VLAN: VLAN Fundamentals and Trunking

Key concepts for broadcasting/vlan justification:

  • Unnecessary use of bandwidth
  • Unnecessary workload on switch to process and send all broadcasts

VLAN Membership of a host depends on 2 factors:

  • Static vlans – depandant on port host is connected to
  • Dynamic vlans – is dependant on the host MAC address (Switch looks at source MAC address 1st on frame)

Trunk ports by default belong to ALL VLANs

dynamic desirable trunking – bad

show vlan id NUMBER – quicker way of viewing ports in 1 particular VLAN. Handy when a large VLAN database.

Dynamic VLANs: VLAN Membership Policy Server – VMPS

VLAN membership is defined by MAC address value, a host can dynamically move from port to another port and still retain VLAN membership. (source mac address used in frame, 1st value!)

VMPS uses a TFTP server to assist in the dynamic port assignment. A database on the TFTP server maps source MAC addresses to VLANs, the downloading occurs every time you power cycle the VMPS server. VMPS uses UDP.

Portfast is enabled automatically on any VMPS port. Interesting default! Further reading required, potentially for the IE.

  • VMPS Server has to be configured BEFORE the ports.
  • Portfast enabled by default.
  • You must disable port security before a port can be made dynamic.
  • Trunking ports cannot be made dynamic. As the ports has to belong to multiple VLANs.

Default VLAN range 1-4094

Transmitting switch tags frame with a VLAN ID, when remote switch receives frame, the switch will examine this ID and forward the frame appropriately.

Trunk negotiation – speed/duplex/encapsulation must match

ISL and dot1q are P2p trunking protocols

  • ISL = Cisco proprietary
  • ISL will encapsulate the frame and add a header and trailer.
  • Has to be de-encapsulated at the other end (Double tagging)
  • ISL does not support the concept of the Native VLAN

Author: drbabbers

ccieme.wordpress.com - my personal journey to ccie

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