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  • Increase security and ease administration and relocation.
  • Break up broadcast domains.
  • VLANS work at layer 2 and 3 of OSI model.
  • Communication between VLANs uses layer 3 routing.

3 ways of assigning a switched port to VLANs:

  • Port-Centric - All nodes connected to ports in the same VLAN are assigned to the same VLAN ID.
  • Static VLANs - Statically assign ports to a VLAN.
  • Dynamic VLANs - Ports that can automatically determine their VLAN assignments based on MAC addresses.
  • Switches segment LANs into individual collision domains, VLANs break up broadcast domains as well.

Cisco CCNA Cramsheet

VLANS and Frame Tagging

Access Link - Only part of one VLAN (native VLAN of the port), any devices attached to an access link are unaware of VLAN membership.

Trunk Links - Trunks can carry multiple VLANS. Supported on fast or gigabit links only. Used to transport VLANS between devices.

Frame Tagging and Frame Tagging formats:

  • ISL (Inter-Switch Link) is Cisco's Proprietary frame tagging format.
  • IEEE 802.1q IEEE standard.
  • ISL - 26 bytes long + 4 bit FCS (frame check sequence)
  • ISL only used on fast and gigabit Ethernet.
  • 802.1q - Use IEEE standard if you are trunking between non-Cisco switches. Since IEEE frame tagging actually inserts a field in the frame.
  • LANE (LAN Emulation) - Used to communicate multiple VLANS over ATM.
  • 802.10 (FDDI) - VLAN communication over FDDI, uses SAID field in frame (Cisco Proprietary)
  • Frames with ISL frame tagging are over the maximum length specified by the Ethernet standard, these will be recorded as giants if frame tagging is not enabled. Maximum Ethernet frame is 1518 bytes, ISL tagged frames can be up to 1522 bytes long.
  • ISL NIC cards are available for servers.
  • ISL information is only added to frames that are forwarded out of configured trunk links.
  • ISL information is removed if it is forwarded out an access link.
  • Trunking makes a single port able to be part of multiple VLANS.
  • Non trunked links between switches forward VLAN1 information only.
  • Dynamic Trunking Protocol (DTP) - Manages trunk negation.
  • Communication between VLANS requires layer 3 devices (Routers)
  • Cisco 1600, 1700, 2500 do not support ISL.

VTP - VLAN Trunk Protocol

  • VTP manages all configured VLANS across a switched network. VTP provides:
    • Tracking and monitoring of VLANS.
    • Consistent VLAN configuration across all switches.
    • VLANS to be trunked across mixed networks.
    • Reporting of new VLANS.
    • Plug and Play VLAN configuration.
  • VTP is not needed if you only have 1 VLAN.
  • VTP advertisements sent every 5 minutes or when a change occurs. Switches only overwrite information with a higher revision number.

VTP Modes of Operation

Server (Default for Catalyst switch) - You need at least one server in a VTP domain. All changes are advertised in VTP domain.

Client - Receives info from VTP servers. Sends and receives updates but cannot make changes. To promote a switch to a server make it a client first, to receive all VTP info, then promote it to a server.

Transparent - Does not participate in a VTP domain, but forwards VTP advertisements through trunked links. Transparent switches keep their own database so you can add or delete VLANS from it (Locally significant).

VTP Pruning

Reduces bandwidth, by reducing broadcasts, multicasts etc that are unnecessary. If a switch does not have any ports configured for VLAN 5 then it will not receive the VTP update.

  • VTP Pruning is disabled by default on all switches.
  • When VTP Pruning is enabled it is enabled across the entire domain.
  • Only VLANs 2-1005 are pruning-eligible.
  • VLAN1 is an administrative VLAN.

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