IRT0030 ANDMESIDE

LOENG 5

Indrek Rokk

Harjutus

• Aadress

2001:db8:aaaa:fc:50a5:8a35:a5bb:66e1/64

• Küsimused

• Interface ID

• Subnet prefix

• Site prefix

• ISP prefix

• ISP prefix kahendkoodis

• Registry number

• Registry number kahendkoodis

2

Harjutus - vastus

• Aadress

2001:db8:aaaa:fc:50a5:8a35:a5bb:66e1/64

• Küsimused

• Interface ID - 50a5:8a35:a5bb:66e1

• Subnet prefix - 00fc

• Site prefix - aaaa

• ISP prefix - 1:0db8

• ISP prefix kahendkoodis - 0001 0000 1100 1011 1000

• Registry number - 000

• Registry number kahendkoodis - 0 0000 0000

3

Routing versus Forwarding

• Routing = building maps and giving directions

• Forwarding = moving packets between interfaces

according to the “directions”

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IP Routing – finding the path

• Path derived from information received from a routing

protocol

• Several alternative paths may exist best path stored in forwarding

table

• Decisions are updated periodically or as topology changes (event

driven)

• Decisions are based on:

• topology, policies and metrics (hop count, filtering, delay, bandwidth,

etc.)

5

IP Forwarding

• Router makes decision on which interface a packet is sent to

• Forwarding table populated by routing process

• Forwarding decisions:

• destination address

• class of service (fair queuing, precedence, others)

• local requirements (packet filtering)

• Can be aided by special hardware

6

Routing Tables Feed the Forwarding

Table

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RIBs and FIBs

• FIB is the Forwarding Table

• It contains destinations and the interfaces to get to those

destinations

• Used by the router to figure out where to send the packet

• Careful! Some people call this a route!

• RIB is the Routing Table

• It contains a list of all the destinations and the various next hops

used to get to those destinations – and lots of other information too!

• One destination can have lots of possible next-hops –only the best

next-hop goes into the FIB

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Router as a Computer

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Routing Table Structure • Routing Table is stored in ram and contains information

about:

Directly connected networks - this occurs when a device is

connected to another router interface

Remotely connected networks - this is a network that is not

directly connected to a particular router

Detailed information about the networks include source of

information, network address & subnet mask, and IP address of

next-hop router

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Routing Table Structure • Dynamic routing protocols

• -Used to add remote networks to a routing table

• -Are used to discover networks

• -Are used to update and maintain routing tables

• Automatic network discovery

• -Routers are able discover new networks by sharing routing

table information

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Routing Table Structure • Maintaining routing tables

• -Dynamic routing protocols are used to share routing information with

other router & to maintain and up date their own routing table.

• IP routing protocols. Example of routing protocols include:

• -RIP

• -EIGRP

• -OSPF

12

Routing Table Structure • 3 principles regarding routing tables:

Every router makes its decisions alone, based on the information it

has in its routing table.

Different routing table may contain different information

A routing table can tell how to get to a destination but not how to get

back

13

Effects of the 3 Routing Table Principles • -Packets are forwarded through the network from one router to

another, on a hop by hop basis.

• -Packets can take path “X” to a destination but return via path “Y”

(Asymmetric routing).

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Router Paths and Packet Switching • A Metric is a numerical value used

by routing protocols help determine the best path to a destination – The smaller the metric value the

better the path

• 2 types of metrics used by routing protocols are: • Hop count - this is the number of

routers a packet must travel through to get to its destination R1 --> R3

• Bandwidth - this is the “speed” of a link also known as the data capacity of a link R1 --> R2 --> R3

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Router Paths and Packet Switching • Path determination is a process used by a router to pick the best

path to a destination

• One of 3 path determinations results from searching for the best path

• Directly connected network

• Remote network

• No route determined

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Using Static Routing

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Dynamic Routing Protocols • Advantages of static routing

• -It can backup multiple interfaces/networks on a router

• -Easy to configure

• -No extra resources are needed

• -More secure

• Disadvantages of static routing

• -Network changes require manual reconfiguration

• -Does not scale well in large topologies

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The Role of Dynamic Routing Protocols

Advantages of dynamic routing

Automatically share information about remote networks

Determine the best path to each network and add this information

to their routing tables

Compared to static routing, dynamic routing protocols require less

administrative overhead

Help the network administrator manage the time-consuming

process of configuring and maintaining static routes

Disadvantages of dynamic routing

Dedicate part of a routers resources for protocol operation,

including CPU time and network link bandwidth

Times when static routing is more appropriate

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Dynamic Routing Protocols • Function(s) of Dynamic Routing Protocols:

• Dynamically share information between routers.

• Automatically update routing table when topology changes.

• Determine best path to a destination.

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Dynamic Routing Protocols • The purpose of a dynamic routing protocol is to:

• -Discover remote networks

• -Maintaining up-to-date routing information

• -Choosing the best path to destination networks

• -Ability to find a new best path if the current path is no longer

available

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Dynamic Routing Protocols • Components of a routing protocol

• Algorithm

• In the case of a routing protocol algorithms are used for facilitating

routing information and best path determination

• Routing protocol messages

• These are messages for discovering neighbors and exchange of

routing information

22

Classifying Routing Protocols • Types of routing protocols:

• -Interior Gateway Protocols (IGP)

• -Exterior Gateway Protocols (EGP)

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Classifying Routing Protocols • Interior Gateway Routing Protocols (IGP)

• -Used for routing inside an autonomous system & used to route

within the individual networks themselves.

• -Examples: RIP, EIGRP, OSPF

• Exterior Routing Protocols (EGP)

• -Used for routing between autonomous systems

• -Example: BGPv4

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Classifying Routing Protocols • IGP: Comparison of Distance Vector & Link State

Routing Protocols • Distance vector

routes are advertised as vectors of distance & direction.

incomplete view of network topology.

Generally, periodic updates.

• Link state

complete view of network topology is created.

updates are not periodic.

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Distance Vector or Link-State Routing Protocols

• Distance vector protocols use routers as sign posts along

the path to the final destination. The only information a

router knows about a remote network is the distance or

metric to reach that network and which path or interface to

use to get there. Distance vector routing protocols do not

have an actual map of the network topology. Rumor based.

• A link-state routing protocol is like having a complete map

of the network topology. The sign posts along the way from

source to destination are not necessary, because all link-

state routers are using an identical map of the network. A

link-state router uses the link-state information to create a

topology map and to select the best path to all destination

networks in the topology.

Routing protocol comparison

• Speed of Convergence - Speed of convergence defines

how quickly the routers in the network topology share

routing information and reach a state of consistent

knowledge. The faster the convergence, the more

preferable the protocol. Routing loops can occur when

inconsistent routing tables are not updated due to slow

convergence in a changing network.

• Scalability - Scalability defines how large a network can

become, based on the routing protocol that is deployed.

The larger the network is, the more scalable the routing

protocol needs to be.

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Routing protocol comparison

• Classful or Classless (Use of VLSM) - Classful routing

protocols do not include the subnet mask and cannot

support VLSM. Classless routing protocols include the

subnet mask in the updates. Classless routing protocols

support VLSM and better route summarization.

• Resource Usage - Resource usage includes the

requirements of a routing protocol such as memory space

(RAM), CPU utilization, and link bandwidth utilization.

Higher resource requirements necessitate more powerful

hardware to support the routing protocol operation, in

addition to the packet forwarding processes.

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Routing protocol comparison

• Implementation and Maintenance - Implementation and

maintenance describes the level of knowledge that is

required for a network administrator to implement and

maintain the network based on the routing protocol

deployed.

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Classifying Routing Protocols • Convergence is defined as when all routers’ routing

tables are at a state of consistency

30

Achieving Convergence

Network converged when all routers have complete and

accurate information about the entire network.

Convergence time is the time it takes routers to share

information, calculate best paths, and update their

routing tables.

A network is not completely operable until the network

has converged.

Convergence properties include the speed of

propagation of routing information and the calculation of

optimal paths. The speed of propagation refers to the

amount of time it takes for routers within the network to

forward routing information.

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Routing Protocols Metrics • Metric

• A value used by a routing protocol to determine which

routes are better than others.

32

Routing Protocols Metrics • Metrics used in IP routing protocols

• -Bandwidth

• -Cost

• -Delay

• -Hop count

• -Load

• -Reliability

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Administrative Distance of a Route • Purpose of a metric

• It’s a calculated value used to determine the best path to a

destination

• Purpose of Administrative Distance

• It’s a numeric value that specifies the preference of a particular

route

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Distance Vector Routing Protocols • Distance Vector Technology

–The Meaning of Distance Vector:

• A router using distance vector routing protocols

knows 2 things:

Distance to final destination

Vector, or direction, traffic should be directed

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• A router using a distance vector routing protocol does not

have the knowledge of the entire path to a destination

network. Instead the router knows only:

• The direction or interface in which packets should be forwarded

• The distance or how far it is to the destination network

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Distance Vector Routing Protocols

Characteristics of Distance Vector routing protocols:

Periodic updates

Neighbors

Broadcast updates

Entire routing table is included with routing update

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Distance Vector Routing Protocols Routing Protocol Algorithm:

-Defined as a procedure for accomplishing a certain task

38

Distance Vector Routing Protocols

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Network Discovery

• Router initial start up (Cold Starts)

-Initial network discovery

Directly connected networks are initially placed in

routing table

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Network Discovery

• Initial Exchange of Routing Information

– If a routing protocol is configured then

-Routers will exchange routing information

• Routing updates received from other routers

-Router checks update for new information

If there is new information:

-Metric is updated

-New information is

stored in routing table

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Network Discovery • Exchange of Routing Information

–Router convergence is reached when

-All routing tables in the network contain the

same network information

–Routers continue to exchange routing information

-If no new information is found then Convergence is

reached

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Network Discovery

• Convergence must be reached before a network is

considered completely operable

• Speed of achieving convergence consists of 2

interdependent categories

-Speed of broadcasting routing information

-Speed of calculating routes

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Network Discovery

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Routing Table Maintenance

• Periodic Updates: RIPv1 & RIPv2

These are time intervals in which a router sends

out its entire routing table.

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Routing Table Maintenance

• Random Jitter

• Synchronized updates

• A condition where multiple routers on multi access LAN segments

transmit routing updates at the same time.

Problems with synchronized updates

• -Bandwidth consumption

• -Packet collisions

Solution to problems with

synchronized updates

- Used of random variable called RIP_JITTER

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Routing Table Maintenance

• RIP uses 4 timers

• -Update timer

• -Invalid timer

• -Holddown timer

• -Flush timer

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Configuring Passive Interfaces

Sending out unneeded updates on a LAN

impacts the network in three ways:

Wasted Bandwidth

Wasted Resources

Security Risk

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Routing Table Maintenance

• Bounded Updates: EIGRP

• EIGRP routing updates are

-Partial updates

-Triggered by topology changes

-Bounded

-Non periodic

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Routing Table Maintenance

• Triggered Updates

–Conditions in which triggered updates are sent

-Interface changes state

-Route becomes unreachable

-Route is placed in routing table

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Routing Loops

• Routing loops are

A condition in which a

packet is continuously

transmitted within a

series of routers

without ever reaching

its destination.

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Routing Loops

• Routing loops may be caused by:

-Incorrectly configured static routes -Incorrectly configured route redistribution -Slow convergence -Incorrectly configured discard routes

• Routing loops can create the following issues

-Excess use of bandwidth -CPU resources may be strained -Network convergence is degraded -Routing updates may be lost or not processed in a timely

manner

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Routing Loops

• Count to Infinity

This is a routing loop whereby packets bounce

infinitely around a network.

53

Routing Loops

• Setting a maximum

• Distance Vector routing protocols set a specified

metric value to indicate infinity

Once a router “counts to infinity” it marks the route

as unreachable

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Routing Loops

• Preventing loops with holddown timers

• -Holddown timers allow a router to not accept any changes to a route for a specified period of time.

• -Point of using holddown timers Allows routing updates to propagate through network with the most

current information.

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Routing Loops

• The Split Horizon Rule is used to prevent routing loops

• Split Horizon rule:

A router should not advertise a network through

the interface from which the update came.

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Routing Loops

• Split horizon with

poison reverse

The rule states that

once a router learns of

an unreachable route

through an interface,

advertise it as

unreachable back

through the same

interface

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Routing Loops • IP & TTL

– Purpose of the TTL field

The TTL field is found in an IP header and

is used to prevent packets from endlessly

traveling on a network

• How the TTL field works

-TTL field contains a numeric value

The numeric value is decreased by one by

every router on the route to the destination.

If numeric value reaches 0 then Packet

is discarded.

58

Routing Protocols Today • Factors used to determine whether to use RIP or EIGRP

include

-Network size

-Compatibility between models of routers

-Administrative knowledge

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Routing Protocols Today

• RIP

Features of RIP:

-Supports split horizon & split horizon with

poison reverse

-Capable of load balancing

-Easy to configure

-Works in a multi vendor router environment

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Routing Protocols Today

• EIGRP

Features of EIGRP: Triggered updates

EIGRP hello protocol used to establish neighbor adjacencies

Supports VLSM & route summarization

Use of topology table to maintain all routes

Classless distance vector routing protocol

Cisco proprietary protocol 2013 - basic functionality of EIGRP released as an open standard

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Marsruutimisprotokollid

• Distance vector

• Marsruudid sisaldavad kaugust ja suunda

• Kaugusel mingi mõõt (hüpete arv)

• Suund on järgmine marsruuter või väljumis võrguliides

• Parim tee naabritelt info

• Naaber X – võrku Z on 4 hüpet

• Naaber Y – võrku Z on 8 hüpet

• Valib tee läbi naabri X

• Ei tea kogu teed lõpp punkti

• Teab, et kas läbi X või Y, kuidas võrk sealt edasi välja näeb ei tea

• RIP, EIGRP

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Marsruutimisprotokollid

• Link state

• The basic concept of link-state routing is that every node constructs

a map of the connectivity to the network, in the form of a graph,

showing which nodes are connected to which other nodes.

• Saadetakse infot ühendatud linkide oleku kohta ja info teiste

marsruuterite kohta

• Protokollid

• Open Shortest Path First (OSPF)

• Intermediate System-to-Intermediate System (IS-IS)

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Link-State Routing Steps

• Each routers learns about its own directly connected networks

• Link state routers exchange hello packet to “meet” other directly connected link state routers.

• Each router builds its own Link State Packet (LSP) which includes information about neighbors such as neighbor ID, link type, & bandwidth.

• After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information.

• Once all the routers have received all the LSPs, the routers then construct a topological map of the network which is used to determine the best routes to a destination

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Directly Connected Networks

• Link - This is an interface on a router

• Link state - This is the information about the state of the links

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Sending Hello Packets to Neighbors • Connected interfaces that are using

the same link state routing protocols

will exchange hello packets.

• Once routers learn it has neighbors

they form an adjacency

-2 adjacent neighbors will

exchange hello packets

-These packets will serve as a

keep alive function

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Establish Neighbor Adjacencies

68

Establish Neighbor Adjacencies

DR and BDR election only occurs on multi-access networks such as Ethernet LANs.

69

OSPF DR and BDR

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OSPF Designated Router

• Designated Router (DR) is the solution to managing

adjacencies and flooding of LSAs on a multiaccess

network.

• Backup Designated Router (BDR) also elected in case DR

fails.

• All other Routers DROTHER only form adjacencies with the

DR and BDR.

• DROTHERs only send their LSAs to the DR and BDR

using the multicast address 224.0.0.6.

• DR uses the multicast address 224.0.0.5 to send LSAs to

all other routers. DR only router flooding LSAs.

• DR/BDR Elections only necessary on multiaccess networks.

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Default DR/BDR Election Process

• The router with the highest interface priority is elected as the DR.

• The router with the second highest interface priority is elected as the BDR.

• Priority can be configured between 0-255. • Priority of 0 - router cannot become the DR.

• If interface priorities are equal then the router with highest router ID is elected DR and second highest the BDR

• 3 ways to determine router ID: • Router ID can be manually configured. • If not configured, ID determined by highest loopback IP

address. • If no loopbacks, ID is determined by the highest active IPv4

address.

• In an IPv6 network, Router ID must be configured manually.

Building the Link State Packet

• Each router builds its own Link State Packet (LSP)

Contents of LSP:

-State of each directly connected link

-Includes information about neighbors such as

neighbor ID, link type & bandwidth.

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Building the Link State Packet

74

Flooding LSPs to Neighbors

• Once LSP are created they are forwarded out to neighbors.

-After receiving the LSP the neighbor continues to forward it throughout

routing area.

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Link-State Routing

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Shortest Path First (SPF) Tree • Building a portion of the SPF tree

Process begins by examining R2’s LSP information

-R1 ignores 1st LSP

Reason: R1 already knows it’s connected to R2

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78

79

80

81

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Building a portion of the SPF tree

• R1 uses LSP from R2 Reason: R1 learns that R2 is connected to 10.9.0.0/16.

This link is added to R1’s SPF tree.

R2 has a network 10.5.0.0/16 with a cost of 2 and no neighbors

This link is added to R1's SPF tree.

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Building a portion of the SPF tree

• R1 uses 3rd LSP Reason: R1 learns that R3 is connected to 10.7.0.0/16.

This link is added to R1’s SPF tree.

R3 has a network 10.6.0.0/16 with a cost of 2 and no neighbors

This link is added to R1's SPF tree.

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Determining the shortest path • The shortest path to a destination determined by adding the

costs & finding the lowest cost

85

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Determining the shortest path

• Once the SPF algorithm has determined the shortest

path routes, these routes are placed in the routing table.

87

Determining the shortest path

88

OSPF

• OSPF does not use a Transport layer protocol, as OSPF

packets are sent directly over IP.

• Protocol Number 89

89

OSPF packet types

• Hello - Hello packets are used to establish and maintain

adjacency with other OSPF routers.

• DBD - The Database Description (DBD) packet contains

an abbreviated list of the sending router's link-state

database and is used by receiving routers to check

against the local link-state database.

• LSR - Receiving routers can then request more

information about any entry in the DBD by sending a Link-

State Request (LSR).

90

OSPF packet types

• LSU - Link-State Update (LSU) packets are used to reply

to LSRs as well as to announce new information. LSUs

contain seven different types of Link-State Advertisements

(LSAs). LSUs and LSAs are briefly discussed in a later

topic.

• LSAck - When an LSU is received, the router sends a

Link-State Acknowledgement (LSAck) to confirm receipt

of the LSU.

91

Hello packets

• Discover OSPF neighbors and establish neighbor

adjacencies.

• Advertise parameters on which two routers must agree to

become neighbors.

• Elect the Designated Router (DR) and Backup

Designated Router (BDR) on multiaccess networks like

Ethernet and Frame Relay.

• Multicast address 224.0.0.5

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• OSPF routers are sending Hello packets on all OSPF-

enabled interfaces to determine if there are any neighbors

on those links.

• Receiving an OSPF Hello packet on an interface confirms

for a router that there is another OSPF router on this link.

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OSPF Authentication –Purpose is to encrypt & authenticate routing

information

–This is an interface specific configuration

–Routers will only accept routing information from other routers that have been configured with the same password or authentication information

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OSPF Metric

• OSPF uses cost as the metric for determining the best route

• The best route will have the lowest cost

• Cost is based on bandwidth of an interface • Cost is calculated using the formula

• 108 / bandwidth

• Reference bandwidth

• defaults to 100Mbps

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Cost: Example Strategy

• 100GE 100Gbps cost = 1

• 40GE/OC768 40Gbps cost = 2

• 10GE/OC192 10Gbps cost = 5

• OC48 2.5Gbps cost = 10

• GigEthernet 1Gbps cost = 20

• OC12 622Mbps cost = 50

• OC3 155Mbps cost = 100

• FastEthernet 100Mbps cost = 200

• Ethernet 10Mbps cost = 500

• E1 2Mbps cost = 1000

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Advantages of Link State

• Builds a Topological Map - Link-state routing protocols

create a topological map, or SPF tree of the network

topology. Because link-state routing protocols exchange

link-states, the SPF algorithm can build an SPF tree of the

network. Using the SPF tree, each router can

independently determine the shortest path to every

network.

• Fast Convergence - When receiving an LSP, link-state

routing protocols immediately flood the LSP out all

interfaces except for the interface from which the LSP was

received. In contrast, RIP needs to process each routing

update and update its routing table before flooding them

out other interfaces.

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Advantages of Link State

• Event-driven Updates - After the initial flooding of LSPs,

link-state routing protocols only send out an LSP when

there is a change in the topology. The LSP contains only

the information regarding the affected link. Unlike some

distance vector routing protocols, link-state routing

protocols do not send periodic updates.

• Hierarchical Design - Link-state routing protocols use the

concept of areas. Multiple areas create a hierarchical

design to networks, allowing for better route aggregation

(summarization) and the isolation of routing issues within

an area.

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OSPF head omadused

• Low Bandwidth Utilisation

• Only changes propagated

• Uses multicast on multi-access broadcast networks

100

OSPF head omadused

• Fast Convergence

• Detection Plus LSA/SPF

• LSA flooded throughout area

• Acknowledgement based

• Topology database

synchronised

• Each router derives routing

table to destination network

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Disadvantages of Link State

• Memory Requirements - Link-state protocols require

additional memory to create and maintain the link-state

database and SPF tree.

• Processing Requirements - Link-state protocols can also

require more CPU processing than distance vector routing

protocols. The SPF algorithm requires more CPU time

than distance vector algorithms such as Bellman-Ford,

because link-state protocols build a complete map of the

topology.

• Bandwidth Requirements - The flooding of link-state

packets can adversely affect the available bandwidth on a

network. This should only occur during initial startup of

routers, but can also be an issue on unstable networks.

102

Single-Area OSPF

Single-area OSPF is useful in smaller networks. If an

area becomes too big, the following issues must be

addressed:

• Large routing table (no summarization by default)

• Large link-state database (LSDB)

• Frequent SPF algorithm calculations

Single-Area OSPF

OSPF and Multiaccess Networks Challenges in Multiaccess Networks

• OSPF defines five network types: – Point-to-point

– Broadcast Multiaccess

– Nonbroadcast Multiaccess (NBMA)

– Point-to-multipoint

– Virtual links

105

OSPF in Multiaccess Networks • 2 challenges presented by multiaccess networks

– Multiple adjacencies

– Extensive LSA flooding

106

Extensive flooding of LSAs

• For every LSA sent out there must be an acknowledgement of

receipt sent back to transmitting router.

• consequence: lots of bandwidth consumed and chaotic traffic

107

Solution to LSA flooding issue

– Designated router (DR)

– Backup designated router (BDR)

• DR & BDR selection

– Routers are elected to send &

receive LSA

• Sending & Receiving LSA

– DRothers send LSAs via multicast

224.0.0.6 to DR & BDR

– DR forward LSA via multicast

address 224.0.0.5 to all other

routers

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Criteria for getting elected DR/BDR

1. DR: Router with the highest OSPF interface priority.

2. BDR: Router with the second highest OSPF interface priority.

3. If OSPF interface priorities are equal, the highest router ID is used to break the tie.

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Tuning OSPF

• Forcibly set your DR and BDR per segment so that they

are known

• Choose your most powerful, or most idle routers, so that

OSPF converges as fast as possible under maximum

network load conditions

• Try to keep the DR/BDR limited to one segment each

110

Multiarea OSPF Multiarea OSPF requires a hierarchical

network design and the main area is

called the backbone area (area 0) and

all other areas must connect to the

backbone area.

Disadvantages of Link-State Protocols

112

OSPF Areas

• Area is a group of contiguous hosts and networks

• Reduces routing traffic

• Per area topology database

• Invisible outside the area

• Backbone area MUST be contiguous

• All other areas must be connected to the backbone

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OSPF Two-Layer Area Hierarchy

Multiarea OSPF is implemented in a two-layer area

hierarchy:

Backbone (Transit) area -

• Area whose primary function is the fast and efficient movement of IP

packets.

• Interconnect with other OSPF area types

• Called OSPF area 0 which all other areas directly connect

Regular (Non-backbone) area -

• Connects users and resources

• A regular area does not allow traffic from another area to use its links

to reach other areas

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Types of OSPF Routers

Types of OSPF Routers

OSPF Route Types

119

Addressing for Areas

• Assign contiguous ranges of subnets per area to facilitate

summarisation

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