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Day 17 VLANs (Part2).md

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Things we will cover.

  • What is a trunk port?
  • What is the purpose of trunk ports?
  • 802.1Q Encapsulation
  • Configure trunk port
  • Router on a stick (ROAS)

What is trunk port?

whereas an access port belongs to a single vlan, trunk ports carry traffic from multiple vlans on a single interface.

How does SW1 know which VLAN the traffic belongs to?

Answer is vlan tagging.

Switches will tag all frames that they send over a trunk link. This allows the receiving switch to know which vlan the frame belongs to

Trunk ports = Tagged ports Access ports = Untagged ports

VLAN Tagging

There are two main trunking protocols :

  • ISL (Inter-Switch Link) = This is a old cisco proprietary and currently not in use
  • IEEE 802.1Q

dot1q inserts 4byte (32bits ) field between source and type/length field

Fields in dot1q tag

TPID (Tag Protocol Identifier)

  • size = 2bytes (16 bits)
  • Always set to a value of 0x8100 which indicates frame is 802.1Q tagged
  • Each hexadecimal id 4bits to 4* 4 = 16bits = Total size

TCI (Tag Control Information)

  • PCP (Priority Code Point)
    • 3bits in length
    • used for Cos (class of service), which prioritizes important traffic in congested networks
  • DEI (Drop Eligible Indicator)
    • 1bit in length
    • used to indicate frames that can be dropped if the network is congested
  • VID (VLAN ID)
    • 12bits in length = 4096 total vlans (2^12), range from 0 - 4095
      • Vlans 0 and 4095 are reserved and cant be used
      • therefore the actual range is 1-4094
    • identifies the vlan the frame belongs to

VLAN Ranges

Range of vlan 1-4094 is divided into tow sections:

  • Normal VLANs: 1-1005
  • Extended VLANs: 1006-4094

Native VLAN

  • It is 1 by default on all trunk ports but can be manually configured
  • Switch does not add an 802.1Q tag on frames in the native vlan
  • When a switch receives an untagged frame on a trunk port, it assumes the frame belongs to the native VLAN
  • It is very important that the vlan matches
  • For security reason it is best to change the native vlan to an unused VLAN

Trunk Configuration

Native Vlan Configuration

SN COMMANDS Description
1 Switchport trunk native vlan {vlan id} change the the vlan
2 do show interfaces trunk
3 show vlan brief shows the access ports assigned to each vlan not the trunk ports that allow each vlan

ROAS (Router on a Stick)

would look like this

Router Configuration

  • ROAS is used to route between multiple VLANs using a single interface on the router and switch
  • The switch interface is configured as a regular trunk
  • We configure the VLAN tag and IP address on each subinterface
  • The router will tag frames sent out of each subinterface with the vlan tag configured on the subinterface