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How are VLANs designed?

VLANs or Virtual local area networks are multiple broadcast domains distinct in nature that have been created by partitioning the single layer 2 network in computer networking.

Design of VLANs

– VLANs are kept in mutual isolation with one another for an effective passing of the data packets between them via routers.
– The number of routers may vary from one to more than one.
– Such a partition can only be achieved on devices such as switches or routers.
– The only thing available on the simpler devices is the port level partitioning.
– The VLANs can also be shared among many devices through the use of dedicated cabling for each of them.
– Whereas in more advanced devices, tagging might be used for the packet marking task.
– This lets transportation of the data within various VLANs through a single trunk.
– VLANs are a great solution for simplifying the network design.
– Using a VLAN, hosts can be grouped together based up on their common requirements.
– This does not depend where the hosts are located.
– The attributes of both LAN and VLAN are same.
– Also, for configuring the VLAN membership, one need not do any relocation of the devices rather a software can be used.
– This concept of VLAN is today employed by a number of enterprise level networks.
– All the interfaces are considered to be in the same broadcast domain by the switch in the absence of a VLAN.
– The physical replication of a VLAN would require a parallel but separate connection of network cables as well as equipment.
– One point to be noted is that the bandwidth is shared by VLANs unlike the physical separate networks.

Designing of a VLAN requires two important things:
1. QoS (quality of service) prioritization
2. Aggregated links

– IEEE 802.1Q is the protocol that is commonly used in the configuration of the VLANs.
– In order to provide multi–vendor VLAN support, this method was introduced by the IEEE committee.
– Explicit tagging is performed by both the IEEE 802. 1Q and ISL.
– An external tagging process is used by the ISL that does not introduce any modification in the existing frame.
– The internal tagging is used by the IEEE 802.1Q that lets it to work up on trunk links as well as the access links.
– The commodity hardware can handle the frames since they are standard Ethernet.
– A given Ethernet can have 4096 VLANs at the maximum as per the IEEE 802.1Q.
– However, by this no limit is imposed up on the number of IP subnets in a network as such.
– This is so because multiple IP subnets exist in a single VLAN.
– With the shortest path bridging, the VLAN limit has been expanded to 16 million.
– For multiplexing VLANs, this is one way involving a high speed backbone.
– In ISL, the header is joined with the frame for transporting the VLAN IDs between the routers and the switches and then both are encapsulated.
– Overhead is also added to the data packet in form of a header of 26 – byte that contains a 10 bit VLAN ID.
– In addition to this, a 4 byte CRC is also added to the frame’s end.
– This is for the checking of the frame.
– The header helps in identifying which frame belongs to which VLAN.
– Only if the packet if forwarded out of a port that has been set as a trunk link, then only the VLAN ID is added.
– In the opposite case i.e., if the port has been set as an access link, no encapsulation is done.
– If the size of the collision domain is reduced in an Ethernet segment, the performance of a VLAN can be improved significantly.

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