-list GROUP_1
  ?

! Apply TLV filter to restrict CDP advertisements
cdp filter-tlv-list GROUP_1

! Apply TLV filter to a specific interface
interface Ethernet1/1
  cdp filter-tlv-list GROUP_1

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Link Layer Discovery Protocol (LLDP)

LLDP is the IEEE standard (802.1AB) for Layer 2 device discovery. It functions similarly to CDP but is supported across multi-vendor environments. It also uses TLVs for information exchange.

Key Points

• LLDP is not enabled by default on some platforms — you must enable it globally and per interface.
• LLDP advertisements include a TTL and are multicast locally on each supported interface.
• LLDP supports selective TLV advertisement using tlv-select.

LLDP Configuration

! Enable LLDP globally
lldp run

! Disable LLDP on a specific interface
interface Ethernet1/1
  no lldp enable

! Adjust advertisement interval and hold time (in seconds)
lldp timer 5
lldp holdtime 10

! View or restrict TLVs from being advertised
no lldp tlv-select ?

! Disable specific TLVs on an interface
interface Ethernet1/1
  no lldp tlv-select ?