network protocols handbook.pdf

268Protocols GuideCisco ProtocolsProtocol NameCDP: Cisco Discovery ProtocolProtocol DescriptionCisco Discovery Protocol (CDP) is primarily used to obtain protocoladdresses of neighboring devices and discover the platformof those devices. CDP can also be used to show informationabout the interfaces your router uses. CDP is media- and protocol-independent,and runs on all Cisco-manufactured equipmentincluding routers, bridges, access servers, and switches.Use of SNMP with the CDP Management Information Base(MIB) allows network management applications to learn the devicetype and the SNMP agent address of neighboring devices,and to send SNMP queries to those devices. Cisco DiscoveryProtocol uses the CISCO-CDP-MIB.CDP runs on all media that support Subnetwork Access Protocol(SNAP), including local-area network (LAN), Frame Relay,and Asynchronous Transfer Mode (ATM) physical media. CDPruns over the data link layer only. Therefore, two systems thatsupport different network-layer protocols can learn about eachother.Each device configured for CDP sends periodic messages,known as advertisements, to a multicast address. Each deviceadvertises at least one address at which it can receive SNMPmessages. The advertisements also contain time-to-live, orholdtime, information, which indicates the length of time a receivingdevice should hold CDP information before discardingit. Each device also listens to the periodic CDP messages sentby others in order to learn about neighboring devices and determinewhen their interfaces to the media go up or down.Commandclear cdp countersclear cdp tableshow cdpshow cdp entry entryname[protocol | version]show cdp interface [typenumber]show cdp neighbors[type number] [detail]show cdp trafficshow debuggingRelated protocolsSNMP, SNAPSponsor SourceCDP is a Cisco protocol.PurposeResets the traffic counters to zero.Deletes the CDP table of informationabout neighbors.Displays the interval between transmissionsof CDP advertisements, the numberof seconds the CDP advertisement is validfor a given port, and the version of theadvertisement.Displays information about a specificneighbor. Display can be limited to protocolor version information.Displays information about interfaces onwhich CDP is enabled.Displays the type of device that has beendiscovered, the name of the device, thenumber and type of the local interface(port), the number of seconds the CDPadvertisement is valid for the port, thedevice type, the device product number,and the port ID. Issuing the detail keyworddisplays information on the native VLANID, the duplex mode, and the VTP domainname associated with neighbor devices.Displays CDP counters, including thenumber of packets sent and received andchecksum errors.Displays information about the typesof debugging that are enabled for yourrouter. See the Cisco IOS Debug CommandReference for more informationabout CDP debug commands.CDP Version-2 (CDPv2), the most recent release of the protocol,provides more intelligent device tracking features. Thesefeatures include a reporting mechanism which allows for morerapid error tracking, thereby reducing costly downtime. Reportederror messages can be sent to the console or to a loggingserver, and cover instances of unmatching native VLAN IDs(IEEE 802.1Q) on connecting ports, and unmatching port duplexstates between connecting devices.Referencehttp://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800ca66d.htmlConfiguring the Cisco Discovery ProtocolProtocol StructureCDPv2 show commands can provide detailed output on VLANTrunking Protocol (VTP) management domain and duplexmodes of neighbor devices, CDP-related counters, and VLANIDs of connecting ports. The following table lists the CDP commands: