Sol22 - CHAPTER 22 Network Layer Delivery Forwarding and...

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1 CHAPTER 22 Network Layer: Delivery, Forwarding, and Routing Solutions to Review Questions and Exercises Review Questions 1. We discussed two different methods of delivery: direct and indirect. In a direct delivery , the final destination of the packet is a host connected to the same physical network as the deliverer. In an indirect delivery the packet goes from router to router until it reaches the one connected to the same physical network as its final destination. 2. The three common forwarding methods used today are: next-hop, network-spe- cific, and default methods. In the next-hop method , the routing table holds only the address of the next hop for each destination. In the network-specific method, the routing table holds only one entry that defines the address of the destination network instead of all hosts on that network. In the default method , a host sends all packets that are going out of the network to a specific router called the default router. 3. A routing table can be either static or dynamic. A static routing table contains information entered manually. A dynamic routing table is updated periodically by using one of the dynamic routing protocols such as RIP, OSPF, or BGP. 4. RIP is an intradomain routing protocol that enables routers to update their routing tables within an autonomous system. 5. A RIP message is used by a router to request and receive routing information about an autonomous system or to periodically share its knowledge with its neigh- bors. 6. The time-out for the expiration time is 6 times that of the periodic timer to allow for some missed communication between routers. 7. The hop count limit helps RIP instability by limiting the number of times a mes- sage can be sent through the routers, thereby limiting the back and forth updating that may occur if part of a network goes down. 8. The two major shortcomings are two-node instability and three-node instability . For the former, infinity can be re-defined as a number such as 20. Another solution is the split horizon strategy or split horizon combined with poison reverse. These methods do not work for three-node instability.
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