Cisco Discovery Protocol and LLDP in Voice networks

Cisco Discovery Protocol (CDP) is a link layer discovery protocol that has been integrated into Cisco switches and routers ever since I can remember. Cisco introduced CDP support into their routers way back in 1993 when Cisco introduced IOS 10.3 on their routers. Cisco went on a buying spree of various switch manufacturers in the 1990s. Soon after acquiring these companies, the CDP protocol was added to the switches to assist in the topology discovery of networks. Cisco acquired the following
switch manufacturers in the 1990s:

• Grand Junction - 2900 series switches
• Kalpana – 3000 series switches.
• Granite – 4000 series switches
• Crescendo – 5000 series switches

CDP allows an administrator to quickly and easily discover Cisco devices in an internetwork. CDP commands are especially useful when doing consulting with limited documentation of the network. The “show cdp neighbor detail” command displays the following information of neighboring switches and routers:

• Local interface connected to neighbor
• Remote interface connected to current device
• Remote device identifier (router, switch)
• Remote device Cisco IOS version

Cisco integrated the CDP protocol stack into every one of the 7900 series Cisco IP phones. The CDP protocol was updated to version 2 when Cisco added this functionality to the Cisco IP phone endpoints. CDPv2 allows switch ports configured with a voice vlan (auxillary vlan in Cat OS) to dynamically propagate the voice VLAN information to the Cisco IP phone. Instead of waiting the standard 60 seconds for the CDP hello, CDPv2 has a trigger (like RIPv2 triggered updates) that allows the Cisco switch to propagate voice vlan information to the Cisco IP phone as soon as the power over Ethernet (PoE) negotiation has occurred. CDPv2 also has a power field where the Cisco switch can dynamically negotiate power requirements with the Cisco IP phones. CDPv2 added a reporting mechanism that will identify duplex mismatches in the console of the switch. Half duplex and duplex mismatches can greatly reduce voice quality during interactive conversations.

Cisco Unified Communications Manager (CUCM) has a CDP protocol stack bound to the network interface card (NIC) of the server. Cisco Unified Communications software clients also loads a CDP stack to the NIC of the computer. The show cdp neighbor details command on the switch will display the firmware version of Cisco IP phones or the software type and revision number for the following applications:

• Cisco IP Communicator (CIPC)
• Cisco Unified Video Advantage (CUVA)
• Cisco Unified Personal Communicator (CUPC)

The CDP feature aids the network discovery procedure that Cisco Emergency Responder (CER) and Cisco Works network management servers use to find devices in the network.

The IEEE 802.1ab standard is an industry standard technique for providing link layer discovery in a multi-vendor environment. The IEEE standard introduced the link layer discovery protocol – media endpoint discovery (LLDP-MED). LLDP-MED is integrated with Microsoft Windows Vista by default. There is also a downloadable client for Microsoft Windows XP in the references link below. LLDP provides for the communication of IEEE 802.3af power levels and VLAN information in a similar manner to that which Cisco pioneered with CDP. LLDP is supported in most of Cisco’s newer switches and phone models. Use the Cisco feature navigator link in the references portion of this blog to see if your Cisco switch has LLDP capabilities. CDP and LLDP are configurable parameters on the Type B Cisco IP phones in CUCM 7.0.

REFERENCES

CDP version 2
http://www.optimumdata.com/shop/files/cisco/3600/3600_CDP_Additions_for_...

Configuring Cisco Discovery Protocol
http://www.cisco.com/en/US/docs/wireless/access_point/1300/12.2_15_JA/co...

LLDP-MED and Cisco Discovery Protocol
http://www.cisco.com/en/US/technologies/tk652/tk701/technologies_white_p...

Configuring LLDP and LLDP-MED
http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/50sg/conf...

Cisco Feature Navigator
www.cisco.com/go/fn

Microsoft Windows LLDP Support
http://social.technet.microsoft.com/Forums/en-US/itprovistanetworking/th...

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