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Chapter Overview

Routing Principles

Understanding Routing--important

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

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/how-to/content/looking-into-routed-versus-routing-protocols-for-t.html#ixzz0nn1fJY6I

We discuss routing protocols in next chapter3

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Understanding Routers 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 data-link 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

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

To route, a router needs to know

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

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

Routing example

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The main example

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

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-network-unreachable-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

A routing table on a router contains 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

Requires administrators to create routing table entries manually

Suitable only for small networks

Creates routing table entries automatically by using routing protocols.

Suitable for large networks

Automatically compensates for network infrastructure changes

Reduces administrative workload

Static Routing Dynamic Routing

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Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP)

IGP

EGP

Border Gateway Protocol (BGP).

More IGP protocols later

IGP and EGP

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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