1 00:00:00,000 --> 00:00:05,000 In a previous video, I configured this switches and routers in this topology 2 00:00:05,000 --> 00:00:11,000 with IP addresses as well as enabling the various ports. 3 00:00:11,000 --> 00:00:15,000 Let’s have a look at CDP or Cisco Discovery Protocol 4 00:00:15,000 --> 00:00:19,000 and LLDP or Link Layer Discovery Protocol 5 00:00:19,000 --> 00:00:22,000 and see if we can discover the links between the devices. 6 00:00:22,000 --> 00:00:30,000 So I'll start with switch 1, here's switch 1 type enable sh cdp neighbor 7 00:00:30,000 --> 00:00:36,000 In the output, we can see switch 2, switch 3 and switch 4 8 00:00:36,000 --> 00:00:42,000 switch 2 is connected on 2 ports gigabit 0/0 9 00:00:42,000 --> 00:00:48,000 this local interface as well as gigabit 0/1, this local interface. 10 00:00:48,000 --> 00:00:56,000 switch 2 is using gigabit 0/0 to connect to our local interface gigabit 0/0 11 00:00:56,000 --> 00:00:59,000 and that’s true in our GNS3 topology 12 00:00:59,000 --> 00:01:06,000 gigabit 0/0 on switch 2 is connected to gigabit 0/0 on switch 1 13 00:01:06,000 --> 00:01:10,000 and the same is true for gigabit 0/1. 14 00:01:10,000 --> 00:01:17,000 Switch 3, this device over here is connected to switch 1 on gigabit0/2 15 00:01:17,000 --> 00:01:25,000 so this local interface gigabit0/2 is the interface that switch 3 is connected to 16 00:01:25,000 --> 00:01:29,000 and switch 3 is using gigabit 0/0, as you can see over here. 17 00:01:29,000 --> 00:01:32,000 By the same token switch 4 18 00:01:32,000 --> 00:01:37,000 this switch over here in the GNS3 topology is connected to switch 1 19 00:01:37,000 --> 00:01:42,000 on gigabit 0/3 and it's using gigabit 0/1. 20 00:01:42,000 --> 00:01:49,000 So Cisco Discovery Protocol or CDP runs by default on Cisco switches 21 00:01:49,000 --> 00:01:51,000 and routers and is a nice way to see 22 00:01:51,000 --> 00:01:54,000 how devices are connected in the topology. 23 00:01:54,000 --> 00:01:59,000 Notice, however, that you only see directly connected devices. 24 00:01:59,000 --> 00:02:05,000 So switch 1 over here doesn’t see router 1 25 00:02:05,000 --> 00:02:09,000 and doesn’t see router 3 it only sees the directly connected switches 26 00:02:09,000 --> 00:02:11,000 that are running CDP. 27 00:02:11,000 --> 00:02:13,000 Let’s have a look at switch 2. 28 00:02:13,000 --> 00:02:17,000 So switch 2 sh cdp neighbor 29 00:02:17,000 --> 00:02:20,000 the commands shows us the neighboring devices 30 00:02:20,000 --> 00:02:26,000 we can see switch 3, router 3, switch 1, switch 1 and switch 4 31 00:02:26,000 --> 00:02:29,000 so notice that on switch 2 32 00:02:29,000 --> 00:02:35,000 this switch, we can see router 3 being connected to gigabit 0/1 33 00:02:35,000 --> 00:02:39,000 using FastEthernet 0/0 34 00:02:39,000 --> 00:02:44,000 we can also see that it’s a 3725 router 35 00:02:44,000 --> 00:02:47,000 and we can see capabilities of that device 36 00:02:47,000 --> 00:02:50,000 in other words, its router, it can act as a switch 37 00:02:50,000 --> 00:02:56,000 and it supports IGMP which is used in multicast environments. 38 00:02:56,000 --> 00:02:58,000 So switch 2 can see router 3 39 00:02:58,000 --> 00:03:03,000 but switch 1 cannot because Cisco Discovery Protocol 40 00:03:03,000 --> 00:03:06,000 only shows you directly connected neighbors 41 00:03:06,000 --> 00:03:11,000 not devices connected behind a device running CDP. 42 00:03:11,000 --> 00:03:13,000 In other words, switch 1 can see switch 2 43 00:03:13,000 --> 00:03:16,000 but cannot see router 3 44 00:03:16,000 --> 00:03:19,000 where switch 2 is able to see router 3 45 00:03:19,000 --> 00:03:23,000 but by the same token switch 2 is not able to see router 2. 46 00:03:23,000 --> 00:03:27,000 To see router 2 we need to connect to switch 4. 47 00:03:27,000 --> 00:03:31,000 So here’s switch 4 sh cdp neighbor 48 00:03:31,000 --> 00:03:38,000 Notice switch 4 is able to see switch 2 up there 49 00:03:38,000 --> 00:03:40,000 it’s able to see switch 1 over there. 50 00:03:40,000 --> 00:03:43,000 And it’s able to see router 2. 51 00:03:43,000 --> 00:03:47,000 It can also see that router 2 using FastEthernet 0/0 52 00:03:47,000 --> 00:03:52,000 switch 1 is using gigabit 0/2 to connect to switch 4 53 00:03:52,000 --> 00:03:58,000 and switch 1 is using gigabit 0/3 to connect to switch 4. 54 00:03:58,000 --> 00:04:00,000 Lastly, let’s have a look at switch 3 55 00:04:00,000 --> 00:04:04,000 switch 3 is over here in our topology. 56 00:04:04,000 --> 00:04:07,000 sh cdp neighbor 57 00:04:07,000 --> 00:04:11,000 it can see router 1, it can see switch 1, as well as switch 2. 58 00:04:11,000 --> 00:04:15,000 So CDP is a very useful protocol once again 59 00:04:15,000 --> 00:04:17,000 to discover devices 60 00:04:17,000 --> 00:04:20,000 that are connected to your local router or switch. 61 00:04:20,000 --> 00:04:22,000 What about the routers? 62 00:04:22,000 --> 00:04:26,000 So on router 1 sh cdp neighbor 63 00:04:26,000 --> 00:04:30,000 Router 1 can only see switch 3 64 00:04:30,000 --> 00:04:32,000 as being directly connected to FastEthernet 0/0 65 00:04:32,000 --> 00:04:40,000 so its local interface and switch 3 is using gigabit 0/2 to connect to router 1. 66 00:04:40,000 --> 00:04:43,000 If you were connected to the console of 1 switch 67 00:04:43,000 --> 00:04:46,000 and you need it to discover your topology 68 00:04:46,000 --> 00:04:50,000 what you could do as an example is type sh cdp neighbors 69 00:04:50,000 --> 00:04:53,000 So like on switch 1 I can see my neighbors 70 00:04:53,000 --> 00:04:55,000 but then I can also do this option 71 00:04:55,000 --> 00:04:57,000 sh cdp neighbors detail 72 00:04:57,000 --> 00:05:03,000 so sh cdp neighbors detail 73 00:05:03,000 --> 00:05:07,000 shows that switch 3 is directly connected to switch 1 74 00:05:07,000 --> 00:05:12,000 but now I can also see the IP address of that switch 75 00:05:12,000 --> 00:05:16,000 so I could telnet from my local switch to that switch 76 00:05:16,000 --> 00:05:22,000 and then use CDP to see the neighbors connected to that device. 77 00:05:22,000 --> 00:05:27,000 Notice I can clean a lot of information about the neighboring device through CDP. 78 00:05:27,000 --> 00:05:30,000 So I can see that it’s a Cisco device 79 00:05:30,000 --> 00:05:33,000 that supports routing and switching in IGMP 80 00:05:33,000 --> 00:05:40,000 I can see my local interface and the outgoing port of the neighboring device. 81 00:05:40,000 --> 00:05:44,000 I can see the version of operating system running on that device 82 00:05:44,000 --> 00:05:47,000 so version 152 as an example. 83 00:05:47,000 --> 00:05:51,000 I can see which version of CDP it's running, so it’s running version 2 84 00:05:51,000 --> 00:05:55,000 I can see its management IP address once again. 85 00:05:55,000 --> 00:05:57,000 See the native VLAN, see the duplex 86 00:05:57,000 --> 00:06:02,000 so a lot of information can be shown through CDP 87 00:06:02,000 --> 00:06:06,000 about neighboring devices and that’s why for security reasons 88 00:06:06,000 --> 00:06:12,000 company turn off CDP typically on Internet facing interfaces 89 00:06:12,000 --> 00:06:15,000 or interfaces facing the outside world. 90 00:06:15,000 --> 00:06:18,000 So on router 3 as an example 91 00:06:18,000 --> 00:06:21,000 sh cdp neighbors 92 00:06:21,000 --> 00:06:25,000 shows me that there is a core 3750 switch 93 00:06:25,000 --> 00:06:32,000 connected out of F0/1, that is a physical Cisco switch 94 00:06:32,000 --> 00:06:36,000 running in my local home network. 95 00:06:36,000 --> 00:06:43,000 So you might not want your networking devices to advertise themselves 96 00:06:43,000 --> 00:06:47,000 through Internet connected interfaces 97 00:06:47,000 --> 00:06:51,000 because now I can discover what IP address that device using. 98 00:06:51,000 --> 00:06:54,000 Now I can see what type of device it is. 99 00:06:54,000 --> 00:07:02,000 So it's 3750 but what I could do now is telnet to that device and try to access it. 100 00:07:02,000 --> 00:07:06,000 So I could log in now 101 00:07:06,000 --> 00:07:12,000 and notice now I’m connected to this physical Cisco switch. 102 00:07:12,000 --> 00:07:15,000 So from a security point of view 103 00:07:15,000 --> 00:07:20,000 you probably wanna hide your devices 104 00:07:20,000 --> 00:07:23,000 and therefore you wanna disable CDP 105 00:07:23,000 --> 00:07:28,000 on internet facing interfaces or interfaces that connects you to the outside world.