LLDP & CDP

CDP (Cisco Discovery Protocol)

• CDP is Cisco proprietary that is used for collecting directly connected neighbor device information like hardware, software, device name details and many more.

• CDP works at Layer 2 (data-link layer), therefore, it's not necessary for the device to have an IP address on it for it to be detected by its neighbors.

• CDP sends CDP packets containing the neighbor information after every 60 seconds and has a hold time of 180 seconds by default (using the multicast MAC address 01-00-0c-cc-cc-cc).

  • E.g, if R1 stop sending CDP packets to R2, R2 will assume that he is down after 180 seconds (hold time).

• CDP is enabled by default on most Cisco equipment. Not just on routers and switches, but also on the firewalls and even the IP Phones will run CDP.

• It aids in troubleshooting by allowing administrators to map out how Cisco devices are connected to each other and to check that the device is there and detected at Layer 2.

  • For example, say that you're in New York and there is a problem in Sydney and you don't know about the network topology in Sydney. As long as you can jump on one device in Sydney, then you could use show CDP neighbors to map out how all the other Cisco devices are connected to each other.

CDP Commands

Command	Description
R# show cdp	It shows if CDP is enabled or not, also the timers, etc.
R# show cdp neighbors	To see all directly connected neighbors. It will give you a nice brief summary view.
R# show cdp neighbors detail	It will show more verbose output, including the IP addresses configured on those devices.
R(config)# no cdp run	To turn it off.
R(config)# cdp run	To turn it back on again.
R(config)# int g0/1 R(config-if)# no cdp enable	To disable it at the interface level.
R(config)# cdp timer [seconds] R(config)# cdp holdtime [seconds]	To change the default time. Note that they won't work on packet tracer.
R# clear cdp table	Delete the CDP table of information about neighbors.

• A reason you would maybe want to disable CDP is it can be seen as a security concern. If you're not in a highly secure environment, you don't want people to be able to see what devices are plugged in there, so you could disable CDP.

  • Let’s say you've got a switch on the edge of your network, and you want CDP to be enabled on the internal facing interfaces. You want to disable it on the external facing interface, so you don't give up information to another organization. Therefore, you can do a no cdp enable at the interface level.

Example

Each router can see his neighbors even if the interfaces don't have IP (L2 Protocol).

Router 1

R1(config)#int g0/0
R1(config-if)#no shut
R1(config-if)#ip address 10.0.0.1 255.255.255.0

R1(config-if)#int g0/1
R1(config-if)#no shut

Router 2

R2(config)#int g0/0
R2(config-if)#no shut

R2(config-if)#int g0/1
R2(config-if)#no shut

Let’s take a look at a network map:

R1#show cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID    Local Intrfce   Holdtme    Capability   Platform    Port ID
Switch       Gig 0/0          175            S       2960        Gig 0/1
R2           Gig 0/1          172            R       C2900       Gig 0/0

• Above you see that R1 is connected to Switch and R2.

• It gives the :

  • Device ID → hostname of the device.
  • Local Intrfce → that is connected on R1 side.
  • Hold time → the amount of time the local device will hold the information before discarding it if no more CDP packets are received.
  • Capability List → shows whether the neighboring device is a router(R), switch(S), server(H) ,Transparent bridge(T),Source-routing bridge(B) or IGMP device (I)..etc.
  • Platform → the hardware platform for the network device (model).
  • Port ID → that is the interface that it is plugged into Switch/R2 side.

If the Hold time less than 120 it means you're losing connection with your neighbor because CDP packets sent every 60 seconds (holdtme >= 120 && holdtme<=180).

• To get more detail, you can do a show cdp neighbor detail and this will give more verbose output like :
  • Router IP address. This is very useful if you need to find out an IP address of a neighbor so that you can Telnet or SSH onto it for troubleshooting.
  • Platform it's running on
  • The IOS version is running on

Switch#show cdp neighbors detail 

Device ID: R1
Entry address(es): 
  IP address : 10.0.0.1
Platform: cisco C2900, Capabilities: Router
Interface: GigabitEthernet0/1, Port ID (outgoing port): GigabitEthernet0/0
Holdtime: 165

Version :
Cisco IOS Software, C2900 Software (C2900-UNIVERSALK9-M), Version 15.1(4)M4, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2012 by Cisco Systems, Inc.
Compiled Thurs 5-Jan-12 15:41 by pt_team

advertisement version: 2
Duplex: full

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LLDP (Link Layer Discovery Protocol)

• LLDP functions like the CDP protocol, but it is an open standard. So, it's supported on most vendors devices, and it provides similar information to CDP.

• It usually sent to one of several multicast MAC addresses (01:80:c2:00:00:0e, 01:80:c2:00:00:03 or 01:80:c2:00:00:00)

• It does have some differences though. CDP will always be enabled by default on Cisco routers and switches, but with LLDP, it depends on the device and version, whether it will be enabled or disabled by default.

• LLDP sends the LLDP messages containing neighbor information after every 30 seconds and has a hold time of 120 seconds.

LLDP Commands

Command	Description
R# show lldp	It will show if LLDP is enabled or not.
R# show lldp neighbors	It will show a summary of our neighbors.
R# show lldp neighbors detail	It will show more verbose output, including the IP addresses configured on those devices.
R(config)# lldp run	To turn it on.
R(config)# no lldp run	To turn it off.
R(config-if)# no lldp transmit R1(config-if)#no lldp receive	To disable it at the interface level, we do it for both transmit and receive separately.
R(config)# lldp timer [seconds] R(config)#lldp holdtime [seconds]	To change the default time.
R# clear lldp table	Delete the LLDP table of information about neighbors.

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CDP vs LLDP

Parameter	CDP	LLDP
Proprietary	Cisco Proprietary	Open Standard
Default Holdtime	180 seconds	120 seconds
CDP packets	every 60 seconds	every 30 seconds
Global Configuration	R1(config)# <no> cdp run	R1(config)# <no> lldp run
Interface Configuration	R1(config-if) <no> cdp enable	R1(config-if) <no> lldp receive R1(config-if) <no> lldp transmit