CS/CCNA

<Jeremy's IT Lab> Day17: VLANs(Part 2)

햄키 2025. 3. 28. 23:50
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VLANs(Part 2)

 

-> small NW is possible to use a seperate interface for each VLAN whe nconnecting switches to switches, and switches to routers.

-> However, when the number of VLANs increases, it is not viable.

-> Use trunk ports to carry traffic from multiple VLANs over a single interface.

-> Switches will 'tag' all sending frames over a trunk link. (allows the receiving switch to know which VLAN frame belongs to)

 

Trunk ports = 'tagged' ports

Access ports = 'untagged' ports

 

Trunking protocolss :

  • ISL (Inter-Switch Link) : old Cisco proprietary protocol 
  • IEEE 802.1Q : industry standard protocol

https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/TCPIP_802.1ad_DoubleTag.svg/1992px-TCPIP_802.1ad_DoubleTag.svg.png

 

 

802.1Q Tag (4 bytes):

-> Inserted between the Source and Type/Length fields on the Ethernet frame

-> consists of :

  • Tag Protocol Identifier (TPID) (2 bytes): indicates that the frame is 802.1Q-tagged
  • Tag Control Information (TCI) :
    • PCP(Priority Code Point) (3 bits): Used for Class of Service(CoS), prioritized important traffic in congested NW
    • DEI(Drop Eligible Indicator) (1 bit): Used to indicate frames that can be dropped if NW is congested
    • VID(VLAN ID) (12 bits): Identifies VLAN belongs to. ( 0 and 4095 can't be used)

 

Acronyms:

  • ISL : Inter-Switch Link
  • ROAS : Router on a Stick
  • TPID : Tag Protocol Identifier
  •  TCI : Tag Control Information
  • PCP : Priority Code Point
  • CoS : Class of Service
  • DEI : Drop Eligible Indicator