Original subtitles

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So let’s do a capture on this link between router 1 and router 2

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we can see some loop messages which are used to ensure that the interface is up

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and we may have to wait a while for CDP message to be sent.

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We can see that by typing sh cdp and pressing enter.

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CDP only sends packets every 60 seconds

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so we may have to wait a while as you can see in the output here

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CDP packets are sent every 60 seconds

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and an entry in the table is held for 180 seconds

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I’ll show you more about that later but scrolling down

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here’s a CDP message

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we can see that it's using 802.3 Ethernet

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and is using a specific Cisco address which I'll talk about in more detail later.

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It's using logical link control with the organizational or unit of Cisco

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in other words, the protocol used at a high layer is CDP

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and here’s the CDP information version 2

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we can see that it's router 2 advertising itself

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with it's platform, port number and other details

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what’s interesting is you can get a lot of detail through a CDP message

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which is great for discovering the network

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but is bad from a security point of view

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you can see version of software, you can see platform

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you can see IP addresses configured, so IP address port number

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you can see its capabilities and so forth

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here's router 1, so you can see the device ID is router 1, as an example

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So the important thing to remember is the CDP does not rely

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on higher layer protocols, messages are sent every 60 seconds by default.

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You can use CDP to map an entire network

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so as an example, if you were connected to router 1

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and you use the command sh cdp neighbors

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you could discover which device you directly connected to

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and then you could use details

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so show CDP neighbor details

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to see detailed information about that neighbor including its IP address

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and then if telnet is enabled on your network

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you could telnet from router 1 to router 2

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and then use the command CDP neighbors

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to discover what devices are connected to that router.

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So as an example, here’s a core 3750 router

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let’s see what's connected to that switch.

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So I could use this command sh cdp entry

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and the name of the device, to just see the output for the specific device

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here’s its IP address, so I'll telnet to that IP address

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I can log in assuming that I know the username and password

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and once again I could use the command

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sh cdp neighbors

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to discover what's connected to that device.

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And here as an example, we can see some other devices

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which we weren't seeing previously, such as the Cisco 1841 router.

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So you could map out the entire network by simply using CDP on 1 device

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and telnetting to the next device using CDP

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to discover what’s connected to that and then telnetting to the next device

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and discovering what’s connected to that device.

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Now the Cisco APIC-EM SDN software does a lot of this for you

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and so does othersSCNMP or Simple Network Management Protocol

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management software such as software from Solarwinds

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you could do this manually but there is software out there

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that can help you automate this process.

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You could also use a combination of protocols.

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So as an example, here’s router 2

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sh cdp neighbors

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shows me that I have this core 3750 switch

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I could then discover the IP address of that switch

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which in this example is this and then I could simply telnet to that switch

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and then use a protocol such as LLDP to discover devices connected to that switch.

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In this example, you can see the 2 Cisco IP phones, and an Ubuntu hypervisor

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I could look at details.

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So sh lldp neighbors detail

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and here I can see the IP address

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and model of phone as well as the firmware being used by that phone

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and other information about it.

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Scrolling down, lot of detail notice firmware revision

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serial number and quite a bit of other information

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such as the layer 2 quality of service and layer 3 quality of service

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set on that phone, we'll talk more about quality of service later.

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Here's the Cisco DX650, so we can see its system name

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we can see an IP address we can see a firmware version

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we can see quality of service information

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we can see once again that it's a Cisco phone

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we can see how much power is being used a lot of detail

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here’s Linux so we can see Ubuntu server running Ubuntu 14.04 LTS

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you can see the name of the server and it's IP address.

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So once again CDP and LLDP allows you to discover devices in your network

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and also allow you to see how the devices

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are connected to each other in your network.