Forwarding and Routing IP Packets Dont ask for

Forwarding and Routing IP Packets Don't ask for a light load, but rather ask for a strong back. - Unknown Chapter 6 1

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 2

Direct versus Indirect Delivery of Packets • Direct Delivery – Source and destination of the packet are on the same network, or – Delivery is between the last router and the destination host Chapter 6 3

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 4

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 5

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) Chapter 6 6

Forwarding Example - Fig. 6. 13 Chapter 6 7

Table 6. 1 Routing table for router R 1 in Figure 6. 13 Show the forwarding process if a packet arrives at R 1 in Figure 6. 13 with the destination address: · 180. 70. 65. 140 (Eg: 8) --> Forwards through m 0 · 201. 4. 22. 35 (Eg: 9) --> Forwards through m 3 · 18. 24. 32. 78 (Eg: 10) --> Forwards through m 2 Chapter 6 8

Address (or, Route) Aggregation • Router R 1 (in Fig. 6. 15) is connected to networks of 4 organizations. R 1’s routing table contains 4 entries (or, routes) for these 4 networks. • In router R 2’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 R 2, this is called Address Aggregation. • As the 4 route entries in R 1 are aggregated into a single route entry in R 2’s routing table, this is also called Route Aggregation. Chapter 6 9

Figure 6. 15 Chapter 6 Address aggregation 10

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 11

Figure 6. 16 Chapter 6 Longest mask matching 12

How does the Routing Table get built? • A route entry can be placed in a routing table in three basic ways 1. Through direct connection. A router knows about its directly connected networks through interface configuration (Connected routes) 2. Manual configuration (Static routes) 3. Dynamically, by using a routing protocol (Dynamic routes) • Chapter 6 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 13

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