Ethernet Basics

Ethernet Basics

Auto-Negotiation

  • Switches can dynamically detect speed setting using Fast Link Pulses (FLP) of the auto-negotiation process.
    • If auto-negotiation is turned off on the other side, speed can still be detected based on the electrical signal.
    • If auto-negotiation is turned off on the other side, the device without a configured duplex setting must assume the default. For Cisco, this is “half-duplex” for 100Mbps interfaces or “full-duplex” for 1Gbps interfaces.

NOTE – Easy way to remember what duplex is to determine history. When 100Mbps interfaces were out, hubs were still in use. Remember the 100Mbps hubs? So, half-duplex was the default. However, there is no such thing as a 1Gbps hub, so it must be a switch. Full-duplex is the default.

  • The only way to disable auto-negotiation on Cisco switches is to set both speed and duplex.

Types of Ethernet Addresses

  • Unicast – Represents communication to a single interface.
    • The I/G bit (The most significant bit of the most significant byte) is set to 0.
  • Multicast – A MAC address that implies a subset of all devices on the LAN.
    • The I/G bit (The most significant bit of the most significant byte) is set to 1.
  • Broadcast – Sends to all devices on LAN with the address FF:FF:FF:FF:FF:FF

Private VLANs

A Private VLAN (PVLAN) is a VLAN that is segregated. In other words, you can isolate ports within a VLAN. 3 modes of communication:

  1. Ports that communicate with all other ports in the VLAN – a.k.a. Primary VLAN
  2. Ports that communicate to certain other ports and with shared devices (e.g. a router) – a.k.a. Secondary community VLAN
  3. Ports that only communicate with shared devices (e.g. a router) – a.k.a. Secondary isolated VLAN.

VLAN Trunking Protocol (VTP) Overview

  • Advertises VLAN ID, VLAN name, and VLAN type to other VTP “clients” or “servers”.
  • Each change to the VLAN database on the “server” device increments the revision number by 1 to tell the other devices whether they have the newest version or sync to get the newer version.
  • 3 modes
    • Server Mode
      • Originates VTP Advertisements
      • Processes received advertisements to update it’s VLAN config
      • Forwards received advertisements
      • Saves VLAN config to vlan.dat or NVRAM
      • Can create, modify, and delete VLANs
    • Client Mode
      • Originates VTP Advertisements
      • Processes Received advertisements to update it’s own VLAN database
      • Forwards VTP advertisements
      • Saves VLAN config to vlan.dat or NVRAM
    • Transparent Mode
      • Forwards received VTP advertisements
      • Saves VLAN config to NVRAM or vlan.dat
      • Can create, modify, and delete VLANs
  • All Cisco switches default to using VTP in server mode, but before advertisments can be sent, a domain name must be given
  • Cisco switches as VTP clients do not have to have a domain specified in order to accept VTP advertisements. If no domain name is specified on a VTP client device, the domain-name specified in the advertisement is used.
  • Before VTP Client devices can accept VTP advertisements, the global command “vtp mode” must be configured.
  • You can (and should) have at least 2 VTP Server switches for high-availability.
  • To prevent DoS attacks using VTP (because updates and revision numbers are clear-text packets), use a password to create an MD5 encoding. Only clients with the same password will accept the updates.

VTP Configuration

  • VTP sends updates out all trunk ports (ISL or 802.1q).
  • Minimum config to use VTP
    • 1 switch to have domain defined (VTP Server)
    • 1 switch to have “vtp mode” defined (VTP Client)
  • VTP Options
    • vtp version” – Sets version 1 or 2. Servers and clients must match version numbers. Transparent switches at version 2 can forward both version 1 or 2 updates
    • vtp pruning” – Prunes VLANs from being created on switches that do not use it.
    • vtp interface” – Specifies the interface with the IP Address used to identify the switch in updates

Normal/Extended VLANS

  • Normal VLANS: id 1 – 1005
    • Can be advertised via VTP versions 1 and 2
    • Stored in vlan.dat
  • Extended VLANs: id 1006-4094
    • Not advertised via VTP
    • Not stored in vlan.dat
    • Only used on Transparent switches

VLAN Trunking

  • ISL – Cisco Proprietary
    • Supports Normal and Extended Range
    • Encapsulates original frame
      • Adds a new 26-byte header
      • Encapsulated frame uses the source address of the device doing the trunking
      • Encapsulated frame uses a multicast destination address of either 0100.0c00.0000 or 0300.0c00.0000
    • Does not support native VLAN
  • 802.1q
    • Supports Normal and Extended Range
    • Inserts Tag inside original frame
      • Inserts a 4-byte header, called a tag, inside the original frame right after the Source Address Field
    • Supports native VLAN
    • IEEE Standard
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