 In order to realize Metro Ethernet and the Metro Ethernet network fully there are certain technologies which work in conjunction with Ethernet technology as such. We'd look at each one of these one by one. The first one being VLAN or the virtual LAN. It's an IEEE standard and it is denoted by IEEE 802.1 Q. In this module we'd briefly explore it. We'd look at what we what do we mean by virtualization? What is a Q tag or a VLAN ID? And we'd see certain problems which arise and the solution which is provided in this standard. So LAN virtualization is actually the provisioning of basically an E-Line service as we just mentioned that the E-Line, E-Line and E-Tree. It is based on the IEEE standard for providing virtual local area network. It means it's not a physical local area network. As we all know a local area network is characterized by the presence of a switching device like a switch and having its own collision domain and a broadcast domain. So it means that the IP addressing scheme of one LAN is going to be different from the IP addressing scheme of another LAN if these two LANs are connected to the same router. So what we understand from here is that if a router actually has to provide two LANs that it needs to have two Ethernet interfaces, each Ethernet interface is in turn connected to a switch to which there are certain other computers connected. This is how a physical Ethernet network or Ethernet networks topology looks like. If we want to virtualize it then what we can do is we can take or procure a switch with say n number of ports, take n as 16, then we earmark and we actually assign these 16 ports belonging to say four different departments. For instance admin, operations, HR and students affairs. Then if we have these four different departments on these 16 ports of a switch, then each of these departments is going to have four ports. It means that four ports of a certain department would be communicating with each other and this is going to be known as the intra LAN traffic. Now if traffic from one department say from student affairs to the operations has to go then it would be the inter VLAN traffic. Now this concept of how VLANs are defined is quite interesting and needs some more lectures but for now we just confine our understanding to how the metro Ethernet uses virtual LANs. So as I've already given you the example, the traffic of different departments actually is segregated or is separated in different VLANs on the same switch. So how is it achieved? It is achieved through assigning each virtual LAN with a unique identifier known as Q tag. The Q tag is a field that is defined by IEEE 802.1 Q as 12 bits. So it means up to 4096 unique identifiers are possible of these two are reserved for this different administrative purposes. So we have 4094 unique VLAN IDs. Now each ID can actually be assigned to a certain LAN, a virtual LAN. So multiple enterprises can actually connect on the same switch by having their own VLAN IDs. If we just assume that the switch is within one administrative domain and the organizations which are connected to that switch or that LAN are managed by a single administrative authority, then there's not going to be any conflict. However, there is a probability that when we look at it in the context of metro Ethernet network, the Q tags or the VLAN IDs of different VLANs may overlap because each administrative authority may assign VLAN IDs and these VLAN IDs may end up being same. So conflicts and some kind of overlaps are going to arise. So there is a recommended solution that is in order to achieve such high scalability. Scalability is one of the requirements for the carrier Ethernet and at the same time, avoiding such overlaps is a compulsory requirement. So individual enterprises actually need to define separate LANs for different departments and there has to be some kind of mechanism that partitions or isolates these Q tags or VLAN IDs. And for that, we have specific methods and protocols known as the end devices known as the provider bridges and the provider backbone bridges. Now, these are the switches with some additional functionality. We devote some time to the understanding of these as well.