Virtual LANs Flashcards
How do VLANs function?
by logically segmenting the network into different broadcast domains so that packets are only switched between ports that are designed for the same VLAN.
Describe the implemeentation of VLANs on a switch.
- the switch maintains a separate bridging table for each VLAN
-if the frame comes in on a port in VLAN 1, the switch searches the bridging table for VLAN 1 - when the frame is received, the switch adds the source address to the bridging table if it is unknown.
- the destination is checked so a forwarding decision can be made.
Describe how Network Administrators configure VLANs statically and dynamically.
Statically: network administratos configure port-by-port. Each port is associated with a specific VLAN. The network administrator is responsible for keying in the mappings between the ports and VLANs.
Dynamically: the ports are able to dynamically work out their VLAN config. Uses a software database of MAC address to VLAN mappings (which the network administrator must first set up)
Name any benefits to VLANs
- they permit the netowrk administrator to organize the LAN logically instead of physically.
- Each VLAN is in a separate broadcast domain and traffic between VLANs is routed. This helps limit the broadcast traffic and keep traffic that ir related in the same VLAN.
What are the 3 types of VLANs.
- Port-based VLANs
- MAC address based VLANs
- Layer 3 based VLANs
Describe how Port-based VLANs work.
switch ports are assigned to VLANs (i.e. ports 1-5 -> VLAN 1). Devices in one VLAN only communicate within that VLAN unless routing connects them. It improves traffic management, increases security and simplifies design. It requires a router or layer 3 switch for inter-VLAN communication. 802.1Q is used on trunk ports for identifying VLAN traffic. Packets don’t “leak” into other domains.
Describe how MAC address based VLANs work.
When VLANs are created, assign devices to VLANs based on their unique MAC address. Devices retain their VLAN no matter which port they are connected to. It offers flexibility, yet adds overhead. It impacts performance, scalability and administation.
Describe how Layer-3 Based VLANs work.
It uses IP routing to enable communication between VLANs. Each VLAN is assigned an IP subnet and a layer 3 switch or router handles inter-VLAN routing. Each VLAN is a separate subnet. This enhances scalability, traffic management and reduces broadcast traffic. Enables efficient communication between network segments.
What does the number of VLANs in a switch depend on?
-> traffic patterns
-> types of application
-> network management needs
-> group commonality
Explain the purpose of the 802.1Q protocol.
Used to support VLAN tagging traffic between LAN switches on Ethernet network. It allows multiple VLANs to coexist on a single physical network link by inserting a VLAN tag into the Ethernet frame header.
This 802.1Q VLAN tag includes:
tag protocol ID (value 0x8100)| tag control info ( each 2 bytes)
the tag control info includes:
user priority (3 bits) | canonical format indicator (1 bit) | VLAN ID (12 bits)
user priority: used by sender to prioritise different types of traffic
canonical traffic: used for compatability between different types of MAC protocols
VLAN ID: specifies the VLAN (1- 4094), 0x000 = frame doesn’t belong to a VLAN, 0xfff = reserved.
What mechanism of protocol could be used to ensure that a broadcast storm could not occur? explain the basic operation of this mechanism.
The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STOp is to prevent bridge loops and the broadcast radiation that results from them.
Bridges running STP:
-> participate with other bridges in the election of a single bridge as the root bridge (lowest ID)
-> calculate the distance of the shortest to the root bridge from all other bridges, choosing the port that will provide that shortest path as each bridges’ “Root port”
-> for each LAN segment (a line between two bridges), elect a designated bridge (port with the best route to the rot bridge on a link) and port on that bridge.
-> select the bridge ports to be included in the spanning tree. the ports selected are the root ports and designated ports, these forward traffic (All others clock traffic)
What is meant by the term Broadcast storm?
a continuous broadcast of frames in looped switches. Never ending that broadcast and switches getting the same message from different ports and continuously updating it.
Which type of VLAN is most useful in a shared hot-desk type offic setup where each user plugs their laptop into a docking station as required?
The MAC address based VLAN is more suitable as it is more flexible and individual switch port VLAN member is not hardwired and will change based on the device plugged in.
