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Links on a Switch

  • Access Ports
  • Trunk Ports

Access Ports​

An Access Port is a conneciton on a Switch that transmits data from a specific VLAN. Because an access port is only assigned to a single VLAN, it sends and receives frames that aren't tagged and only have the access VLAN value.

Configuration​

  1. Create a VLAN
  2. Assign VLAN to port
  3. As soon as VLAN becomes part of that single VLAN, it becomes an Access Port.

Trunk Ports​

A Trunk Port can transmit data from multiple VLANs.

In constrat to an Access Port, it must use Frame Tagging in order to allow signals to get to the correct endpoint.

Frame Tagging / Trunking Protocols​

  • ISL: Inter Switch Link (CISCO proprietary)
  • dot1q: IEEE 802.iq

Takes original Frame and encapsulates it into a new Frame with a different header, with a field that identifies the VLAN the frame belongs to. This inclues a new 26 bytes header, meaning more overhead that dot1q protocol.

dot1q: IEEE 802.iq​

Inserts a 4 bytes field in original frame that identifies the VLAN the frame belongs to.

This protocol has a feature called "Native VLAN", this VLAN is not tagged over the trunk.

Dynamic Trunking Protocol (DTP)​

Performs automatic trunk negotation between switches that are connected on ports.

Can run in one of three modes:

  • Dynamic Desirable: it will send DTP frames and it will respond to DTP frames.
    • Port initiates trunking.
    • Usually default for CISCO devices ending in 50.
  • Dynamic Auto: port will respond to DTP frames from the other send, but it will not send.
    • Port will not initiate trunking.
    • Usually default for CISCO devices ending in 60.
  • ON: set port to trunking unconditionally (it doesn't care about the other side).

The only scenario where you don't automatically get a Trunk, is if two sides have Dynamic Auto.

Trunking - CISCO IOS​

Administrative Mode (dynamic/trunk)​

configure terminal
interface fastEthernet 0/1
do show interface fastEthernet 0/1 switchport

If ports by default are in Administrative Mode: Dynamic Auto mode, ports won't try to initate trunking. For ports to initiate trunking, they need to be setup to Dynamic Desirable.

You won't see anything from do show interface trunk

switchport mode dynamic desirable
do show interface trunk

Or if don't want it to be dynamical/negotiable

switchport mode trunk
switchport no negotiate

Trunk protocol​

switchport trunk encapsulation ?
switchport trunk encapsulation dot1q

Allowed VLANs​

Don't let all VLANs allowed (security).

switchport trunk allowed vlan 1,150,200-220
runk allowed vlan none // remove all
do show interface trunk

Native VLAN​

switchport trunk native vlan 150

Save configuration​

do show start
copy running-config startup config