Sunday, December 31, 2017

Cisco CCNP:300-115 - 1.2 Configure and verify Layer 2 protocols: 1.2.a CDP, LLDP

Recently I needed to renew my Cisco CCNPs, that is both CCNP Routing and Switching as well as CCNP Security. While working with Cisco products (well now they own SourceFire, so exclude these) is not within my daily duties, I still thought it was important for me to maintain these two credentials. As a result, I've put together my notes below focusing on the key points I used to study. I believe that someone else may find them useful.

            - Layer 2, media-independent and network-independent
            - Runs on Cisco devices
            - Allows networking applications to learn about directly connected neighbours
            - Allows systems using different network player protocols to learn about each other
            - Not supported on Frame Relay multipoint subinterfaces
            - Leverages Type-Length-Value (TLV) fields
            - Some of the TLVs supported in CDP v2 includes
              - Address TLV: Contains address for both sending and receiving devices
              - Application TLV - Send application specific TLV through CDP
              - Capabilities: Device capability, such as switch, router, etc
              - Device-ID-TLV: Device name as a string
              - Full or Half Duplex TLV: Duplex configuration of the CDP broadcast interface. Can be used to diagnose connectivity issues with adjacent neighbours
              - IP Network Prefix TLV: List of prefixes for which a sending device can forward IP packets. Includes the interface, protocol and port number. i.e. Ethernet 1/0
              Location TLV: Delivers location based information to endpoint devices: Can send Civic (Postal info, etc) and ELIN (phone nuber which routes to a local public safety answering point (PSAP) and which the PSAP can use to call back the emergency caller
              - Location-Server TLV: Mechanism for location server to transfer the necessary information to neighbouring devices
              - Native VLAN TLV: Indicates on a per interface basis the assumed VLAN for untagged packets on that interface. Only supported on IEEE 802.1Q protocol
              - Platform TLV: Hardware platform of the host
              - Port-ID TLV: The port on which the CDP protocol was sent
              - Version TLV:  Device software release/12-2_52_se/configuration/guide/3750scg/swsdm
              - VTP Management Domain TLV: Advertises the configured VTP domain name of the system
              - CDP is enabled by default on all Cisco devices
              - Use well-known multicast address "01:00:0C:CC:CC:CC" that hosts listen on
              - Advertisements contain the time-to-live information which tells a host how long it should hold this information for
              - Advertisements are sent every 60 seconds by default
              - 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
              - Cisco devices never forward CDP packets
              - CDP received information is stored in a table. This table is refreshed every time a new advertisement is received       
              - Information about a device is discarded after 3 advertisements from that device is missed
              - Information contained in CDP advertisements can vary between systems. However, typical information found in the advertisements include:
                  - Running IOS version
                  - Hardware Platform
                  - IP address of interface on devices
                  - locally connected devices advertising CDP
                  - Interfaces active on Cisco devices, including encapsulation type
                  - Hostname
                  - Duplex setting
                  - VTP domain
                  - Native VLAN
            - CDP v2 does more than CDP v1
            - CDP v2 provides rapid error checking.
            - Can provide errors on mismatched native VLANs IDs on connected ports
            - Can provide errors on mismatched port-duplex state between connected devices
            - CDP Error messages can be sent to the console or to a logging server
            - CDP can be used with SNMP
            - CDP & SNMP allows network management application to learn the type of devcie and the SNMP agent address of neighboring device
            - CDP can be used with ATM PVCs
            - On-Demand Routing (ODR) uses CDP to propogate IP address information in hub-and-spoke topologies.
            - When ODR is eabled, spoke route automatically advertise their subnets using CDP
            - CDP is disabled by default on ATM PVC interfaces
        - CDP also supports IPv6
        - CDP in IPv6 operates the same way as it does in IPv4
        - CDP in IPv6 allows for the exchange of IPv6 and neighbor addressing information
      Benefits of CDP
        - Systems running different network layer protocols can learn about each other
        - Facilitates management of Cisco devices
        - Assists with troubleshooting with TLVs
        - Works with SNMP by learning SNMP agent addresses and sending SNMP queries
      - Useful commands
        - cdp run: Enable CDP globally
        - no cdp run: Disable CDP globally
        - cdp enable: Enable CDP on the interface
        - no cdp enable: Disable CDP on the interface
      - default CDP advertisements timeer 60 seconds
      - default CDP holdtime 180 seconds


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