Forwarding and Routing IP Packets Don't ask for a light load, but rather ask for a strong back. - Unknown Chapter 6

Objectives Describe how packets are delivered from a source to a destination using direct and indirect delivery methods Explain how routers forward packets based on the entries in their routing table(s) Explain what is meant by address aggregation (or, route aggregation) Describe how routers build their routing tables Chapter 6

Direct versus Indirect Delivery of Packets Source and destination of the packet are on the same network, or Delivery is between the last router and the destination host Chapter 6

Indirect Delivery of Packets If source and destination of the packet are NOT on the same network, the packet is forwarded from one router to another until it reaches a router connected to the same network as the destination host Chapter 6

Routing and Forwarding Routing is the task of finding a path from a sender to a desired destination. Forwarding means sending the packet towards its destination, based on the routing information. Routing Table: A compilation of all the networks (and sometimes hosts) that the router can reach Entries in the routing table are known as “routes” and consist of a network address, a “next hop” (the IP address of the next router in the path to the destination), and a metric indicating how “good” the path is Chapter 6

Understanding Forwarding When a packet is received on a router interface, after checking its validity (by checking the Header Checksum, etc.) the router must find out the packet’s final destination The router reads the Destination IP Address, and then looks in the Network field of its routing table for a match If a match is found, then the packet is forwarded to the corresponding next hop, which is usually another router on a directly connected network If a match is not found, the packet is sent to the router listed as the default next hop in the routing table (normally with a network address of 0.0.0.0) Chapter 6

Forwarding Example - Fig. 6.13 Chapter 6

Table 6.1 Routing table for router R1 in Figure 6.13 Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address: 180.70.65.140 (Eg: 8) --> Forwards through m0 201.4.22.35 (Eg: 9) --> Forwards through m3 18.24.32.78 (Eg:10) --> Forwards through m2 Chapter 6

Address (or, Route) Aggregation Router R1 (in Fig. 6.15) is connected to networks of 4 organizations. R1’s routing table contains 4 entries (or, routes) for these 4 networks. In router R2’s routing table, these 4 separate route entries are aggregated into a single route entry. As the 4 blocks of addresses for the 4 organizations are aggregated into one larger block (i.e., 140.24.7.0/24) in router R2, this is called Address Aggregation. As the 4 route entries in R1 are aggregated into a single route entry in R2’s routing table, this is also called Route Aggregation. Chapter 6

Figure 6.15 Address aggregation Chapter 6

Longest Mask (or, Prefix) Matching When there are routes with different masks (or, network prefixes), the routing table is sorted from the longest mask to the shortest mask. This ensures that, when forwarding a packet, if there are multiple matches to the destination network in a routing table, the route with the longest mask (or, prefix) is chosen. Chapter 6

Figure 6.16 Longest mask matching Chapter 6

How does the Routing Table get built? A route entry can be placed in a routing table in three basic ways Through direct connection. A router knows about its directly connected networks through interface configuration (Connected routes) Manual configuration (Static routes) Dynamically, by using a routing protocol (Dynamic routes) Routers use routing protocols to exchange information periodically about various networks they are connected to, and use that information to build up their routing tables Chapter 6

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