 Having understood the operation of the metropolitan Ethernet networks in the context of the technology such as the Ethernet passive optical networking, we know that metropolitan Ethernet network is an amazing way to extend the coverage of Ethernet LANs. However, when it comes to different customer premises, different administrative domains, we actually have to isolate the metro Ethernets into multiple Ethernet domains. For that, the concept of bridging is important. In this module, we shall understand what these bridges do, specifically the service provider bridges to the customer networks, and then we look at the operation specifically. So the goal is to extend the scalability and the coverage of Ethernet networks for metropolitan areas. For that, the provider networks use the concept of provider bridges. These bridges isolate, like if you recall, the bridges be used for isolating the broadcast domains in Ethernet. So these provider bridges extend the network coverage to different customer networks. It means multiple customer networks can be connected to each other through various service provider bridges. The implementation of provider bridges is based on certain technological features. The first one is similar to VLAN ID, here an additional tag called the S tag is introduced. It is again 12 bits in length. So giving a total of 4096 different IDs of which two are reserved for administrative purposes. So it means using these identifiers, then the multiple instances can be created from the customer network to another customer network through the user-to-network interface. Here the user-to-network interface is actually the interface between the customer network and the provider network through the bridges. Since there isn't much change, as far as the addressing is concerned, so the source and destination MAC addresses remain the same. The source MAC, say, is from the customer network on one end. The destination MAC is on the customer network on the other end. So it means that these two MAC addresses remaining the same, the additional ID, the S ID is used to establish a link between these end-to-end networks. So we see now we have the provider network using these 12 bits in addition to the VLAN ID that is there in the customer network. The VLAN ID essentially identifies different customers belonging to different administrative domains, the VLAN, the virtual LANs. The provider network actually appears as one large Ethernet network to the devices within the provider. So it means the provider bridges are now going to utilize that S tag only to take the traffic from one end of the provider network to the other end of the provider network, that is from one customer network to another customer network. Consequently, we have two LAN IDs essentially making 24-bit VLAN ID. That is why sometimes it is also known as the queue-in-queue tagging network. Let's look at the figure of it. Here we see we have multiple customer networks. These customer networks are connected to the provider network through a user-to-network interface. Till the traffic stays within the customer network for the intra-customer network traffic, we have the source and destination MAC addresses. We have the VLAN ID here known as the customer VLAN tag or the typical VLAN ID as we know it. But when that traffic enters into the provider network, then in order to make the bridges within the provider network understand what to do with this traffic, then a specific S tag is associated as you can see in here. So this is the only addition that we see once the provider bridges are used to relay traffic between the customer networks connected to each other via a provider network.