Where is the VLAN header located?
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Virtual local area networks, or VLANs, can be used to segment traffic within a network in combination with subnetting. VLANs keep traffic from different networks separated when traversing shared links and devices within a topology. This process, also known as VLAN tagging, is invaluable to limiting broadcast network traffic and securing network segments. VLAN tagging is an integral part of networks of all sizes and is supported on MX security appliances, MR access points, and MS series switches. This can be done for both data, and management traffic independently. Show Best practices are to use a single subnet per VLAN ID Common TermsVLAN - Virtual local area network; logical identifier for isolating a network - A port enabled for VLAN tagging - A port that does not tag and only accepts a single VLAN Encapsulation - The process of modifying frames of data to include additional information 802.1Q - The most common encapsulation method for VLAN tagging. This is the method used by Meraki devices. Native VLAN - The VLAN associated with all untagged traffic on a trunk Subnet - A logical network which may be derived from a larger network ID Best PracticesVLAN-enabled ports are generally categorized in one of two ways, tagged or untagged. These may also be referred to as "trunk" or "access" respectively. The purpose of a tagged or "trunked" port is to pass traffic for multiple VLAN's, whereas an untagged or "access" port accepts traffic for only a single VLAN. Generally speaking, trunk ports will link switches, and access ports will link to end devices. Trunk ports require more steps to successfully negotiate as a trunk. Both ends of the link must have the following in common:
While a link may successfully establish as up with mismatched allowed or native VLANs, it is best practice to have both sides of the link configured identically. Mismatched native VLANs or allowed VLANs can have unforeseen consequences. Recall that the native VLAN is the VLAN associated with untagged traffic. Mismatched native VLANs on opposite sides of a trunk can inadvertently create "VLAN hopping." This is often a method of intentional attack used to sneak into a network and is an open security risk. Consider the following example and diagram. A client is plugged in to a VLAN 1 access port and desires an address from the DHCP server on the VLAN 1 subnet (192.168.1.0/24). There is a native VLAN mismatch on the trunk link between the two switches, which will prevent the client from receiving the appropriate address. Coming from an access VLAN 1 port, when the DHCP request gets to the trunk on the switch, it will be untagged traffic, as the native VLAN is 1. When the traffic gets to the other switch on the other side of the trunk, the native VLAN is 10. The untagged traffic from the switch on the right will be treated as VLAN 10 on the switch on the left. The DHCP server will reply to the DHCP request for VLAN 10 (192.168.10.0/24) and send the address back to the client. Once again, as VLAN 10 is untagged on the left switch, it will be treated as VLAN 1 on the right switch because of the native VLAN mismatch, and the client will ultimately obtain an address in the wrong subnet.
This, along with all other trunk configuration, must be identical for the entire path through the network that traffic will follow. For example, if there are three switches between a client and a gateway on VLAN 100, it must be trunked through all the switches' connecting links (shown below). 
While VLANs are effective for separating network segments and limiting broadcast traffic, it is often a requirement for subnets separated by VLANs to be able to communicate. This can be accomplished only through a layer 3-enabled device that can route between the VLANs. Even if both VLANs exist on a device, their traffic will be segregated unless mediated by a layer 3 routing device. With any single shared media LAN segment, transmissions propagate through the entire segment. As traffic activity increases, more collisions occur and transmitting nodes must back off and wait before attempting the transmission again. While the collision is cleared, other nodes must also wait, further increasing congestion on the LAN segment. The left side of depicts a small network in which PC 2 and PC 4 attempt transmissions at the same time. The frames propagate away from the computers, eventually colliding with each other somewhere in between the two nodes as shown on the right. The increased voltage and power then propagate away from the scene of the collision. Note that the collision does not continue past the switches on either end. These are the boundaries of the collision domain. This is one of the primary reasons for switches replacing hubs. Hubs (and access points) simply do not scale well as network traffic increases. The use of switches at Layer 2 eliminates much of the scaling problem because they filter out problems such as collisions. Instead, transmissions are now governed by the behavior of the switches and the broadcast domain. A broadcast domain defines the area over which a broadcast frame will propagate. For example, an ARP request issued by PC 3 results in a broadcast frame that propagates through the switches all the way to the routers as shown in . A broadcast frame has the broadcast address ( With the improved performance and filtering resulting from the use of switches, there is a temptation to create large Layer 2 topologies and add lots of nodes, but this creates a large broadcast domain. The problem is that all devices on a network (computers, printers, switching equipment, etc.) generate broadcast and multicast frames that traverse the entire broadcast domain, competing with data traffic for bandwidth. Much of this traffic is for management of the network and includes protocols for address resolution (ARP), dynamic host configuration (DHCP), spanning tree (STP), and an assortment of Windows tasks. illustrates the potential difficulty. Assume that PC1 has generated the following requests: ARP, Windows registration, and DHCP. Because all of the requests use a broadcast frame, as they are received at Switch 1, the frames are forwarded in all directions. As the other switches in the topology follow suit, the frames traverse the entire network and are received at all other nodes and the routers. As the number of network nodes increases, the amount of overhead also increases. Each switch might be connected to dozens of nodes, with each node generating the several broadcast frames. If enough traffic is created, even a switched network can have poor performance. Deploying VLANs can help solve this problem by breaking up the broadcast domain and separating the traffic. Where is the VLAN located?A VLAN is identified on network switches by a VLAN ID. Each port on a switch can have one or more VLAN IDs assigned to it and will land in a default VLAN if no other one is assigned. Each VLAN provides data-link access to all hosts connected to switch ports configured with its VLAN ID.
Where is an 802.1Q tag inserted in a frame?An Ethernet frame can contain an 802.1Q tag, with fields that specify VLAN membership and user priority. The VLAN tag is inserted between the source MAC address and the Type/Length fields in the Ethernet frame.
Which layer encodes the VLAN tag?To enable a network device to identify frames of different VLANs, a VLAN tag field is inserted into the data link layer encapsulation.
What is VLAN on router?A virtual LAN (VLAN) is a local area network that maps devices on a basis other than geographic location, for example, by department, type of user, or primary application. Traffic that flows between different VLANs must go through a router, just as if the VLANs are on two separate LANs.
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