What if devices are scattered? Try link layer Discovery Protocol

What if devices are scattered? Try link layer Discovery Protocol

In a large network, it is very important to know what is connected to the network. The larger the network, the more logical and physical it is-the harder it is to clarify its topology. In addition, redundant links and link aggregation will greatly increase the difficulty of clarifying the interconnection between devices in the network. In addition, the problem persists. Hybrid use of network products from different vendors will make device discovery protocols at a loss. However, it is not impossible to clarify the environment. Let me explain it in detail.

First, I want to tell a background story: I used to be in a team. Most of the environments we managed were Cisco devices. The word "majority" is important for device discovery. Like other network vendors, Cisco provides a private protocol that allows network administrators to view which devices are directly connected to a specific device. Of course, you must have administrator access to this device. Many readers must be familiar with the Cisco Discovery Protocol (CDP) below:

Figure 1 Cisco Discovery Protocol

Run the "show CDP neighbors" command to display the table of connected devices. Run "show CDP neighbors details" to expand the table and view the detailed connection information between vswitches (a VLAN matching error exists, right ?), And the detailed information of each device, including its management IP address information.

Figure 2

Now everything goes well, right? But what happens when a Cisco switch connects to another vendor's hardware and does not support CDP? Fortunately, we can also use the Link Layer Discovery Protocol (LLDP), which has the same powerful query functions as CDP and similar private Discovery protocols. LLDP is suitable for viewing the connection of adjacent network devices in a multi-vendor network, but people do not pay enough attention to it. Let's look at a real example.

I mentioned earlier that most of my environments are Cisco devices, but we also use products from other vendors in some special locations. We use Meraki products in most branch networks in the United States and overseas. However, like other third-party devices, Meraki access terminals (APS) cannot view the connection of switches through CDP. In other words, the "show CDP neighbors" command cannot view their information.

Use LLDP to view network changes

If we use LLDP, we will immediately see different results-the results are much better.

By default, the Cisco Catalyst Switch disables LLDP. We need to open the Protocol first, and then execute "show LLDP neighbors" to view these Meraki APs.

Then, Run "show LLDP neighbors detail" to show the details.

This is the result! Use LLDP to get better results. This Protocol is mature and is already a real standard (IEEE 802.1AB). It also displays information not only from the interconnection network infrastructure of different vendors. Many VoIP phones and analog terminal adapters support LLDP, and some CCTV (closed-circuit television) cameras and tool-type network hosts also support LLDP. This is one of the features that are easily ignored during the vswitch configuration process, because it is disabled by default, but as long as you start to use it, it will play a huge role in fault repair and environment ing.

Some environments use LLDP to allocate VoIP phones and other devices, but this is another topic. Now you must remember that like CDP, LLDP is a convenient tool for viewing devices. It can directly view connected devices (the next switch in the network ), however, its functions are not limited to this. To view all interconnect devices-from a core switch to a building aggregation switch and then to a boundary switch, run the "show LLDP neighbors" command for each switch ".