mang lại cho người sử dụng càng gần đến các máy chủ càng tốt. Ví dụ, nó sẽ làm cho tinh thần để gửi một người sử dụng ở Bắc Carolina đến một máy chủ ở thành phố New York, chứ không phải hơn là một máy chủ ở San Jose, California. 2. GSLB cung cấp khả năng dự phòng trong trường hợp trang web nào không. | Multipurpose Devices 49 you ll probably need your own Layer 2 switches but you will not need a router since the colocation company would provide this. Since it s much more cost effective to aggregate several clients off of a router port rather than dedicate a port to each customer most providers use Layer 2 switches to distribute router-port traffic see Figure 5-11 . A client s Layer 2 switch would plug directly into the provider s Layer 2 switch resulting in a configuration known as the six pack two routers two colocation provider switches and two client switches . VRRP or similar protocols provide the redundancy on the routers. Figure 5-11. Six-pack design To prevent a bridging loop shown in Figure 5-12 some form of port blocking must be done on one of the ports to prevent multiple Layer 2 paths from existing. Spanning-Tree Protocol STP is a protocol that can take care of this automatically. Each port has a cost associated with it with the lowest cost being preferred. If STP detects multiple paths it shuts off all but the highest priority port with the lowest number . It can be set up on the provider s end the site s end or both. Figure 5-13 shows an example of STP on the provider s right switch blocking the path between the provider s right switch and the site s right switch. Multipurpose Devices It is now possible and even advantageous to merge two or more typically separate functions into one. For example a multitude of Layer 2 3 switches on the market incorporate the port capacity and Layer 2 functionality of a switch with the Layer 3 routing functions of a router. With the switch-based load balancers incorporating Layer 4 5-7 with Layer 2 3 functionality is also possible. For the purposes 50 Chapter 5 Introduction to Architecture Figure 5-12. Six-pack bridging loop Figure 5-13. Six pack with STP blocking of this book however I ll keep devices and their layered functions separate. For example a Layer 2 device will be only a Layer 2 device. This is to keep .
