Chapter Overview
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Routing Principles
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Understanding Routing--important
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Routing refers to the process of taking a
packet from one device and sending it
through the network to another device on a
different LAN using the network address
The router on the same network as the host
then uses the physical address to deliver the
packet
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Routed protocol vs. Routing protocol
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routed protocols (IP, etc) route your data and
routing protocols send routing updates between
routers about the status of the network so that
your routed protocol data can be routed.
Read more: http://www.dummies.com/howto/content/looking-into-routed-versus-routingprotocols-for-t.html#ixzz0nn1fJY6I
We discuss routing protocols in next chapter
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Understanding Routers
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A router is a system connected to two or more
networks that forwards packets from one network to
another.
Routers operate at the network layer of the Open
Systems Interconnection (OSI) reference model.
Routers can connect networks running different datalink layer protocols and different network media.
Large internetworks often have redundant routers,
providing multiple routes to a destination.
Routers select the most efficient route to each
destination.
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Redundant Routers
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Router Products
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The Microsoft Windows 2000/2003/2008, Microsoft
Windows NT, and Novell NetWare operating
systems include routing capability. Linux?
A stand-alone router is a hardware device that is
essentially a special-purpose computer (with CPU,
RAM, HD, and NICs).
Cost can go from $30 to $3M
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To route, a router needs to know
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Destination address
Neighbor routers and possible routes to all
remote networks (routing table)
The best route to a remote network
How to maintain and verify routing info
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Direct and Indirect Routes
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Direct route. The route taken when a computer
running TCP/IP transmits a packet to a destination
on the local network
Indirect route. The route taken when a computer
running TCP/IP transmits a packet to a destination
on another network by forwarding the packet to a
router on the local network
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Routing example
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The main example
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Host_A sends a ICMP (Internet Control Message
Protocol) package to Host_B
Per https://en.wikipedia.org/wiki/Internet_Control_Message_Protocol,
it is an IP layer protocal
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Internet Control Message Protocol
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ICMP creates an echo request (ping 172.16.20.2) and
passes it to IP
IP realizes the IP address is not local, it gets the default
gateway, which it has the IP address 172.16.10.1
If IP does not have the MAC address, it uses ARP to get
that
Data Link layer protocol is invoked to send a frame to the
router
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All devices on the LAN get the message. Each runs CRC,
and checks the destination MAC address if CRC checking is
a go. Discards otherwise .
Router finds the match, removes envelope, passes it to IP
IP checks the destination IP address against its routing
table
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The touting table has an entry for the 172.16.20.2, either
as an IP address, network, or default.
If the routing table does not have a matching entry, the
message is discarded and a “destination-networkunreachable-message is sent back to the requester.
In this case, router sends the message to 172.16.20.1
interface/port and knows it is directly connected
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Interface 172.16.20.1 needs to either know the MAC
address of 172.16.20.2 already or use ARP to resolve it
Again Data Link and Physical layers deliver the IP packet
Host_B receives the frame, runs CRC, checks the MAC
address – it is for itself, checks protocol and passes it to IP
IP passes the package to ICMP, which generates an echo
reply message, trigging a message from B to A 
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Selecting a Table Entry
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A Routing Table: Sample
Network
Address
Netmask
Gateway
Address
Interface
0.0.0.0
0.0.0.0
192.168.2.99
192.168.2.2
127.0.0.0
255.0.0.0
127.0.0.1
127.0.0.1
192.168.2.0
255.255.255.0
192.168.2.2
192.168.2.2
192.168.2.2
255.255.255.255
127.0.0.1
127.0.0.1
192.168.2.255
255.255.255.255
192.168.2.2
192.168.2.2
224.0.0.0
224.0.0.0
192.168.2.2
192.168.2.2
255.255.255.255
255.255.255.255
192.168.2.2
192.168.2.2
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Routing Table Entries for a Router
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A routing table on a router contains
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Entries for all of the networks that the router is
attached to
Entries provided manually by administrators or
dynamically by routing protocols
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Static Routing and Dynamic Routing
Static Routing
 Requires administrators
to create routing table
entries manually
 Suitable only for small
networks
Dynamic Routing
 Creates routing table entries
automatically by using
routing protocols.
 Suitable for large networks
 Automatically compensates
for network infrastructure
changes
 Reduces administrative
workload
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Interior Gateway Protocols (IGP) and
Exterior Gateway Protocols (EGP)
Border Gateway Protocol
(BGP).
EGP
IGP
More IGP protocols later
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IGP and EGP
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An interior gateway protocol (IGP) is a routing
protocol that is used within an autonomous system
(AS).
In contrast an Exterior Gateway Protocol (EGP) is
for determining network reachability between
autonomous systems and makes use of IGPs to
resolve routes within an AS.
The interior gateway protocols can be divided into
two categories: 1) Distance-vector routing protocol
and 2) Link-state routing protocol.
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