9-Understanding IP Routing Concept

8/12/2019 9-Understanding IP Routing Concept

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Understanding IP Routing Concept

Understanding IP Routing ConceptArticle covers the following CCNA/ICND1 Exam Topics:

Under Operation of IP Data Networks:

1. Recognize the purpose and functions of various network devices such as Routers, Switches, Bridges, and Hubs.

2. Select the components required to meet a given network specification.

3. Predict the data flow between two hosts across a network.

Under IP Routing Technologies:

1. Differentiate methods of routing and routing protocols such Static vs. Dynamic.

Recommended Study Plan:

1. Download the ICND1v2 Exam Topics Sheet from Cisco Website.

2. Follow the Steps and the Articles under IP Networks Fundamentals by order.

Average Time Required Studying this Article: 1 Hour

Packets Routing and Frame Forwarding Concept

Please Note:I have used the word Routing Packets with Layer 3 (which it does) and the word Forwarding Frame

with Layer 2 to distinguish between the 2 operations. In the end, Routing and Forwarding means the same thing, bu

it doesnt make sense to use Routing at the Data-link Layer, therefore, I used Forwarding instead.

So what is IP Routing?

IP Routing is an umbrella term, it refers to a set of protocols to determine the best path that an IP Packet can follow

order to travel across multiple networks from its source to its destination. The set of protocols run together at the

Network Layer to help hosts and routers route IP packets. IP Routing will involve too many network devices to

accomplish an IPPackets Routing. Switches, Firewall, and Routers all of them will be involved to Route the IP

Packet.

Network Layer deals with ONE major protocol to Route Packets

Network Layer defines set of functions based on One Major Logical Protocol. There are two versions of this logical

protocol where other layer 3 functions revolve. IPv4 and IPv6, both define network layer routing functions, but with

different details for each. This article focuses on IPv4. Internet Protocol (IPv4), and as a Layer 3 protocol, focuses o

Routing or guiding the IP packets that carry the Data from the upper Layers as source device to the destination

device using Dotted Decimal Notation known by Logical Addressing or IP Addressing System, then IPv4 Routing

handle the Packets to the Data-Link for Framing and finally Physical Layer for transmitting frames as Bit Stream.
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What are the required components for IPv4 Routing to happen?

1.The Router itself the Hardware Piece

2.Router Operating System Internet Operating System (IOS)

3.IP Addressing System based on Dotted Decimal Notation

4.Routing Protocols such OSPF or EIGRP creates and maintains an Internet Road Map using IP Addressing

System as reference Points

Now, Routers can guide what is called IP Packet to reach its final destination using an Internet Road Map built by

OSPF or EIGRP using IPv4 Addressing System.

IP Routing vs. GPS Routing

If GPS Routing guide Human to drive from point A to point B, then IP Routing guide Routers to Route IP Packets fro

point A to point B. If Human understand how to use GPS Road Map, then Routers running Routing Protocols such

OSPF or EIGRP understand how to use Internet Road Map.

Routing Protocols

Create, maintain, and present to the Router an Internet Road Map using IP Addressing System similar to GPS Map

Reference Points. Every Router loaded with the required components understand IP Routing Internet MAP. Routers

can be dedicated Hardware like Cisco or Software base running in Linux box using the same exact Routing Protoco

to create, maintain, and present Internet MAP for the Router to use.

A Road MAP system like GPS device wont really care which method you would use to reach point B, rather, it cares

how to represent the right and the closest path to reach point B; its up to you to choose which moving method to us

you can walk, fly, ride a bus, car, or train, hence, different Layer! So, Network Layer uses a Universal logical Protoco

called either IPv4 or IPv6 to manage the IP Addressing System; Routing Protocols in the other hand such OSPF and

EIGRP Create and Maintain an Internet Road MAP using the IP Addressing System as Reference Points called:

Public IP Addresses.

IPv4 or IPv6 logical addresses are used to route packets from point A to point B across different types of networks

regardless of their physical structure type. Imagine that your job is to route people from point A to point B as tourist

guide, what will always concern you the most is your MAP accuracy and not how the people will be riding or walking

from point A to Point B. There will be another guy (A Data-Link Protocol) who would instruct the tourist to dress spec

shoes maybe at certain areas and to ride a special car maybe (Physical Cabling)to tour around special Mountain

areas.

Finally IPv4 manage Logical Addressing or IP Addressing combined with Routing Protocols such OSPF or EIGRP is

similar to GPS System combined with Satellite geographical reference points, but it guides IP Packets instead of

Humans.

How Routers pick the Best Route from their IP Routing Table

Routers logic uses a database table called IP Routing Table,to route Packets from Public IP Address to another

Public IP Address, or from Network to Network; the routing table lists IP addresses as groups or blocks, called IP

Networks (referring to Classful Addressing) or IP Subnets (referring to Classless Addressing) and some of these

addresses were directly learned due to directly and physically connected Networks, and some were learned using

Dynamic and Multicasting Routing Protocols such OSPF or EIGRP.

IP Routing Table can be filled as follows:


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1. Static Route Entries:Routing tables can be filled manually by the Network Engineer.

2. Directly Connected Route Entries:Physically connected links entries automatically get populated by the router

once you configure the IP address on the interface.

3. Dynamic Route Entries:Routing Protocol such OSPF or EIGRP is used to shout and inform all neighbors route

about its directly connected routes

*One of the main and major jobs of the any Routing Protocol is: to dynamically shout and Multicast the physically

connected routes to other routers, so they can fill their Routing Table with Entries to build an Internet MAP.

When a router receives an IP packet at its Network Layer, it compares the packets destination IP address to the

entries on the Routing Tableand makes a match. This matching entry also lists directions such the exit or via

interface to tell the router where to route the packet next.

How Routers Route IP Packets?

When the frame arrives, the router uses the following logic on the data link frame:

Step 1:Use the data link Frame Check Sequence (FCS) field to ensure that the frame had no error; of errors

occurred, discard the frame.

Step 2:If the frame was not discarded at Step 1, de-encapsulate the frame by striping and discarding its old data lin

header and trailer, leaving the IP packet.

Step 3:Compares the IP packets destination IP address to the routing table, and find the route that best matches t

destination address. In addition, this route will identify its outgoing interface or exit at the router.

Step 4:Once routing decision is made, encapsulates the IP packet inside a new data link header and trailer (New

Frame), with source MAC addressof the outgoing interface and a destination MAC address based on Address

Resolution Protocol (ARP) table, and forward the frame to the next-hop. The next-hop is the receiving router, and its

destination MAC address indicates its interface.

Note:keep in mind that routers sometimes uses WAN links with protocol either HDLCor PPP, in this case they don

use source or destination MAC address, instead, they use different method then MAC address. Routers use MAC

addresses to route Packets when Ethernet Standardsis used.

PC Routing the Packet

All network devices such Routers and PCs work together to perform IP routing. The hosts OS has software TCP/IP

stack, PCs use this software networking model to choose where to send IP packets, oftentimes to a nearby router if

the host they are trying to reach is located on different Subnet and Routers make choices of where to send the IP

packet next. Together, hosts and routers deliver the IP packet to the correct destination.

How a PC uses some simple routing logic when choosing where to send an IP Packet:

Step 1:If the destination IP address is in the same IP subnet as I am, forward the packet directly to the destination

host using MAC forwarding. No need for default gateway routing. Hint:IP is always needed in either case, so hosts

can learn MAC addresses from each other.

Step 2:Otherwise, route the IP packet to my default gateway as next-hop (using MAC forwarding)for further routing

Address Resolution Protocol (ARP)

Any network device initially needs the help of Address Resolution Protocol (ARP) to be able to Route Packets either
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MAC address of R3 as next-hop in the data link header to ensure that nearby router receives the frame for further

processing.

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About Imad Daou

He is the founder of CCNA HUB, a CCNA Training HUB to help CCNA students get certified. Imad has more than 1

years of IT experience as Field Service and Consulting Engineer. A+, Network+, Server+, Security+, Storage+, HP,

Dell, and IBM Hardware Certified. He's a Professional SMB IT Consultant.

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9-Understanding IP Routing Concept