Introduction to networking(Yarnfield)

Introduction to routing

Objectives Describe a router’s primary aim Differentiate between ports and interfaces Describe the basic router configuration Describe the routing table

Router overview Routers are the centre of a network They can connect multiple networks They are responsible for the efficient delivery

of packets Routers have interfaces that belong to

different networks Routers can forward packets to the network of

the final destination, or to another router that is used to reach the final destination

Each network typically requires a separate physical interface

Routes determine the best path The primary activity of the router is to forward

packets to local or remote networks This is done by

Determining the best path to send packets Forwarding packets to their destination

This is achieved via the routing table Received packets are referenced with the

routing table When a match is found the router will

encapsulate the packet into the corresponding data-link frame of the outgoing interface and forward the packet toward its destination

Ports and interfaces Ports usually refer to management ports, such

as the console port Interfaces usually refer to the connectors that

send and receive user data Interfaces come in two major categories

LAN interfaces E.g. Fast Ethernet – must have an IP and subnet mask

WAN interfaces ISDN, Frame relay – must have an IP address

AUX Task You are looking at the back of a router and

find a network cable which has a RJ-45 connector plugged into one of the ports/interfaces on the router.

You read the label where the cable is plugged in and it says AUX above it.

Is AUX a port or an interface? What is the purpose of AUX?

You have 10 minutes to come up with an answer

Forwarding packets Routers are considered layer 3 devices

because their forwarding decision is based upon the IP packet

Routing tables are searched for matching IP addresses based upon a received packet, and forwarded to the next router if no match has been found

Eventually a router will receive a packet that can forward it to a directly connected interface, which will be the packets final destination

Routers operate at layers 1, 2 and 3 of the OSI model

Switches/Hubs Task You are using switches and hubs within your

organisation. Your manager has approached you to ask you the following:

At what OSI layer(s) do switches work? At what OSI layer(s) do hubs operate? How do switches/hubs differ from routers?

You have 10 minutes to come up with the answers

Basic router configuration Name the router Set passwords

Console, AUX, telnet and secret Configure interfaces

IP address, subnet mask, description (240 chars), clock rate, turn it on!

Banner Save changes Verify configuration

Show running-config Show ip route Show ip interface brief Show interfaces

The routing table Routing tables store information about directly

connected and remote networks They tell a router what the association between the

‘next hop’ and a network is A directly connected network is one that is directly

attached to the routers interface This happens when the interface is configured with an IP

address and subnet mask of a given network A remote network is one where the router cannot

connect to it without the help of another router This is possible via static and dynamic routing protocols

Directly connected routes – visiting a neighbour Static routes – used by trains Dynamic routes – driving a car

Example

Routing table Task Using the routing table from the previous

slide, could you explain what will happen when the router receives a packet destined to the 172.16.3.0 network?

You have 10 minutes to come up with the answer and provide justifications for your logic

Routing table principles1. Every router makes its decision alone, based

on the information it has in its own routing table

2. The fact that one router has certain information in its routing table does not mean that other routers have the same information

3. Routing information about a path from one network to another does not provide routing information about the reverse, or return path

Asymmetric routing – packets can traverse a network in one direction, using one path and return through another path

Routing table principles

PC1 R1 R2PC2

1 2 3

45

1. PC1 sends ping to PC22. RA has a route to PC2’s network3. R2 is directly connected to PC2’s network4. PC2 sends a reply ping to PC15. R2 does NOT have a route to PC1’s network, so it drops the packet

Routing factors Equal cost load

balancing Unequal cost paths

(IGRP and EIGRP) Packet forwarding

involves Path determination via

metrics Directly connected

network – packet sent to host directly

Remote network – packet sent to next router

No route determined – packet discarded

Metrics Bandwidth – the data capacity of a link Delay – the length of time required to move a

packet along each link from source to destination

Load – the amount of activity on a network resource such as a router or a link

Reliability – an error rate of each network link Hop count – the number of routers that a

packet must travel through before breaching its destination

Cost – an arbitrary value, usually base don bandwidth, monetary expense, or other measurement, that is assigned by an administrator

Routing factors Switching function

Accept packet on one interface and switch it out of another

1. De-encapsulate layer 3 packet2. Examine destination IP address to find best path3. Encapsulate layer 3 packet into new layer 2

frame and forward frame out of exit interface If TTL field reaches 0 then packet is discarded

Layer 3 IP addresses do not change during routing, but layer 2 data-link addresses change at every hop

Summary Routers have multiple network interfaces

representing multiple networks The primary activity of a router is to forward

packets via the best path Routers use a routing table to determine the best

path Ports and interfaces refer to two different aspects Routers work at layers 1, 2 and 3 of the OSI model,

with layer 3 being the primary operating layer Show ip route – shows the routing table Each router interface must belong to a different

network

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