It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. Again a basic question. It's MAC address table is as one of the follows. Suppose Switch A has 24 ports can be any number , this switch is has been configured to have 3 vlans, namely vlan 10 for ports vlan 20 for port and the default vlan - vlan 1 for ports Now if this switch receives a frame destined for port 11 vlan 20 , but the frame does not have a vlan tag.
The switch will assume it to be a part of the default vlan and look up in the part its L2DA table for the default Vlan. Ofcourse it wouldn't find an entry for port 11 in the table because entry for port 11 is in the part L2DA table reserved for vlan 20 Now as the switch coudn't find the required MAC address in its table, it ll do what switches do best!
But remember vlans do not receive broadcast messages. So the frame never reaches its destination. Long story short, for correct delivery, Switch needs to look in the correct portion of its L2DA table and for that we need vlan tags. An access port is a port which carries traffic for only one VLAN. On Trunk ports, all the frames are still carried across the wire in the form of 1 s and 0 s. Something has to exist in order for the sending switch to indicate to the receiving switch which 1 s and 0 s belong to which VLAN.
The VLAN Tag will be added whenever a frame is crossing a trunk port and removed when the other switch receives the frame. It will look like this:. You can read more about VLANs and how they work in this article , and how to configure them on Cisco switches in this article.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why is VLAN tagging required? Also, switches could keep track of the port to which devices were connected to. This means that switches do not need to flood packets out all ports except to the port on which a device is connected. Note : Flooding still occurs for broadcast packets and also for unicast packets for which the switch does not know about the destination MAC address.
However, switches were still limited to a single broadcast domain which means that broadcast packets are sent to all ports on that switch. It also meant that segmentation was on a per-device basis: if you wanted to differentiate between sets of users on the network, you need to connect them to different switches. While this is not a big deal on smaller networks, it is clearly inefficient on larger networks.
Enter the world of VLANs. Being logical, VLANs are not restricted to the physical location of devices and can even span multiple switches. This means that devices within a certain group do not have to be connected to the same switch for local layer 2 communication to occur between them.
We will not be discussing interVLAN communication in this article. Apart from providing logical segmentation of devices, VLANs are also useful for addressing security, easing network management, and also improving the performance of a network e. While different vendors have their own proprietary method for creating this tag e. The first 16 bits in this field TPID are used to identify the frame as an This means that all the ports on that switch will belong to the default VLAN by default pun intended.
This is the reason you can buy a new switch, connect multiple devices to this switch, assign these devices IP addresses, and they can immediately communicate with themselves. When these devices send packets to the switch, they send plain Ethernet frames i. It means these devices tag the packets they send and can also understand when they received a tagged packet. Since VLANs can span multiple switches, it means there needs to be a way for tagged packets to travel from one switch to another.
In this case, the switch will need to tag packets correctly for their correct VLANs as they exit the port and the receiving device e. Depending on the vendor, tagged ports are able to carry traffic for all VLANs by default but a filter can be applied on such ports to limit the allowed VLANs. There is the last scenario we have not considered: what should a tagged port do if it receives an untagged packet? The switches need to be configured beforehand for working properly with the process of VLAN tagging.
With this system, multiple broadcast systems can be segregated into individual domains. Bridging traffic can be forwarded with the use of this system. Clients and information can be organized, configured and grouped logically.
Overall, the functionality of the system is optimized. By: Justin Stoltzfus Contributor, Reviewer. By: Satish Balakrishnan. Dictionary Dictionary Term of the Day. Natural Language Processing.
0コメント