DSL là một dịch vụ không đối xứng cung cấp băng thông ngược dòng tương đối ít, đó sẽ là không đủ cho các máy chủ Web Internet trong việc xây dựng. Một T-1 cho thuê đường dây sẽ là một kết nối thích hợp cho việc xây dựng này bởi vì nó cung cấp băng thông đủ cả thượng nguồn và hạ nguồn và hoạt động suốt ngày đêm. | 5-16 Chapter 5 Using Routing and Remote Access The following sections compare the characteristics and capabilities of RIP and OSPF providing the information you need to select the appropriate one for your network. Understanding Routing Metrics One of the most important functions of dynamic routing protocols is to evaluate the relative efficiency of routes to a specific destination. On a network with redundant routers there might be several paths that packets can take from a particular source to a particular destination. When this is the case a router might have multiple entries for the same destination in its routing table and it is up to the router to forward packets using the most efficient route available. Routing table entries all include a numeric qualifier called a metric which the router uses to evaluate routes to the same destination. The lower the metric value the more efficient the route. Although IP routers all use the metric the same way there is no standardized definition for what the metric actually represents if anything. On a network that uses static routing network administrators can arbitrarily assign metrics to the routing table entries they create. As long as the routes the administrators want the traffic to take have lower metric values the routers will choose them instead of routes with higher values. Keeping track of the relative metric values for all the routing tables on the network is another chore that falls to the network administrator who opts to use static routing on a large network. In dynamic routing the metric values must represent a specific attribute for routing protocols to compute them. However different routing protocols use different algorithms to compute the metric for each routing table entry this is one of the main characteristics that differentiates between routing protocols. Distance Vector Routing RIP uses one of the simplest and most obvious methods for computing routing table metrics. The metric value for each entry in
