CCIE RS – Written – L2 – Implement and Troubleshoot Layer 2 Protocols

Implement and Troubleshoot Layer 2 Protocols

CDP – Cisco Discovery Protocol

  • Propriety, runs on all Cisco equipment
  • Used to obtain protocol address of neighboring devices and discover the platform of those devices
  • Media and protocol-independent
  • Sends periodic messages – advertisements, every 60 seconds
  • Multicast Address: 01-00-0C-CC-CC-CC
  • CDPv2 is latest version

TLV – Type Length Value: Embedded in CDP advertisements

  • Device-ID TLV: Identifies the device name in the form of a character string.
  • Address TLV: Contains a list of network addresses of both receiving and sending devices.
  • Port-ID TLV: Identifies the port on which the CDP packet is sent.
  • Capabilities TLV: Describes the functional capability for the device in the form of a device type, for example, a switch.
  • Version TLV: Contains information about the software release version on which the device is running.
  • Platform TLV: Describes the hardware platform name of the device, for example, Cisco 4500.
  • IP Network Prefix TLV: Contains a list of network prefixes to which the sending device can forward IP packets. This information is in the form of the interface protocol and port number, for example, Eth 1/0.
  • VTP Management Domain TLV: Advertises the system’s configured VTP management domain name-string. Used by network operators to verify VTP domain configuration in adjacent network nodes.
  • Native VLAN TLV: Indicates, per interface, the assumed VLAN for untagged packets on the interface. CDP learns the native VLAN for an interface. This feature is implemented only for interfaces that support the IEEE 802.1Q protocol.
  • Full/Half Duplex TLV: Indicates status (duplex configuration) of CDP broadcast interface. Used by network operators to diagnose connectivity problems between adjacent network elements.

LLDP – Link Layer Discovery Protocol

  • Open Standard – IEEE 802.1AB
  • Switch supports basic management TLV’s
    • Port description TLV
    • System name TLV
    • System description TLV
    • System capabilities TLV
    •  Management address TLV
    • These organizationally specific LLDP TLVs are also advertised to support LLDP-MED.
      • Port VLAN ID TLV ((IEEE 802.1 organizationally specific TLVs)
      • MAC/PHY configuration/status TLV(IEEE 802.3 organizationally specific TLVs)

Link Layer Discovery Protocol – Media Endpoint Discovery (LLDP-MED)

  • Extension of LLDP that operates between endpoint devices (IP Phones)
  • TLVs
    • Capabilities
    • Network Policy
    • Power Management
    • Inventory
    • Location

UDLD – UniDirectional Link Detection

  • Cisco Proprietary
  • Allows devices connected through fiber or copper to monitor the physical configuration of the cables and detect when a unidirectional link exists
  • Layer 2 protocol that works with layer 1 protocol to determine physical status of a link
  • RFC 5171

Aggressive Mode

  • Disabled by default
  • Configure only on p2p links
  • Bidirectional lnik send UDLD messages, if stops receiving, UDLD tries to reestablish connection. After 8 retriesthe port is disabled
    • Error Disables

Normal Mode

  • Default
  • Does not disable the port when unidirectional link is detected

Configuration Defaults

  • UDLD global enable state — Globally disabled
  • UDLD aggressive mode — Disabled
  • UDLD per-port enable state for fiber-optic media — Enabled on all Ethernet fiber-optic LAN ports
  • UDLD per-port enable state for twisted-pair (copper) media — Disabled on all Ethernet 10/100 and 1000BASE-TX LAN ports

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