frame relay và ATM là ví dụ về liên kết NBMA. Các mạng chuyển tiếp địa chỉ cho các tuyến đường trong bảng định tuyến được ánh xạ để nhận dạng mạch ảo bằng cách sử dụng một bảng duy trì bởi các nút gửi. Inverse ARP được sử dụng để khám phá các địa chỉ mạng lưới của các nút trên đầu khác của các mạch thức định tuyến | Routing 93 point unicast services. Packet-switched WAN links such as frame relay and ATM are examples of NBMA links. The forwarding network address for the route in the routing table is mapped to the virtual circuit identifier using a table maintained by the sending node. Inverse ARP is used to discover the network addresses of nodes on the other ends of the virtual circuits. Routing protocols Figure Router functions. 94 Connecting Networks Together Router Figure is a functional diagram of a router. A database of routes is stored and maintained by all routers. Called a routing table it contains information concerning routes between the node owning the table and the potential destination nodes. At a minimum it includes the destination ID intermediate interface ID s and forwarding address es and information to distinguish the best route to use when multiple routes are possible. It is significantly more complex than the table maintained by bridging devices. However its extent is limited to the immediately reachable nodes that surround it so that it is significantly smaller. Searching a routing table is a relatively simple task. For each route a typical routing table will include the following fields Destination address The IP address of the node to which the source directs the packet to be delivered. For direct deliveries the destination IP address carries the same network ID as the router. For indirect deliveries the destination address does not carry the same network ID as the router and the datagram is sent to the forwarding address contained in the table entry. Network mask A bit mask is used to determine the network ID of the destination IP address. An IP datagram with a destination IP address that contains the specific network ID for this route will be forwarded over it. Forwarding IP address For indirect deliveries the IP address of a directly reachable router to which the IP datagram is forwarded for eventual delivery to the destination
