Mar 4, 2014Articles

Link Layer Discovery Protocol, LLDP-Med Discovery Methods & Provisioning

The IEEE 802.1ab standard defines a method for Media Endpoints such as IP Phones and Ethernet network devices such as switches and routers to exchange information with each other and to store this information in a Management Information Base (MIB).

LLDP provides administrators with the ability to inventory and map network infrastructure, giving architects vital information for design and deployment,  as well as providing a diagnostic tool for common Voice over IP (VoIP) issues. LLDP-MED (LLDP Media Endpoint Discovery) can also be used by administrators to generate Enhanced 911 maps for IP endpoints. Additionally, through LLDP an administrator can verify that all IP Phones are configured with the correct settings as well as validating speed and duplex checks to ensure a resilient voice network.

LLDP Discovery methods

LLDP Name TLV, Network Policy Discovery TLV

Participation in the Voice VLAN can be assigned by use of the Media Endpoint Discovery(802.1ab, LLDP-MED) to interpret the LLDP Name TLV (Type Length Value) packet delivered by the phone. e.g., when the IP Phone sends an LLDP-MED packet (LLDPDU or LLDP Data Unit) to the data switch identifying itself and requesting participation in the “voice” VLAN, the name is interpreted by the data switch and the LLDP-MED response from switch-to-phone instructs (via Network Policy) the IP Phone to assign itself certain configuration values.

NOTE: LLDP is not supported on the PC port of the Avaya CS1000 IP Phone.

Automatic Detection, Automatic Configuration

Avaya ADAC is a proprietary extension of the LLDP standard which facilities endpoint recognition and configuration. Utilizing the MAC address of the endpoint and the LLDPprotocol, ADAC can speed discovery and configuration in LLDP environments. Avaya data switches can be configured with ADAC and come preconfigured with a list of MAC addresses that are standard for the Avaya 2000, 1100 and 1200 series IP Phones. Additional MAC addresses and ranges can be configured to  support other devices. ADAC is largely outside of the scope of this series, but for more information, see the Ethernet Routing Switch documentation.

LLDP can be used to configure:

  • VLAN Membership (802.1q)
  • Differentiated Service Control Point (DSCP) values for Layer-3 Quality of Service(QoS)
  • Control pBits (802.1p)
  • Media pBits (802.1p)

DHCP mnemonic for LLDP enable state

To enable LLDP, use the DHCP keyword=value


To disable use


TIP: As mentioned in the last DHCP configuration article, LLDP can be disabled via DHCP. Disabling LLDP provisioning when not used can speed the IP Phone boot up process by up to 18% by removing the boot delay caused by the phone engaging in LLDP-MED.

TIP: While on the topic of the network, another tip is to use Spanning Tree Group Fast Learning (i.e., Cisco Spanning Tree Group PortFast) on all VoIP ports to decrease boot up times on IP Phones. Spanning Tree Group normal behavior is to block all IP traffic for the first 30 seconds after a device becomes active on a port (powers up, plugged in, etc.). Spanning Tree Group can interfere with and/or delay boot up processes. Fast Learning (i.e., Cisco PortFast) skips the 30 second learning state and goes immediately to forwarding. Under Fast Learning, a network port only enters Learning state after a Bridge Protocol Data Unit (BPDU) is received and forwarded. Use of Fast Learning can cause hiccups in the network when ports enter a learning state and is best used when the port configured with Fast Learning will only be used for endpoints (i.e., not connected to another data switch.)

Provisioning Precedence Order

  1. Manual entry
  2. Automatic provisioning via 802.1ab (LLDP), including Avaya ADAC (Auto Detection, Auto Configuration).
  3. Info block via TFTP
  4. Info block via DHCP
  5. Info block via HTTP
  6. LPR (Last provisioned value)
  7. UNIStim (via Element Manager configuration)
  8. Factory default

WARNING: IP 2000 Series IP Phones do not support the precedence rule and use the last value received.

We’ll talk more about TFTP and HTTP provisioning in the next article.


If you enable stickiness, then the IP Phone will remember previous automatic configuration values. e.g., if you configure LLDP to be disabled via DHCP, then configure stickiness, then the first time the phone boots (or any boot after the NVRAM is cleared) it will seek LLDP, but subsequent boots will disable LLDP.

However, making lldp=n part of your DHCP string provides administrators with the ability to re-enable LLDP via DHCP. (As opposed to manually disabling LLDP on each phone, which would require touching each phone to re-enable LLDP.)

WARNING: For refurb phones, this can cause configuration issues if the phone remembers it’s NVRAM settings from the last deployment having values that the new environment does not configure. Whenever troubleshooting Refurbished IP Phones, always include a reset to factory default as part of your troubleshooting processes.

DSCP Override

In LLDP environments an administrator may sometimes want to manually set the DSCPpriority of certain phones. DSCP Override provides this functionality by allowing administrators to override DSCP configuration obtained from LLDP with other configuration settings (e.g., DHCP, TFTP or HTTP).

DSCP Override changes the precedence order by removing LLDP automatic provisioning from the precedence list (for DSCP settings only.)

For example, an administrator wants to classify some IP Phonesets as business critical for QoS while others are less critical. The administrator decides that during a congestion event where prioritization of media traffic is critical, some phones should degrade before others. The administrator can use DSCP Override to apply VLAN, DSCP and pBit settings to all phones via LLDP, then the administrator can utilize another provisioning method to override the LLDP policy (e.g., the NOC IP Phones are configured to override standard LLDP-MED provisioning with an increased DSCP priority. This way, during a congestion event the NOC will retain phone QoS at a higher level for longer than other business phones.  Likewise, C-level IP Phonesets could be similarly prioritized so their calls are less likely to be impacted by QoS issues.)

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