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The term routingis used for taking a packet

from one device and sendingit through thenetwork to another device on a different network.

Routers dont really care about hoststhey

only care about networks and the best path toeach network.

What is Routing ?

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Routers route traffic to all the networks in your

internetwork. To be able to route packets, a router mustknow, at a minimum, the following:

Destination address

Neighbor routers from which it can learn aboutremote networks

Possible routes to all remote networks

The best route to each remote network

How to maintain and verify routing information

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Routing Example :

Routing is taking place from Host_A to Host_B

through the Lab_A Router.

To be able to route, the router must know how to get into

the network 172.16.20.0.

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The different types of routing are :

Static routing

Default routing

Dynamic routing

Routing Types :

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Static Routing

Static routing occurs when you manually add routes

in each routers routing table.

Static routing has the following benefits:

There is no overhead on the router CPU

There is no bandwidth usage between routers

It adds security, because the administrator can chooseto allow routing access to certain networks only.

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ip route [destination_network] [mask]

[next-hop_address or exitinterface]

[administrative_distance]

Configuring Static Route :

ip route : The command used to create the static route.

destination_network : The network youre placing in the routing table.

mask : The subnet mask being used on the network.

next-hop_address : The address of the next-hop router

Exitinterface : You can use it in place of the next-hop address

administrative_distance : By default, static routes have an

administrative distance of 1

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DTE DCEDTE DCEE0

S0 S0 S1 S0

E0

DCE : Data Communication EquipmentDCE : Data Communication Equipment

DTE : Data Terminal EquipmentDTE : Data Terminal Equipment

Static Route LAB :

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Default Routing

default routing is usedto send packets with aremote destination network not in the routing

table to the next-hop router.

You can only use default routing on stubnetworksthose with only one exit path out of

the network.

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Visual(config)#ip route 0.0.0.0 0.0.0.0 20.142.26.1

Visual(config)#ip route 0.0.0.0 0.0.0.0 s0/0

Visual(config)#ip default-network 20.142.26.0

Default Routing Configuration

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Default Routing LAB :

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Dynamic Routing

Dynamic routing is when protocols are used to find networks

and Update routing tables on routers.

A routing protocol defines the set of rules used by a router

when it communicates routing information between neighbor

routers.There are two types of routing protocols used in internetworks:

Interior Gateway Protocols (IGPs) and

Exterior Gateway Protocols (EGPs).

IGPs are used to exchange routing information with routers in the same

autonomous system (AS) EGPs are used to communicate between ASes.

AnAS is a collection of networks under a common administrative domain, which

basically means that all routers sharing the same routing table information are in

the same AS.

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Routing Protocol Basics

Some basic things to understand in Routing Protocols :

Administrative Distances

Three different kinds of routing protocols

Routing loops

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Administrative Distances

The administrative distance (AD)is used to rate the trustworthinessof routing information received on a router from a neighbor router. An

administrative distance is an integer from 0 to 255, where 0 is the

most trusted and 255 means no traffic will be passed via this route.

If a router receives two updates listing the same remote network, the

first thing the router checks is the AD. If one of the advertised routeshas a lower AD than the other, then the route with the lowest AD will

be placed in the routing table.

If both advertised routes to the same network have the same AD,

then routing protocol metrics (such as hop countor bandwidth of the

lines) will be used to find the best path to the remote network.Theadvertised route with the lowest metric will be placed in the routing

table.

But if both advertised routes have the same AD as well as the same

metrics, then the routing protocol will load-balance to the remote

network.

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Default Administrative Distances

Route Source Default AD

Connected interface 0

Static route 1

EIGRP 90

IGRP 100OSPF 110

RIP 120

External EIGRP 170

Unknown 255 (this route will never be used)

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Routing Protocols

There are three classes ofrouting protocols:

Distance vector: The distance-vector protocols find the bestpath to a remote network by judging distance. Each time a packetgoes through a router, thats called a hop. The route with the least

number of hops to the network is determined to be the best route.

The vector indicates the direction to the remote network. They send

the entire routing table to directly connected neighbors.Ex:RIP,IGRP

Link state : Also called shortest-path-first protocols, the routerseach create three separate tables. One keeps track of directly attached

neighbors, one determines the topology of the entire internetwork, and

one is used as the routing table. Linkstate routers know more about the

internetwork than any distance-vector routing protocol.Link stateprotocols send updates containing the state of their own links to all

other routers on the network. Ex : OSPF

Hybrid : Hybrid protocols use aspects of both distance vector andlink state. Ex: EIGRP.

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Distance-Vector Routing Protocols

The distance-vector routing algorithm passes complete

routing table contents to neighboring routers

A router receiving an update from a neighbor router

believes the information about remote networks without

actually finding out for itself.

Its possible to have a network that has multiple links tothe same remote network, and if thats the case, the

administrative distance is checked first. If the AD is the

same, the protocol will have to use other metrics to

determine the best path to use to that remote network.

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Routing Loops

Distance-vector routing protocols keep track of any

changes to the internetwork by broadcasting periodic

routing updates out all active interfaces. This broadcastincludes the complete routing table.

Routing loops can occur because every router isnt

updated simultaneously.

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Routing Loop Example :

The interface to Network 5 fails. All routers know about

Network 5 from RouterE. RouterA, in its tables, has a

path to Network 5 through Router B.

When Network 5 fails, Router E tells Router C. This causes Router C to stop

routing to Network 5 through Router E. But Routers A, B, and D dont know

about Network 5 yet, so they keep sending out update information. Router C will

eventually send out its update and cause B to stop routing to Network 5, but

Routers A and D are still not updated. To them, it appears that Network 5 is stillavailable through Router B with a metric of 3.The problem occurs when Router

A sends out its regular 30-second Hello, Im still herethese are the links I

know about message, which includes the ability to reach Network 5 and now

Routers B and D receive the wonderful news that Network 5 can be reached

from Router A, so Routers B and D then send out the information that Network 5

is available. Anypacket destined for Network 5 will go to Router A, to Router B,and then back to Router A.This is a routin loo .

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Loop Avoidance

Maximum Hop Count

Split Horizon

Route Poisoning

RIP permits a hop count of up to 15, so anything that requires

16 hops is deemed unreachable. Thus, the maximum hop count

will control how long it takes for a routing table entry to become

invalid.

This reduces incorrect routing information and routing overhead

in a distance-vector network by enforcing the rule that routing

information cannot be sent back in the direction from which it was

received.

when Network 5 goes down, Router E initiates route poisoning by

advertising Network 5 as 16, or unreachable. When Router C

receives a route poisoning from Router E, it sends an update,

called a poison reverse, back to Router E. This ensures all routes

on the segment have received the poisoned route information.

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Routing Information Protocol (RIP) is a true distance-vector routing

protocol.

It sends the complete routing table out to all active interfaces

every 30 seconds.

RIP only uses hop count to determine the best way to a remote

network, but it has a maximum allowable hop count of 15 by

default, meaning that 16 is deemed unreachable.

RIP version 1 uses only classful routing, which means that all

devices in the network must use the same subnet mask.

RIP version 2 provides something called prefix routing, and does

send subnet mask information with the route updates. This is

called classless routing.

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RIP Timers

RIP uses three different kinds of timers to regulate its performance:

Route update timerS

ets the interval (30

seconds) between periodic routingupdates, in which the router sends a complete copy of its routing table out to all

neighbors.

Route invalid timerDetermines the length of time that must elapse (180

seconds) before a router determines that a route has become invalid. It will

come to this conclusion if it hasnt heard any updates about a particular route for

that period. When that happens, the router will send out updates to all its

neighbors letting them know that the route is invalid.

Holddown timerThis sets the amount of time during which routing information

is suppressed. Routes will enter into the holddown state when an update packet

is received that indicated the route is unreachable. This continues until either an

update packet is received with a better metric or until the holddown timer

expires. The default is 180 seconds.

Route flush timerSets the time between a route becoming invalid and its

removal from the routing table (240 seconds). Before its removed from the

table, the router notifies its neighbors of that routes impending demise. The

value of the route invalid timer must be less than that of the route flush timer.

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RIP LAB 1 :

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RIP LAB 2:

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RIP Version 2 (RIPv2)

Both RIPv1 and RIPv2 are distance-vector protocols, which

means that each router running RIP sends its complete routing

tables out all active interfaces at periodic time intervals.

The timers and loop-avoidance schemes are the same in both RIPversions

Both RIPv1 and RIPv2 are configured as classful addressing, (but RIPv2

is considered classless because subnet information is sent with each

route update)

Both have the same administrative distance (120)

RIP is an open standard, you can use RIP with any brand of router.

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RIP V1 Vs RIP V2

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IGRP vs. RIP

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IGRP Timers

To control performance, IGRP includes the following timers with default

settings:

Update timers :These specify how frequently routing-update messages

should be sent. The default is 90 seconds.

Invalid timers : These specify how long a router should wait beforedeclaring a route invalid if it doesnt receive a specific update about it.

The default is three times the update period.

Holddown timers : These specify the holddown period. The default is

three times the update timer period plus10

seconds.

Flush timers : These indicate how much time should pass before a

route should be flushed from the routing table. The default is seven

times the routing update period. If the update timer is 90 seconds by

default, then 7 90 = 630 seconds elapse before a route will be flushed

from the route table.

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IGRP LAB 1 :

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Enhanced IGRP (EIGRP) is a classless, enhanced distance-vector protocolthat gives us a real edge over IGRP.

Like IGRP, EIGRP uses the concept of an autonomous system to describe

the set of contiguous routers that run the same routing protocol and share

routing information.

But unlike IGRP, EIGRP includes the subnet mask in its route updates.

The advertisement of subnet information allows us to use VLSM and

summarization when designing our networks.

EIGRP is sometimes referred to as a hybrid routing protocolbecause it

has characteristics of both distance-vector and link-state protocols.

EIGRP has a maximum hop count of255.

It sends traditional distance-vector updates containing information about

networks plus the cost of reaching them from the perspective of the

advertising router.

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Powerful features that make EIGRP a real standout from IGRP

Support for IP, IPX, and AppleTalk via protocol-dependent

modules Considered classless (same as RIPv2 and OSPF)

Support forVLSM/CIDR

Support for summaries and discontiguous networks

Efficient neighbor discovery

Communication via Reliable Transport Protocol (RTP)

Best path selection via Diffusing Update Algorithm (DUAL)

Note: Cisco calls EIGRP a distance vector routing protocol, or sometimes an

advanced distance vector or even a hybrid routing protocol.

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EIGRP supports different Network layer protocols through the use of

protocol-dependent modules (PDMs).

Each EIGRP PDM will maintain a separate series of tables containing the

routing information that applies to a specific protocol.

It means that there will be IP/EIGRP tables, IPX/EIGRP tables, and

AppleTalk/EIGRP tables.

Neighbor Discovery

Before EIGRP routers are willing to exchange routes with each other,

they must become neighbors.There are three conditions that must be met

for neighborship establishment:

Hello orACK received

AS numbers match

Identical metrics (K values)

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To maintain the neighborship relationship, EIGRP routers must

also continue receiving Hellos from their neighbors.

EIGRP routers that belong to different autonomous systems (ASes)

dont automatically share routing information and they dont become

neighbors.

The only time EIGRP advertises its entire routing table is when itdiscovers a new neighbor and forms an adjacency with it

through the exchange of Hello packets. When this happens,

both neighbors advertise their entire routing tables to one

another. After each has learned its neighbors routes, only

changes to the routing table are propagated from then on.

EIGRP maintains three tables containing information about the internetworks.

1. NeighborTable (records information about routers with whom neighborship relationships have been formed.)

2. Topology Table (stores the route advertisements about every route in the internetwork received from each neighbor.)

3. Routing Table (stores the routes that are currently used to make routing decisions.)

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Feasible distance

This is the best metric along all paths to a remote network, including the metric

to the neighbor that is advertising that remote network. This is the route that you will find

in the routing table, because it is considered the best path. The metric of a feasible

distance is the metric reported by the neighbor(called reported distance), plus the metricto the neighbor reporting the route.

Reported distance (AdvertisedDistance )

This is the metric of a remote network, as reported by a neighbor. It is also

the routing table metric of the neighbor.

Neighbor table

Each router keeps state information about adjacent neighbors. When a newly

discovered neighbor is learned, the address and interface of the neighbor are recorded,

and this information is held in the neighbor table, stored in RAM. There is one neighbor

table for each protocol-dependent module.

Topologytable

The topology table is populated by the PDMs and acted upon by the Diffusing Update

Algorithm (DUAL). It contains all destinations advertised by neighboring routers, holding

each destination address and a list of neighbors that have advertised the destination.

For each neighbor, the advertised metric is recorded, which comes only from the

neighbors routing table. If the neighbor is advertising this destination, it must be using

the route to forward packets.

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Feasible successor

A destination entry is moved from the topology table to the routing table

when there is a feasible successor.A feasible successor is a path whose reporteddistance is less than the feasible distance, and it is considered a backup route. EIGRP

will keep up to six feasible

successors in the topology table. Only the one with the best metric (the successor).is

placed in the routing table.

Successor

A successor route is the best route to a remote network. A successor route is used by

EIGRP to forward traffic to a destination and is stored in the routing table. It is backed up

by a feasible successor route that is stored in the topology tableif one is available.

A feasible successor is a backup route and is

stored in the topology table.

A successor route is stored in the topology

table and also placed in the routing table.

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EIGRP uses a proprietary protocol, called Reliable Transport Protocol (RTP), to manage

the communication of messages between EIGRP-speaking routers.

EIGRP sends updates via multicast traffic, it uses the

Class

Daddress 224.

0.0.10

.

IfEIGRP doesnt get a reply from a neighbor, it will switch to using unicasts to resend

the same data.

If it still doesnt get a reply after16 unicast attempts, the neighbor is declared dead.

This process is reffered to as reliable multicast

EIGRP uses Diffusing Update Algorithm (DUAL) for selecting and maintaining the best

path to each remote network. This algorithm allows for the following:

Backup route determination if one is available

Support of Variable-Length Subnet Masks (VLSMs)

Dynamic route recoveries

Queries for an alternate route if no route can be found

DUAL provides EIGRP with possibly the fastest route convergence time among all protocols.

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EIGRP Metrics

Another really sweet thing about EIGRP is that unlike many other protocols that use a

single factor to compare routes and select the best possible path,E

IGRP can use acombination of four:

Bandwidth

Delay

Load

Reliability

MTU

Like IGRP, EIGRP uses only bandwidth and delay of the line to determine the

best path to a remote network by default.

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Maximum Paths and Hop Count

By default, EIGRP (and IGRP) can provide unequal cost load balancing of up to fourlinks.However, you can have EIGRP actually load balance across up to six links.

EIGRP (and IGRP) has a maximum hop count of100, but can be set up to 255.

If you need to stop EIGRP from working on a specific interface, such as

Ethernet interface or a serial connection to the Internet. Configure with passive-

interface interface command.

By using the no auto-summarycommand, EIGRP will advertise all the subnets

between the two routers. If the networks were larger, you could then providemanual summarization on these same boundaries.

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Open Shortest Path First (OSPF)is an open standards routing protocol thats

been implemented by a wide variety of network vendors, including Cisco.

This works by using the Dijkstra algorithm. First, a shortest path tree is

constructed, and then the routing table is populated with the resulting best

paths. OSPF converges quickly, although perhaps not as quickly as EIGRP,

and it supports multiple, equal-cost routes to the same destination. But unlikeEIGRP, it only supports IP routing.

OSPF provides the following features:

Consists of areas and autonomous systems

Minimizes routing update traffic Allows scalability

Supports VLSM/CIDR

Has unlimited hop count

Allows multi-vendor deployment (open standard)

OSPF is the first link-state routing protocol that most people are introduced to.

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OSPF and RIP comparison

Characteristic OSPF RIPv2 RIPv1

Type of protocol Link-state Distance Vector Distance-vector

Classless support Yes Yes No

VLSM support Yes Yes No

Auto summarization No Yes Yes

Manual summarization Yes No No

Discontiguous support Yes Yes No

Route propagation Multicast on change Periodic multicast Periodic broadcast

Path metric Bandwidth Hops Hops

Hop count limit None 15 15

Convergence Fast Slow Slow

Peer authentication Yes Yes NoHierarchical network Yes (using areas) No (flat only) No (flat only)

Updates Event Triggered Route table updates Route table updates

Route computation Dijkstra Bellman-Ford Bellman-Ford

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OSPF is supposed to be designed in a hierarchical fashion, which basically means

that you can separate the larger internetwork into smaller internetworks called areas.

This is the best design for OSPF.

The reasons for creating OSPF in a hierarchical design include:

To decrease routing overhead

To speed up convergence

To confine network instability to single areas of the network

Each router in the network connects to the backbone called area 0,or the backbone area

OSPF must have an area 0, and all routers should connect to this area if at all possible.

But routers that connect other areas to the backbone within an AS are called

Area Border Routers (ABRs). Still, at least one interface must be in area 0.

OSPF runs inside an autonomous system, but can also connect multiple autonomous

systems together. The router that connects these ASes together is called an

Autonomous System Boundary Router (ASBR).

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Topology database The topology database contains information from all of the

Link StateAdvertisement packets that have been received for an area. The router

uses the information from the topology database as input into the Dijkstra algorithmthat computes the shortest path to every network. LSA packets are used to update

and maintain the topology database.

Link State Advertisement ALink State Advertisement (LSA) is an OSPF data

packet containing link-state and routing information thats shared among OSPF

routers. There are different types of LSA packets. An OSPF router will exchange

LSA

packets only with routers to which it has established adjacencies.Designated routerAdesignated router (DR) is elected whenever OSPF routers

are connected to the same multi-access network.A prime example is an Ethernet

LAN.

Backup designated routerAbackup designated router (BDR) is a hot standby for

the DR on multi-access links The BDR receives all routing updates from OSPF

adjacent routers, but doesnt flood LSA updates.

OSPF areas An OSPF area is a grouping of contiguous networks and routers. All

routers in the same area share a common Area ID.

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Broadcast (multi-access) Broadcast (multi-access) networks such as Ethernet

allow multiple devices to connect to (or access) the same network, as well as

provide a broadcastability in which a single packet is delivered to all nodes on

the network. In OSPF, a DR and a BDR must be elected for each broadcast

multi-access network.Non-broadcast multi-access Non-Broadcast Multi-Access (NBMA) networks

are types such as Frame Relay, X.25, and Asynchronous TransferMode (ATM).

These networks allow for multi-access, but have no broadcast ability like

Ethernet. So, NBMA networks require special OSPF configuration to function

properly and neighbor relationships must be defined.

Point-to-point Point-to-pointrefers to a type of network topology consisting of adirect connection between two routers that provides a single communication

path. The point-to-point connection can be physical, as in a serial cable directly

connecting two routers, or it can be logical.

Point-to-multipoint Point-to-multipointrefers to a type of network topology

consisting of a series of connections between a single interface on one router

and multiple destination routers.All of the interfaces on all of the routers sharingthe point-to-multipoint connection belong to the same network. As with point-to-

point, no DRs or BDRs are needed.

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Within an area, each router calculates the best/shortest path to every

network in that same area.This calculation is based upon the informationcollected in the topology database and an algorithm called shortest path

first (SPF)

OSPF uses a metric referred to as cost. A cost is associated with every

outgoing interface included in an SPF tree. The cost of the entire path is the

sum of costs of the outgoing interfaces along the path.

Cisco uses a simple equation of108/bandwidth.The bandwidth is the

configured bandwidth for the interface. Using this rule, a 100Mbps Fast

Ethernet interface would have a default OSPF cost of1 and a 10Mbps

Ethernet interface would have a cost of10. An interface set with a

bandwidth of 64,000 would have a default cost of1563.