Monday 18 September 2023

What is Cisco discovery protocol? | How to configure CDP? | CDP



In most networks, we have several routers, switches, and more. We want to make management for networks easy, most of the admins take the help of CDP and LLDP to know about what type of devices are present in the network, the IP addresses, and how all the devices are connected with each other (which means the interface or port) and the information about which VLAN they belong.


Cisco discovery protocol

 

CDP (Cisco discovery protocol) is a Cisco proprietary protocol which is designed by Cisco. CDP is used to collect information about directly connected devices. We can collect the hardware and protocol information about neighboring devices. This information is very helpful when we do troubleshoot or document the network.

What information do we collect from neighbors?

  • Device ID means the hostname of the neighbor.
  • The local interface is the port on which CDP enables the device to receive the CDP packets.
  • Hold time means the router's remaining amount of time is left to hold the information before the router is discarding it. The router discards the information if no more CDP packets are received. {You can choose the length from 10 – 255 seconds.}
  • Capability means what is the capability of the neighbor it’s a router or switch or it’s a repeater.
  • Platform means the type of Cisco device directly connected. In the previous output,
  • Port ID means the neighbor device’s port or interface on which the CDP packets are multicasting.

 Let's see the configuration and we will see some interesting show commands.


Topology: -



  • configure the topology as per the diagram 
  • configure the Hostname 
  • assign the IPs to their respective ports as per the topology
  • configure CDP in global mode. or you can also configure CDP on an interface. 
  • make sure the router-1-core will collect all the information.


R1(config)#hostname router-1-core-


R1(config)#interface serial 2/0

R1(config-if)#ip address 10.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface serial 2/1

R1(config-if)#ip address 20.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface fastEthernet 0/0

R1(config-if)#ip address 30.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface fastEthernet 6/0

R1(config-if)#ip address 40.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface fastEthernet 1/1

R1(config-if)#ip address 30.1.1.1 255.0.0.0

R1(config-if)#no shu

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface fastEthernet 0/0

R1(config-if)#ip address 50.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface ethernet 3/0

R1(config-if)#ip address 60.1.1.1 255.0.0.0

R1(config-if)#no shu

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface ethernet 3/2

R1(config-if)#ip address 70.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface ethernet 3/1

R1(config-if)#ip address 80.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit


R1(config)#interface fastEthernet 1/0

R1(config-if)#ip address 90.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit




R6(config)#hostname router-6-

router-6-(config)#interface serial 0

router-6-(config-if)#ip address 10.1.1.2 255.0.0.0

router-6-(config-if)#no shutdown

router-6-(config-if)#exit





R5(config)#hostname router-5-

router-5-(config)#interface serial 2/0

router-5-(config-if)#ip address 20.1.1.2 255.0.0.0

router-5-(config-if)#no shutdown

router-5-(config-if)#exit




R2(config)#hostname router-2-

router-2-(config)#interface fastEthernet 0/0

router-2-(config-if)#ip address 50.1.1.2 255.0.0.0

router-2-(config-if)#no shutdown

router-2-(config-if)#exit




R3(config)#hostname router-3-

router-3-(config)#interface fastEthernet 0/0

router-3-(config-if)#ip address 90.1.1.2 255.0.0.0

router-3-(config-if)#no shutdown

router-3-(config-if)#exit



  • configure CDP in global mode. or you can also configure CDP on an interface. 


router-1-core-#configure terminal

router-1-core-(config)#cdp ?

  advertise-v2      CDP sends version-2 advertisements

  holdtime          Specify the holdtime (in sec) to be sent in packets

  log               Log messages generated by CDP

  run               Enable CDP

  source-interface  Insert the interface's IP in all CDP packets

  timer             Specify rate (in sec) at which CDP packets are sent


router-1-core-(config)#cdp advertise-v2

router-1-core-(config)#exit


(now let's collect information about our neighbors)


router-1-core-#show cdp

Global CDP information:

        Sending CDP packets every 60 seconds

        Sending a holdtime value of 180 seconds

        Sending CDPv2 advertisements is  enabled

(From the above output you see the default time)




router-1-core-#show cdp ?
  entry      Information for specific neighbor entry
  interface  CDP interface status and configuration
  neighbors  CDP neighbor entries
  traffic    CDP statistics
  |          Output modifiers
  <cr>



router-1-core-#show cdp entry ?
  *     all CDP neighbor entries
  WORD  Name of CDP neighbor entry

router-1-core-#show cdp entry *
-------------------------

-------------------------
Device ID: router-3-
Entry address(es):
  IP address: 90.1.1.2
Platform: Cisco 7206VXR,  Capabilities: Router
Interface: FastEthernet1/0,  Port ID (outgoing port): FastEthernet0/0
Holdtime : 162 sec

Version :
Cisco IOS Software, 7200 Software (C7200-ADVENTERPRISEK9-M), Version 12.4(24)T, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2009 by Cisco Systems, Inc.
Compiled Thu 26-Feb-09 00:31 by prod_rel_team

advertisement version: 2
Duplex: half

-------------------------
Device ID: router-2-
Entry address(es):
  IP address: 50.1.1.2
Platform: Cisco 7206VXR,  Capabilities: Router
Interface: FastEthernet0/0,  Port ID (outgoing port): FastEthernet0/0
Holdtime : 153 sec

Version :
Cisco IOS Software, 7200 Software (C7200-ADVENTERPRISEK9-M), Version 12.4(24)T, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2009 by Cisco Systems, Inc.
Compiled Thu 26-Feb-09 00:31 by prod_rel_team

advertisement version: 2
Duplex: half

-------------------------
Device ID: router-6-
Entry address(es):
  IP address: 10.1.1.2
Platform: Cisco 1720,  Capabilities: Router Switch
Interface: Serial2/0,  Port ID (outgoing port): Serial0
Holdtime : 111 sec

Version :
Cisco IOS Software, C1700 Software (C1700-ADVENTERPRISEK9-M), Version 12.4(8), RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2006 by Cisco Systems, Inc.
Compiled Mon 15-May-06 12:42 by prod_rel_team

advertisement version: 2
VTP Management Domain: ''

-------------------------
Device ID: router-5-
Entry address(es):
  IP address: 20.1.1.2
Platform: Cisco 3725,  Capabilities: Router Switch IGMP
Interface: Serial2/1,  Port ID (outgoing port): Serial2/0
Holdtime : 151 sec

Version :
Cisco IOS Software, 3700 Software (C3725-ADVSECURITYK9-M), Version 12.4(15)T12, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2010 by Cisco Systems, Inc.
Compiled Fri 22-Jan-10 03:58 by prod_rel_team

advertisement version: 2
VTP Management Domain: ''

-------------------------



(You can see we have collected lots of information about neighbors)






router-1-core-#show cdp interface
FastEthernet0/0 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
FastEthernet1/0 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
FastEthernet1/1 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial2/0 is up, line protocol is up
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial2/1 is up, line protocol is up
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial2/2 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial2/3 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/0 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/1 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/2 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/3 is administratively down, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/4 is administratively down, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/5 is administratively down, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/6 is administratively down, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Ethernet3/7 is administratively down, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/0 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/1 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/2 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/3 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/4 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/5 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/6 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
Serial4/7 is administratively down, line protocol is down
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
FastEthernet6/0 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds



router-1-core-#show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
switch-1         Eth 3/2            135         R S I     Linux Uni Eth 0/0
router-3-        Fas 1/0            171           R       7206VXR   Fas 0/0
router-2-        Fas 0/0            162           R       7206VXR   Fas 0/0
router-6-        Ser 2/0            140          R S      1720      Ser 0
router-5-        Ser 2/1            168         R S I     3725      Ser 2/0
vIOS-L2-01       Eth 3/0            20          R S I     IOSv      Gig 0/0
vIOS-L2-01       Eth 3/1            5           R S I     IOSv      Gig 0/0
IOU2             Fas 6/0            153         R S I     Linux Uni Eth 0/0
IOU1             Fas 1/1            173         R S I     Linux Uni Eth 0/0

router-1-core-#show cdp traffic
CDP counters :
        Total packets output: 406, Input: 308
        Hdr syntax: 0, Chksum error: 0, Encaps failed: 0
        No memory: 0, Invalid packet: 0, Fragmented: 0
        CDP version 1 advertisements output: 0, Input: 0
        CDP version 2 advertisements output: 406, Input: 308

(If you want to collect information about a specific neighbor, not the whole network you can enable CDP on that particular port) (if you want to completely disable CDP)


router-1-core-(config)#no cdp run

router-1-core-#show cdp neighbors
% CDP is not enabled


router-1-core-(config)#cdp ?
  advertise-v2      CDP sends version-2 advertisements
  holdtime          Specify the holdtime (in sec) to be sent in packets
  log               Log messages generated by CDP
  run               Enable CDP
  source-interface  Insert the interface's IP in all CDP packets
  timer             Specify rate (in sec) at which CDP packets are sent

  • here you can change the timer.

  • let's change the holdtime from 180 sec to 15 sec.

router-1-core-#show cdp
Global CDP information:
        Sending CDP packets every 60 seconds
        Sending a holdtime value of 180 seconds
        Sending CDPv2 advertisements is  enabled


router-1-core-(config)#cdp holdtime ?
  <10-255>  Length  of time  (in sec) that receiver must keep this packet


router-1-core-#show cdp
Global CDP information:
        Sending CDP packets every 60 seconds
        Sending a holdtime value of 180 seconds
        Sending CDPv2 advertisements is  enabled


router-1-core-(config)#cdp holdtime ?
  <10-255>  Length  of time  (in sec) that receiver must keep this packet


router-1-core-(config)#cdp holdtime 15

router-1-core-#show cdp
Global CDP information:
        Sending CDP packets every 60 seconds
        Sending a holdtime value of 15 seconds
        Sending CDPv2 advertisements is  enabled

  • let's change the CDP packets time from 60 sec to 20 sec

router-1-core-(config)#cdp timer ?
  <5-254>  Rate at which CDP packets are sent (in  sec)

router-1-core-(config)#cdp timer 20
router-1-core-(config)#end

router-1-core-#show cdp
Global CDP information:
        Sending CDP packets every 20 seconds
        Sending a holdtime value of 15 seconds
        Sending CDPv2 advertisements is  enabled




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