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Understanding Cisco Discovery Protocol on the Cisco TelePresence endpoints

Introduction

Cisco Discovery Protocol (CDP) is a proprietary layer-2

management protocol developed by Cisco in the early 1990s

to provide enhanced automation of network discovery and

management. It is broadly deployed on millions of existing

Cisco products and provides countless benefits to network

administrators for managing router and switch interfaces. With

the introduction of IP Telephony in the late 1990s and early

2000s, CDP was enhanced to provide additional automation

capabilities for IP-based telephones, including automatic

VLAN discovery, Power over Ethernet (PoE) negotiation,

Quality of Service (QoS) automation, location awareness

(to automate the discovery of the physical location of an

IP telephone for management and emergency services

purposes), Ethernet speed and duplex mismatch detection,

and more.

NOTE: The IETF, IEEE and TIA, in cooperation with Cisco

and numerous other networking vendors, have since

created the IEEE 802.1AB standard, known as Link-Layer

Discovery Protocol (LLDP), with extensions developed for

Media Endpoint Discovery (LLDP-MED) for voice and video

endpoints. LLDP-MED will eventually subsume CDP, but this

may take years to unfold due to the enormous installed-base

and widespread use of CDP.

History

Cisco acquired TANDBERG in April 2010. The TANDBERG

portfolio of video endpoints compliments Cisco’s existing

Telepresence and Unified Communications solutions. CDP

support was introduced on the Cisco E20 in release TE4.0;

on the Cisco TelePresence MX series, EX series, Codec C

Series, Profile series and Quick Set C20 in release TC5.0.

The Cisco TelePresence SX20 Quick Set is supported

from release TC5.1. The Cisco TelePresence EX Series is

supported in TE6.0.

However, because there is already an installed-base of these

endpoint models (prior to the Cisco acquisition) that are not

running CDP, introducing CDP in a software release requires

careful consideration of how the new automation functionality

will affect that existing installed-base. Enabling CDP by

default could cause undesired behavior for those existing

deployments when they upgrade to a CDP-enabled release

and the devices suddenly begin using VLAN automation, so

CDP is being introduced in a phased approach.

Benefits provided by CDP

As mentioned in the introduction above, CDP provides

numerous automation benefits for network administrators

deploying IP-based voice and video

endpoints on their networks. This

section briefly highlights some of the

most pertinent benefits for IP-based

voice/video endpoints like the Cisco

TelePresence MX series, EX series,

Codec C Series, Profile series, Quick Set

C20 and SX20 Quick Set.

Automatic VLAN discovery

Virtual LANs (VLANs) allow a network

administrator to introduce IP-based

telephones and video terminals onto

their network without the need for re-

addressing their existing data sub nets,

or adding additional Ethernet ports to

their switches. Leveraging the 802.1Q

standard, a device such as the endpoint

can tag its Ethernet frames with the VLAN

ID that its traffic belongs to, placing its

traffic into the voice /video VLAN (known

as the auxiliary VLAN); while Ethernet

frames sent by a PC are not tagged,

and therefore end up in the data VLAN

(known as the native VLAN). This allows

the endpoint to be inserted in between

an existing PC and the Ethernet switch

to which it is attached, allowing for a

single Ethernet port per user, thereby

eliminating the need to add additional

ports in the wiring closet, and allowing

the endpoint to be assigned to a different

(new) IP sub net rather than consuming IP addresses in the

existing PC VLAN. VLANs also allow the network administrator

to apply different security and Quality of Service (QoS) policies

on a per-VLAN basis.

Figures 1 and 2 illustrate these concepts.

Without CDP (or LLDP-MED), the user must manually configure

each endpoint with the 802.1Q VLAN ID it should use. CDP

automates this task, allowing the Ethernet switch to advertise

to the endpoint the ID of the VLAN it should belong to.

Fig. 1: Without VLANs

Fig. 2: With VLANs

Cisco TelePresence Endpoints running TC6 and Cisco Unified Communications Manager 9.0 Quick Reference Guide

Contents

Introduction

Endpoint configuration

CUCM configuration

Setting passwords

AppendicesAppendices