What is Layer 2 and Layer 3 discovery and its significance in simple terms?

[Suggy]

ServiceNow Discovery -

1. can anyone explain what is Layer 2 and Layer 3 discovery in simple terms?

2. OOB layer 3 discovery is enabled via system properties [glide.discovery.L3_mapping]. What will happen if I disable them? I mean what use case I will miss out?

[James Behrens]

I suspect that corresponds to OSI layers, I'd be surprised if it did not.

What is the OSI Model? | Cloudflare

[Suggy]

Anyone please

[James Behrens]

@Suggy , are you looking to speed up your discovery? Why would you disable it?

from the Tivoli website, this one liner might sum it up. 

A Layer 3 discovery is quicker, but the results of a Layer 3 discovery cannot be used for root cause analysis. A Layer 2 discovery is more detailed and the results can be used for root cause analysis.

From the CloudFlare about Layer 2

The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the same network. The data link layer takes packets from the network layer and breaks them into smaller pieces called frames. Like the network layer, the data link layer is also responsible for flow control and error control in intra-network communication (The transport layer only does flow control and error control for inter-network communications).

From ServiceNow Docs

During the discovery of a network device, Discovery creates records in the Router Interface [dscy_router_interface] table and the Switchport [dscy_switchport] table. This information contains network adapter information for that device. For SNMP-enabled devices, Discovery gathers the information from a routing probe during the exploration phase. The Layer 2 protocol cache probe runs next to collect neighbor data from the device.

...

When device discovery is complete, the system runs the Service Discovery - Device Complete business rule on the Device [discovery_device_history] table. This business rule uses several strategies, together with information from various tables to create Connects to:Connected by relationships in the CMDB

I suspect from all that, if you disabled it, it would not run that Layer 2 protocol cache probe and that probably WOULD speed up your discovery. You would not get those Connects to:Connected by relationships it seems, at a minimum.

It's an interesting question to be sure. Curious what the more experienced will say about the topic.

[James Behrens]

@Suggy do you know what sys_properties those are? I am not finding a reference to them?