Original subtitles

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Sometimes you believe the network is cabled in a certain way

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whereas in reality it's cabled differently

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so you may think that 1 device is connected to another device

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but that’s not actually how it's cabled.

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Cisco discovery protocol or CDP

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and the industry standard version Link Layer Discovery Protocol or LLDP

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are protocols that run at layer 2 in the OSI model

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and allow you to discover how devices are connected to each other

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this runs independently of protocol such as IPv4 and IPv6

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CDP and LLDP are a great way to discover how devices are connected

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and allow you to verify documentation as well as great new documentation.

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CDP is Cisco propriety and well typical only discover other Cisco devices.

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please be aware that some vendors such as HPE do not send CDP messages

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but will receive CDP messages

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and hence they can see which Cisco devices are connected directly to them

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but a Cisco devices will not be able to see the switches

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and routers from other vendors

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such as HPE unless the Cisco devices are enabled for LLDP.

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CDP uses multicast frames which I'll demonstrate in a moment

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but once again doesn’t rely on higher layer protocols.

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We'll start with the simple GNS3 topology

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and then I’ll show you a more complex 1 in a different video.

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In this GNS3 topology, I’ve got 3 Cisco routers.

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one of them is connected to a hub which is bridge to my Local Area Network

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which will allow me to see physical devices in the network.

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Router 1 and router 2 are configured with IP addresses

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on their FastEthernet 0 interfaces, router 2 is using DHCP on this interface

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but this network hasn’t been configured.

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So let’s see how that affects CDP.

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Firstly on router 1, sh cdp neighbors

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What you'll notice is it sees the directly connected neighbor router 2

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but no others neighbors.

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I could also use the details command

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to see detailed information about that neighboring router.

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you can see as an example the IP address of the neighbor

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the platform, the capabilities, in other words, this supports routing

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it supports switching and it supports IGMP

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IGMP is used in multicast applications.

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We could see the version of operating system that that router is using.

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on router 2 sh cdp neighbors this shows a lot more detail

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we can see router 1 once again, connected via FastEthernet 0/0

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we can see its a 3725 router

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but in addition, we can see a Cisco Unified Communication Manager

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this is a Linux server that runs the Cisco Unified Communication Manager software

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that’s running within the VMware and is using Ethernet 0

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somewhere in the cloud here to connect back through the hub to router 2.

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Now CDP only shows you directly connected devices that are running CDP

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so as an example, router 1 doesn’t see these devices

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because router 1 is separated from them by router 2

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but CDP messages are sent through a hub

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and any other layer 2 switches in the topology that are not running CDP

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so there are actually some Cisco switches in this cloud

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that connect router 2 to some of these devices such as the IP phones

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but those Cisco switches are not running CDP

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and hence are not seen in the topology.

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So be careful CDP shows you directly connected devices that are running CDP.

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This hub as an example is not running CDP

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it's layer 1 device so CDP messages are transparently sent through it

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to devices in the cloud

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and they transparently send CDP messages back to router 2.

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In addition, there are layer 2 Cisco switches

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these are small personal switches that are not running CDP

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and they don’t show up in the topology

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these 2 phones as an example are powered by Cisco switch.

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So CDP does show you directly connected neighbors

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that would be the exam answer, in the real world, it’s not as simple as that.

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Let’s look at some details so sh cdp neighbor detail

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We can see some information about the Cisco Unified Communication Manager server

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you can see as an example that it’s running on Linux

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we can see a Cisco 3750 switch here

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scrolling down, we can see a Cisco IP phone

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notice it's Cisco DX650 that’s it's IP address.

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We could as an example, open up a web browser to that phone.

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In this case, the phone is supporting a web browser

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so we can see details about the phone including its telephone number.

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CDP is great for discovering devices on the network

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but also is a security risk because you can discover the devices on the network.

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That may be good for you but maybe bad if you have a hacker

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or someone inquisitive to find out what you’re running.

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So as an example, I can connect to the Cisco Communication Manager

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and if I knew the username and password or hack it

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I could then discover other devices in the topology.

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So back on router 2, there’s a CUCM, there’s a 3750

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here’s my Cisco phone DX650, I can see as an example

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the power that's drawn by that phone

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I can see the operating system that it's running

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I can see which port it's using to connect back to router 2

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so router 2 is using F0/1 that phone is using port 1.

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Here’s another phone Cisco 7970, its port is 1

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the local interface on the router is FastEthernet 0/1

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here’s router 1 its outgoing port is FastEthernet 0/0

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and router 2 is local interface is FastEthernet 0/0.

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In this case, the router can see itself

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because CDP message is being loop back to the router in the cloud

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but once again sh cdp neighbors

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we can see the local interface that's receiving CDP messages

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and we can see devices

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and their local interfaces is used to connect back to this local router.

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now, what about R3? It’s not showing in the topology

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and that’s because the interface isn't up, this interface is shutdown

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so I’m gonna no shut that interface

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but I’m not going to enable an IP address on the interface.

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So we'll enable this link but configure no IP addresses.

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So interface f0/0 no shut but that’s all we’ve done.

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sh cdp neighbor

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no neighbors are shown yet

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it takes a while for CDP messages to be sent

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you might have to wait 30 seconds before you see any CDP messages.

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but sh ip int brief

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will allow us to see the state of the interface, FastEthernet 0/0 is up up

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on router 3, router 3 is connected and FastEthernet 0/0 to router 1

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and FastEthernet 0/1 to router 3

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router 3 is connected using FastEthernet 0/0

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to router 2 is fast Ethernet interface 1/0.

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sh cdp neighbors

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we can now see router 2

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the reason why the domain is showing up is I've configured

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a domain name of cisco.com on router 2

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so the local router interface is F0/0 and the interface of router 2 is F1/0

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but please be aware that there are no IP addresses configured on this router.

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so sh run int f0/0 sh ip int brief

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what you’ll see is no IP addresses are assigned to any interfaces on this router

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but CDP still works because CDP does not rely on IP.

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Let’s have a look at detail

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so on router 3 sh cdp neighbor detail

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we're seeing router 2, we're seeing it's IP address

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the local interface of router 3 is F0/0

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and the interface used on router 2 is F1/0

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the outgoing port is the port of the remote device in this output

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which can be very confusing

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That’s a basic example of CDP and the next video I'll do a Wireshark capture

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and then on subsequent videos, we'll look at a more complex topology.