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Link Layer Discovery Protocol (LLDP) format

Link Layer Discovery Protocol (LLDP) format, Link Layer Discovery Protocol (LLDP) is a modular technique in which the Link Layer Discovery Protocol (LLDP) allows network devices operating at the lower layers of the protocol stack, such as Layer 2 bridges and switches, to learn some of the capabilities and characteristics of LAN devices available to higher-layer protocols, such as IP addresses. “IP”, where the information collected through the operation of the Link Layer Discovery Protocol (LLDP) is stored in a network device and queried using SNMP.


Link Layer Discovery Protocol (LLDP) format


Link Layer Discovery Protocol (LLDP) format


In LLDP, each device sends information from each of its interfaces at a fixed interval, in the form of an “Ethernet” frame, where each frame contains a single LLDP data unit, knowing each “ LLDPDU” is a series of Type Length Structures (TLV), where the “EtherType” field is set to “0x88cc”, each Link Layer Discovery Protocol (LLDP) frame starts with a topology ID, a port ID and a “TTL” of the lifetime or limit Leap.


Knowing that the frame ends with the end of a special “TLV” called “LLDPDU” where both the type and length fields are “0”, where the Link Layer Discovery Protocol (LLDP) specification allows organizations to define and encrypt their own “TLVs”, where these are called “TLVs”. Regulatory defined and starts with a value of “LLDP TLV Type” from “127”.


When the transmission countdown timer expires or if the LLDP information changes, it sends the LLDP proxy to LLDP frames and to LLDP-enabled neighbors ), where the Link Layer Discovery Protocol (LLDP) manager takes the information inside the “MIB” management information base and formats it into “TLVs”.


They are included in the Link Layer Discovery Protocol (LLDP), and when the proxy receives it checks to make sure it has the correct sequence of mandatory TLVs and then validates optional TLVs, if there are errors it will be dropped while the TLVs are stored valid in the neighborhood database.


1. Types of Link Layer Discovery Protocol (LLDP)


There are two types of regular LLDP and OFF advisory LLDP, where the regular LLDP provides management information about a local device to the device next door to that device, and this is the one that transmits” TLVs” mandatory and optional At the same time when a port is disabled Link Layer Discovery Protocol (LLDP) is disabled or the switch is restarted, the LLDP shutdown frame is sent to neighboring units, indicating that the LDP information The link (LLDP) is no longer valid.


2. Link Layer Discovery Protocol (LLDP) message structure


The Link Layer Discovery Protocol (LLDP) exchanges information through certain units of data called “LLDPDU.” This data unit consists of “TLVs” and each “TLV” field corresponds to a specific type and length. The “LLDP IEEE 802.1AB” standard contains Three “TLVs” are mandatory at the start of the Link Layer Discovery Protocol (LLDP) in the following order:


  • type 1 = the identifier of the chassis where it identifies the device.
  • type 2 = port identifier where it identifies the port.
  • Type 3 = where it tells the receiving device how long the received information should remain valid.


The LLDP IEEE 802.1AB standard contains three additional and optional TLVs at the start of the Link Layer Discovery Protocol (LLDP) in the following order:


  • Type 4 = port description where it displays details about the port.
  • Type 5 = system name where it displays the first name of the device.
  • Type 6 = System Description where it displays the software version.
  • Type 7 = System capabilities It will tell the basic function and capabilities of the device.
  • Type 8 = Administration Address where it displays the “IP” or “MAC” address of the device.
  • Type 0 = end where it indicates the end of the data unit.


3. Link Layer Discovery Protocol (LLDP) operating modes


Transmission Only Mode: Only the agent can transmit information about the capabilities and current state of the local system.


Receive-only mode: Only the agent can receive information about the capabilities and current state of remote systems.


Transmit and receive mode: the agent can transmit local system capabilities and status information, and receive remote system capabilities and status information.


4. LLDP Media Endpoint


LLDP-MED is an extension of the Link Layer Discovery Protocol (LLDP). This protocol is used specifically to support VOIP applications. LLDP-MED allows network discovery between network communication devices and media endpoints such as smartphones, phones By default, network devices send Link Layer Discovery Protocol (LLDP) packets only until they receive LLDP-MED packets from the endpoint device, which will then continue to send LLDP-MED packets until The remote device to which it is connected ceases to be able to "LLDP-MED".


Knowing that the Link Layer Discovery Protocol “LLDP” is a one-way protocol, where the basic process of the Link Layer Discovery Protocol “LLDP” consists of a device that is enabled to send the Link Layer Discovery Protocol of information that sends periodic advertisements of information in the frames of the “LLDP” protocol to Receiver.


5. Link Layer Discovery Protocol (LLDP) timing


Wait timer indicates the time or duration that an LLDP device keeps neighbor information before it aging defaults to “120 seconds”, if the timer expires and no LLDP packet is received, the other party information will be discarded.


Where the frequency timer relates to the time interval in which network devices send LLDP updates to neighboring devices, the default is “30 seconds”, knowing that “Reinit timer” is the delay time for LLDP initialization on any given device. interface, the default is “2 seconds”, and the Link Layer Discovery Protocol (LLDP) is disabled by default and can be activated via the “LLDP” run command.


How Link Layer Discovery Protocol (LLDP) works


The Link Layer Discovery Protocol must be enabled on the devices first, then the LLDP-enabled devices will send LLDP advertisements to each other and the device information is stored in MIB databases on the devices. Any network management software based on this data via “SNMP” and using this data a network map is created.


With the LLDP enabled, device information such as topology identification, port identifier, port description, system name and description, and device capacity is transmitted.