2.8 Configure and verify Layer 2 protocols: CDP and LLDP

Alright, so let me just introduce CDP or cisco discovery protocol. It is very obvious that CDP is a cisco proprietary protocol and the goal of this protocol is to provide a specific cisco device an insight of the topology. I have here a show command for cdp:

show cdp neighbors

R1#en
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
R3 Ser 3/0 160 R PT1000 Ser 3/0
R2 Ser 2/0 122 R PT1000 Ser 2/0
Switch Fas 0/0 172 S 2960 Fas 0/1

First, let me define the terms you see above:

  1. Device ID- name of the cisco device connected on the device reference( device reference for this show command is R1) so device id refers to cisco devices connected to R1
  2. Local interface- interface of device reference ( R1 as of the moment) which is connected to neighbor devices

Let’s say , from show commands above, R1 has interface Serial 3/0 which is connected to R3, ok mate?

3. Capability–> there is a label above that says the capability of the device whether it is router or switch

4.Platform —> the model of either the switch or router or any cisco devices connected. From the figure above we can conclude that Switch model used is catalyst 2960

5. Port ID –> the interface of the neighboring device wherein the device reference is connected.

Let us conclude the show commands above:

1st. R1 which has serial interface 3/0 is connected to R3 also with serial interface 3/0 and both of them has routing capabilities and is cisco model PT1000

2nd R1 which has serial interface 2/0 is connected to R3 also with serial interface 2/0 and both of them has routing capabilities and is cisco model PT1000

3rd R1 which has fastethernet interface 0/0 is connected to switch which has fastethernet interface 0/1 and switch has switching capabilities ? hahaha and the model is cisco catalyst 2960.

And the network topology is:

eeeeeeeee

So, we conclude that using CDP, we can have a clear picture of the topology. If you want more details specially the ip address connected, you can use show  cdp  neghbors details

show cdp neighbors detail

R1#show cdp neighbors detail

Device ID: R3
Entry address(es):
IP address : 172.16.14.2
Platform: cisco PT1000, Capabilities: Router
Interface: Serial3/0, Port ID (outgoing port): Serial3/0
Holdtime: 163

Version :
Cisco Internetwork Operating System Software
IOS ™ PT1000 Software (PT1000-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Wed 27-Apr-04 19:01 by miwang

advertisement version: 2
Duplex: full
—————————

Device ID: R2
Entry address(es):
IP address : 192.168.31.2
Platform: cisco PT1000, Capabilities: Router
Interface: Serial2/0, Port ID (outgoing port): Serial2/0
Holdtime: 124

Version :
Cisco Internetwork Operating System Software
IOS ™ PT1000 Software (PT1000-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Wed 27-Apr-04 19:01 by miwang

advertisement version: 2
Duplex: full
—————————

Device ID: Switch
Entry address(es):
Platform: cisco 2960, Capabilities: Switch
Interface: FastEthernet0/0, Port ID (outgoing port): FastEthernet0/1
Holdtime: 175

Version :
Cisco IOS Software, C2960 Software (C2960-LANBASE-M), Version 12.2(25)FX, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2005 by Cisco Systems, Inc.
Compiled Wed 12-Oct-05 22:05 by pt_team

advertisement version: 2
Duplex: full

 

show cdp neighbors detail just give you more details like the ip addresses involved and the IOS involved but it is basically almost the same. It can be helpful specially if the network topology design has lost.

Let me take first my breakfast mate!

****************************After Breakfast***************************

LLDP ( Link-Layer Discovery Protocol)

Yeah! I’m back after that heavy breakfast that I had. Seriously, I am planning to discuss LLDP and the next blog post. Upon researching, I realized that CDP is an IEEE standardized neighbor discovery protocol as a counterpart of CDP. It is registered as IEEE 802.1AB and it almost function as the same but LLDP is created to support non-cisco devices including VOIP phones and switchs.

Use show lldp to verify

Most of the cases, CDP is used to discover neighbor devices but it is a good to know thing  for further troubleshooting purposes what is LLDP just like I’ve learned it 30 minutes ago.

******************************End of Blog*******************************

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