Transcript Chapter 6

Forwarding and Routing IP Packets
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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
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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
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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
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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
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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)
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Forwarding Example - Fig. 6.13
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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
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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.
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Figure 6.15 Address aggregation
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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.
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Figure 6.16 Longest mask matching
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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)
•
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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
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