Routing BasicsRouting Basics

ByBy

Craig LindstromCraig Lindstrom

OverviewOverview

Routing ProcessRouting ProcessDefault RoutingDefault RoutingStatic RoutingStatic RoutingDynamic RoutingDynamic RoutingRouting ProtocolsRouting Protocols

The RouterThe Router A router is a device that forwards data packets A router is a device that forwards data packets

from one network to another.from one network to another. Hardware Based RoutersHardware Based Routers

Device’s primary function is to route network traffic.Device’s primary function is to route network traffic. Usually have specialized ASICs to increase Usually have specialized ASICs to increase

performance.performance. Cisco Systems Inc. Is major supplier.Cisco Systems Inc. Is major supplier.

Software Based RoutingSoftware Based Routing Routing is performed by a software add-on to a Routing is performed by a software add-on to a

generic system (LINUX, Windows2000, etc.)generic system (LINUX, Windows2000, etc.) All routers must have more than one network All routers must have more than one network

interface.interface.

Routing Process DetailRouting Process Detail

1.1. A data packet is received at a network A data packet is received at a network interface.interface.

2.2. The destination address is decodedThe destination address is decoded

3.3. The address is compared to the routers The address is compared to the routers addresses to see if the packet’s addresses to see if the packet’s destination is the router.destination is the router.

4.4. If If YESYES the router passes the data up the the router passes the data up the protocol stack.protocol stack.

Routing Process Detail cont.Routing Process Detail cont.

5.5. If If NONO the router checks it’s routing table the router checks it’s routing table for the best route to the destination IP for the best route to the destination IP addressaddress

a)a) If the destination network is directly If the destination network is directly connected to the router. The router forwards connected to the router. The router forwards the packet to the destination hostthe packet to the destination host

b)b) If the destination network is not directly If the destination network is not directly connected to the router the packet is connected to the router the packet is forwarded to the “best” next-hop router as forwarded to the “best” next-hop router as determined by the routing tabledetermined by the routing table

Routing Process Detail cont.Routing Process Detail cont.

a)a) The routing process continues from router to The routing process continues from router to router until the destination network is found. router until the destination network is found. If the destination network is never found an If the destination network is never found an error message is returned to the sending error message is returned to the sending host.host.

Three Types of RoutingThree Types of Routing

StaticStaticAll routes must be manually configuredAll routes must be manually configured

DefaultDefaultOne way street routingOne way street routing

DynamicDynamicLet the computer do the workLet the computer do the work

Static RoutingStatic Routing

Route table is manually built by the Route table is manually built by the administrator.administrator.

Each router knows the networks that it is Each router knows the networks that it is physically connected to.physically connected to.

The route to all networks not directly The route to all networks not directly connected to the router must be connected to the router must be configured by the administrator.configured by the administrator.

What Static Routes Are Needed?What Static Routes Are Needed?

LabServer

TR2

10.0.20.0/24

10.0.20.254

T0 T1 S0 S110.0.20.2

Host

TR1

T0 S0

S1

10.0.30.2

10.0.30.110.0.30.0/24

10.0.10.0/2410.0.10.254

10.0.10.1

Default RoutingDefault Routing

Lets say we connect our small network to Lets say we connect our small network to the internet. The internet has thousands the internet. The internet has thousands of networks. It is not possible for an of networks. It is not possible for an administrator to enter a route to every administrator to enter a route to every possible networkpossible network

Default routing is the answer.Default routing is the answer.A default route is the route used when a A default route is the route used when a

route to the destination network is not route to the destination network is not found in the route tablefound in the route table

Default Route ExampleDefault Route Example

Router

63.230.23.0/24

IP 63.230.23.10Mask 255.255.255.0Default Gateway 63.230.23.254

E0 DSL

63.230.23.254

HostHost

IP 63.230.23.20Mask 255.255.255.0Default Gateway 63.230.23.254

When to use a default routeWhen to use a default route

Default routes are used to “push” data from a Default routes are used to “push” data from a stub network to a number of networks.stub network to a number of networks.

Most routers use a combination of a default Most routers use a combination of a default route and other routes.route and other routes.

Hosts use a default route (the default gateway) Hosts use a default route (the default gateway) so they don’t have to maintain a large route so they don’t have to maintain a large route table.table.

A network with only two routers can be A network with only two routers can be configured using only default routesconfigured using only default routes

How Can You Configure This Network How Can You Configure This Network Using Only Default Routes?Using Only Default Routes?

LabServer

TR2

10.0.20.0/24

10.0.20.254

T0 T1 S0 S110.0.20.2

Host

TR1

T0 S0

S1

10.0.30.2

10.0.30.110.0.30.0/24

10.0.10.0/2410.0.10.254

10.0.10.1

Dynamic Routing OverviewDynamic Routing Overview

Dynamic routing uses routing protocols to Dynamic routing uses routing protocols to exchange route information between routers.exchange route information between routers.

Each router always knows it’s connected Each router always knows it’s connected interfaces.interfaces.

If two routers share a network they can If two routers share a network they can exchange their known routes with each other. exchange their known routes with each other. Combining the new routes with the router’s Combining the new routes with the router’s existing route table. After a few exchanges all existing route table. After a few exchanges all routes are known.routes are known.

Routing ProtocolsRouting Protocols

A routing protocol is a protocol that A routing protocol is a protocol that automatically discovers known routes and automatically discovers known routes and exchanges the route information with exchanges the route information with neighbor routers.neighbor routers.

Some examples are:Some examples are:Routing Information Protocol (RIP)Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP)Interior Gateway Routing Protocol (IGRP)Open Shortest Path First (OSPF)Open Shortest Path First (OSPF)

Administrative Distance (Best Route)Administrative Distance (Best Route)Route Source Default Distance

Connected interface 0

Static route 1

Enhanced Interior Gateway Routing Protocol (EIGRP) summary route

5

External Border Gateway Protocol (BGP) 20

Internal EIGRP 90

IGRP 100

OSPF 110

Intermediate System-to-Intermediate System (IS-IS) 115

Routing Information Protocol (RIP) 120

Exterior Gateway Protocol (EGP) 140

External EIGRP 170

Internal BGP 200

Unknown 255

SummarySummary All devices within a LAN can communicate directly with All devices within a LAN can communicate directly with

all other devices in the LAN.all other devices in the LAN. Congestion occurs when too much data is trying to be Congestion occurs when too much data is trying to be

sent on a given network.sent on a given network. Congestion is often solved by segmenting the network.Congestion is often solved by segmenting the network. Routing is the process of moving data packets between Routing is the process of moving data packets between

networks.networks. Routers must maintain a route table (network map) to Routers must maintain a route table (network map) to

determine best route.determine best route. Routing protocols are used to automatically maintain Routing protocols are used to automatically maintain

route tables.route tables.