What is LLTD (Link Layer Topology Discovery)?

LLTD is a link-layer protocol used to determine the discovery of devices available in a network and quality of service, and it was developed and registered by Microsoft Inc.

What is LLTD (Link Layer Topology Discovery)?

The LLTD Protocol

LLTD protocol has two components, Mapper and Responder. Mapper controls network discovery and generates a network map. Responder provides Mapper with information about devices on the network.

This protocol determines the state of the network using an algorithm called RepeatBAND and devices on the network use this algorithm to calculate the amount of information or blocks transmitted over the network.

Microsoft developed LLTD based on Rally technology, and its purpose is to transfer the physical structure of the LAN and WLAN networks to which a computer is connected to the graphics environment.

Additionally, LLTD helps to optimize a network so that more resource usage can be determined for such high bandwidth-demanding operations such as limited bandwidth, media streaming.

Available internally on Windows Vista, Windows 7, and Windows Server 2008, this protocol graphically creates the topology of both local and wireless networks. In Windows XP, this is not available as an internal component, but for Service Pack 2 and Service Pack 3 this feature can be downloaded and installed on the 32-bit architecture of the system if desired. LLDP is not included for 64-bit versions of Windows XP.

How Does It Work?

LLTD operates on the link layer of the OSI model and can only discover devices on the local network. Therefore, if the network environment is divided into subnets via a Router, this protocol cannot reach devices on other subnets and discover information, as this protocol works only at the link layer.

Since the mapper is the main component that plays a major role in the mapping of the network, devices can be restricted or allowed only for a domain, public or private networks with the settings configured in the operating system during the discovery process.

In addition to discovering computers, switches, or hubs in the LAN, it also discovers other information such as MAC addresses, IP addresses.

It configures the physical state of the network as a visual topology with the information it obtains and adds features such as icons and connections on devices. For example, when the gateway includes the device in the topology, clicking this device on the interface automatically opens the default web browser installed in the system to log into the management interface and initiates an HTTP or HTTPS session.

It also diagnoses the network functionality in detail and determines the amount of bandwidth available in different parts of the network by evaluating the data flow. In this way, certain processes can be prioritized to maximize bandwidth.

Working Principle

For the link-layer topology discovery to work, both Mapper and Responder services must be installed and enabled on the computers in the network. For example, in Windows 7, clicking on the network adapter connected to the network in the network and sharing center can check whether these options are enabled or not from the Ethernet Properties section.

In Windows 8 and Windows 10 systems, this feature has been removed, but with 3rd party tools, devices on the network can be scanned and a visual map of the network can be created. However, these services must be enabled on the network adapters.

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