Introduction to LANs - JustAnswer(1).pdf · ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public. 3. Link-State Routing Link state routing protocols-Also

TDC 363Introduction to LANs

Greg BrewsterDePaul University

OSPF

TDC 363 Greg Brewster, DePaul University 1

Link State Routing Algorithms Open Shortest Path First (OSPF)

Message Types Operations Configuration and Verification

OSPF

Greg Brewster, DePaul University 2TDC 363

ITE PC v4.0Chapter 1 3 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Routing Link state routing protocols

-Also known as shortest path first algorithms

-These protocols built around Dijkstras SPF


Link-State RoutingLink-State Routing Process How routers using Link State Routing Protocols reach

convergence-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 links and 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 entire network which is used to determine the best routes to all destinations.


Link-State RoutingSending 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


Link-State RoutingBuilding 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.


Link-State Routing LSPs are sent out under the following conditions

-Initial router start up or routing process-When there is a change in topology


Link-State RoutingConstructing a link state data base

Routers use a database to construct a topology map of the network


Link-State Routing


Link-State Routing Determining the shortest path

The shortest path to a destination determined by adding the costs & finding the lowest total path cost


Link-State Routing Once the SPF algorithm has determined the shortest

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


Link-State Routing Protocols

Routingprotocol

BuildsTopological

map

Router can independently determine the

shortest path to every network.

ConvergenceA periodic/

event driven routing updates

Useof

LSP

Distance vector

No No Slow Generally No No

Link State Yes Yes Fast Generally Yes Yes

Advantages of a Link-State Routing Protocol


Link-State Routing ProtocolsRequirements for using a link state routing protocol Memory requirements

Typically link state routing protocols use more memory

Processing RequirementsMore CPU processing is required of link state routing protocols

Bandwidth RequirementsInitial startup of link state routing protocols can consume lots of bandwidth

1. A Link State Internal Gateway Protocol used within most Autonomous Systems on the Internet

2. Advantages over RIP (distance-vector protocol) a) Supports many distance metrics, including physical distance, transmission

delay, queueing delay, etc. b) Adapts to changes in topology quickly. c) Supports multiple types of service such as least delay, highest throughput,

etc. d) Does load balancing over multiple paths to the same destination. e) Supports hierarchical systems:

i) Autonomous Systems ii) Areas iii) Networks iv) Subnets

f) Improved security.

Open Shortest Path First (OSPF)


1. A Link State Internal Gateway Protocol used within most Autonomous Systems on the Internet

2. Advantages over RIP (distance-vector protocol)

a) Supports many distance metrics, including physical distance, transmission delay, queueing delay, etc.

b) Adapts to changes in topology quickly.

c) Supports multiple types of service such as least delay, highest throughput, etc.

d) Does load balancing over multiple paths to the same destination.

e) Supports hierarchical systems:

i) Autonomous Systems

ii) Areas

iii) Networks

iv) Subnets

f) Improved security.

OSPF Link State Database Each OSPF router maintains a Link State

Database containing, for each link in the network: The Link Type Router IDs of routers connected to this link IP subnet address for this link IP subnet mask for this link Link cost Link status (up or down)



Basic OSPF Configuration

Router IDThis is a single IP address used to identify a router3 methods for determining the router IDConfigure with OSPF router-id command

-Takes precedence over other methodsIf router-id command not used then router chooses highest IP address of any loopback interfaces Commonly set up Loopback0 IP to become Router ID

If no loopback interfaces are configured then the highest IP address on any active interface is used Using active interface IP address is not best practice because Router ID may change if interface goes down.


Basic OSPF ConfigurationOSPF Router ID Commands used to verify current router ID

Show ip protocolsShow ip ospfShow ip ospf interface

OSPF Hierarchical Networks Autonomous Systems

Owned by single organization Areas

Collection of subnets and routers within an AS. Each area has an Area Number.

Routers inside an area flood information to all other routers in area

Area border routers summarize area information to pass to another area

All areas connect to backbone area (0). For TDC 363, we only study single-area OSPF all

subnets and routers will be in Area 0.Greg Brewster, DePaul University 18TDC 363

Figure 13-17

OSPF Hierarchical Networks

Backbone router: connect only to networks in the backbone area (Area 0)

Area border router: attaches to multiple areas.

Internal router: all interfaces connected to networks in the same area.

AS boundary router (Also known as Border Router): connect to other ASes



Introduction to OSPFOSPF Packet Types

OSPF HELLO Packet

When router first powers up or comes online, it will first discover neighbors Multicast HELLO messages on all its

interfaces Listen for HELLO messages in response

from neighbor routers Store neighbor router information in

Adjacency Database.


Chapter 322 2007 2010, Cisco Systems, Inc. All rights reserved. Cisco Public

OSPF Router Tables / Databases

OSPF maintains three databases which are used to create three tables.

Database Table Description

Adjacency Database

Neighbor Table

List of all neighbors routers to which a router has established bidirectional communication.

This table is unique for each router. Can be viewed using the show ip ospf neighbor command.

Link-state Database

Topology Table

List of information about all other routers in the network. The database shows the network topology. All routers within an area have identical link-state databases. Can be viewed using the show ip ospf database command.

Forwarding Database

Routing Table

List of routes generated when an algorithm is run on the link-state database.

Each routers routing table is unique and contains information on how and where to send packets to other routers.

Can be viewed using the show ip route command.


Introduction to OSPF Hello Packets

Maintain adjacencies and set Designated Router

OSPF Hello IntervalsUsually multicast (224.0.0.5)Sent every 10 seconds in generalSent every 30 seconds for NBMA

OSPF Dead IntervalsThis is the time that must transpirebefore the neighbor is considered down

Default time is 4 timesthe hello interval

OSPF DATABASE DESCRIPTION Message

When other routers receive the HELLO message: If this is the first time they have heard from

this router, they will send DATABASE DESCRIPTION message

This message gives a summary outline of all information in the Link State Database of the router


OSPF LINK STATE REQUEST Message

When a router wants more information about a particular set of links, it will send a LINK STATE REQUEST message to its neighbor.

Neighbor will respond with LINK STATE UPDATE message containing requested information


OSPF LINK STATE UPDATE Message

When any link state changes (link goes up or down, cost changes, etc.) a router will flood a LINK STATE UPDATE message to all other routers in the Area. Link State Update contains one or more

Link State Advertisements (LSAs) from the router.


LSU Flooding When any router receives a Link State

Update message, it checks whether this is a new update message If it is a New LSU message, then this router

will flood the message (transmit a copy out all interfaces except the one it arrived on)

If it is an Old LSU message (seen before), then this router will drop the message


New or Old? A router determines whether an arriving LSU

message is new or old using two header fields Sequence Number

Each router remembers the Sequence Number of the last LSU seen from every other router

Each router increments Sequence Number when sending out a new LSU

Age Routers update Age field with estimated transit

time. Messages dropped when Age gets too large


OSPF Types of Links Point-to-Point Link

Directly connects two routers with no other host or router inbetween.

Transient Link A Link with multiple routers on it (example: a

LAN) Stub Link

A link with only one router on it Virtual Link

A multi-hop connection between two routers (a tunnel) used for backup if direct link goes down.


Point-to-Point Links Each of the two routers measures the

metric from itself to its neighbor. Result: there are 2 metrics for each point-

to-point link one for data sent in each direction. May be same value or different.


Transient Links and DRs Many routers connect to the same link

(multiaccess link), such as in a LAN. Problem: Its too much work to measure metrics

between all possible pairs of routers Solution:

Choose one router to be designated router (DR) Other routers only form adjacencies with DR. Cost metric is assigned from each router to designated

router. No cost from designated router to others. Designated Router is elected based on Priority value. Backup Designated Router is also elected and takes over

if DR fails.



OSPF in Multiaccess Networks Solution to LSA flooding issue

is the use of Designated router (DR)Backup designated router (BDR)

DR & BDR selectionRouters are elected to send & receive LSA

Sending & Receiving LSADRothers send LSAs via multicast 224.0.0.6 to DR & BDRDR forward LSA via multicast address 224.0.0.5 to all other routers


OSPF in Multiaccess Networks

Criteria for getting elected DR/BDR1. 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.


OSPF in Multiaccess Networks

Timing of DR/BDR ElectionOccurs as soon as 1st router has its interface enabled on multiaccess networkWhen a DR is elected it remains as the DR until one of the following occurs

-The DR fails.-The OSPF process on the DR fails.-The multiaccess interface on the DR fails.


OSPF in Multiaccess NetworksOSPF Interface Priority Manipulating the DR/BDR election process continued

Use the ip ospf priority interface command.Example:Router(config-if)#ip ospf priority {0 - 255}Priority number range 0 to 255

0 means the router cannot become the DR or BDR1 is the default priority value

Stub Links A stub link is just a special case of a

transient network where the number of routers on the network is 1

Stub links are advertised in LSAs sent by connected router.


OSPF Protocol Overview When router first powers up, it will

Determine who its neighbor routers are. Download a complete Link State Database

from a neighbor router When something changes in the network

(link goes up or down, cost metric changes, etc.) Discovering router will flood Link State

Advertisements (LSAs) to all other routers in the Area to update their databases


Link State Advertisement

An LSA packet announces the state of all links connected to a router

Four types of LSAs: Router Link LSA Network Link LSA Summary Link LSA (multi-area OSPF) External Link LSA (multi-area OSPF)


Router Link LSA Advertises the state of the links connected

to a true router Can be any of the 4 link types


Figure 13-25

Network Link LSA Advertises the state of the links connected

to a designated router


Summary Link LSA

Two Types (not required for TDC 363): Summary Link to Network LSA

Used to provide a summary of all link information for one particular Area to all routers in another Area in the same Autonomous System

Summary Link to AS Boundary Router LSA Advertises a path to an AS Boundary router so that

Area routers can learn how to get outside of their own Autonomous System



OSPF Metric OSPF uses path cost as metric for determining best route.

Path Cost = sum of link costs on path to destination. -Link Cost is based on bandwidth of link interfaceInterface bandwidth parameter can be changed by admin. It is not affected by clock rate command.Link Cost is calculated using the formula

Link Cost = 108 / (Interface Bandwidth)Default Link Costs

Eth interface: 10FastEth interface: 1GigEth interface: 1Serial Interface: 64

(regardless of clock rate)


OSPF Metric COST of an OSPF route

Is the accumulated value from one router to the next


Basic OSPF ConfigurationModifying the Cost of a link Both sides of a serial link should be configured with the

same bandwidthCommands used to modify bandwidth valueBandwidth command

Example: Router(config-if)#bandwidth bandwidth-kbpsip ospf cost command allows you to directly specify interface cost

-Example:R1(config)#interface serial 0/0/0R1(config-if)#ip ospf cost 1562

OSPF Configuration router ospf (global config)

Creates OSPF route database, starts OSPF routing process number that manages OSPF messages, maintains database, and copies routes from database into routing table.

network netmask area Causes OSPF to advertise Subnet ID with wildcard

(inverse subnet mask) in OSPF area number and to send/receive OSPF Hellos and LSUs on all interfaces that directly connect to this subnet.

The subnet specified in the network command should be directly connected to this router.

Two routers connected to same link will only become OSPF neighbors if both of them execute network statements for the common link subnet in their OSPF configurations.


Presentation_ID 46 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential

Configure Single-area OSPFv2The network Command

This activates OSPF (sends out HELLOs) on interfaces connected to subnets 172.16.1.0/24, 172.16.3.0/30 and 192.168.10.4/30. It also includes these 3 subnets in OSPF database and OSPF Update messages.

Specific Network Statements Type a separate specific OSPF network statement for

each directly-connected subnet on the router. Do not type a network statement for any remote subnet that is

not directly connected. Do not combine multiple subnets into a single network

statement. Example: 172.16.5.0/24 &172.16.6.0/24 are directly attached:

Correct: network 172.16.5.0 0.0.0.255 area 0 network 172.16.6.0 0.0.0.255 area 0

Wrong: (this will work in some cases, not others - you lose points in any case) network 172.16.0.0 0.0.255.255 area 0

It will always be wrong to type: network 0.0.0.0 255.255.255.255 area 0 Sometimes this works, sometimes it messes things up. But it is always sloppy and

not acceptable.


Basic OSPF Configurationenableconfigure terminalinterface loopback0 ! For OSPF IDip address 10.1.1.1 255.255.255.255no shutdowninterface ethernet0/0ip address 130.88.55.1 255.255.255.0no shutdowninterface serial0/0ip address 130.88.56.1 255.255.255.0no shutdownrouter ospf 1network 130.88.55.0 0.0.0.255 area 0network 130.88.56.0 0.0.0.255 area 0exitexit


OSPF Configuration - Optional router ospf (global config)

passive-interface Disables sending route updates out the specified interface

router-id A.B.C.D Sets OSPF router-id to A.B.C.D

ospf log-adjacency-changes Shows a message on router console any time an OSPF

neighbor goes up or down.



By default, OSPF messages are forwarded out all OSPF-enabled interfaces. However, these messages really only need to be sent out interfaces connecting to other OSPF-enabled routers.

Sending out unneeded messages on a LAN affects the network in three ways: Inefficient Use of Bandwidth Inefficient Use of Resources Increased Security Risk

OSPF messages are not sent out a Passive Interface, but its IP subnet is included in OSPF database and in Update messages sent out other interfaces.

The Passive Interface feature helps limit the scope of routing update advertisements.

Configure Single-Area OSPFv2Passive Interface


Configure Single-area OSPFv2Configuring Passive Interfaces

Use the passive-interface router configuration mode command to prevent the transmission of routing messages through a router interface, but still allow that network to be advertised to other routers.

OSPF Interface Configuration(optional)

ip ospf cost Sets the cost for the link connected to this interface to

ip ospf retransmit-interval Sets the time between Link State Advertisement retransmissions

to seconds.

ip ospf priority Sets the OSPF priority of this interface to . Highest

priority interface on transient network will be designated router.

ip ospf hello-interval Sets the time between HELLO packets to seconds.

ip ospf dead-interval If no HELLO packets seen for seconds, then neighbor

router is declared down.Greg Brewster, DePaul University 52TDC 363

OSPF Interface Configuration

enableconfigure terminalinterface ethernet0/0ip address 130.88.55.1 255.255.255.0ip ospf cost 10ip ospf priority 5no shutdowninterface serial0/0ip address 130.88.56.1 255.255.255.0no shutdownrouter ospf 1network 130.88.55.0 0.0.0.255 area 0network 130.88.56.0 0.0.0.255 area 0exitexit


OSPF Verification/Debug show ip protocols show ip ospf interface show ip ospf database show ip ospf router debug ip ospf ? adj - OSPF adjacency events events - OSPF events flood - OSPF flooding lsa-generation - OSPF lsa generation packet - OSPF packets retransmission - OSPF retransmission events tree - OSPF database tree

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TDC 363Introduction to LANsLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State Routing ProtocolsLink-State Routing ProtocolsOpen Shortest Path First (OSPF)OSPF Link State DatabaseBasic OSPF ConfigurationBasic OSPF ConfigurationOSPF Hierarchical NetworksSlide Number 19Introduction to OSPFOSPF HELLO PacketOSPF Router Tables / DatabasesIntroduction to OSPFOSPF DATABASE DESCRIPTION MessageOSPF LINK STATE REQUEST MessageOSPF LINK STATE UPDATE MessageLSU FloodingNew or Old?OSPF Types of LinksPoint-to-Point LinksTransient Links and DRsOSPF in Multiaccess NetworksOSPF in Multiaccess NetworksOSPF in Multiaccess NetworksOSPF in Multiaccess NetworksStub LinksOSPF Protocol OverviewLink State AdvertisementRouter Link LSANetwork Link LSASummary Link LSAOSPF MetricOSPF MetricBasic OSPF ConfigurationOSPF Configuration Configure Single-area OSPFv2The network CommandSpecific Network StatementsBasic OSPF ConfigurationOSPF Configuration - OptionalConfigure Single-Area OSPFv2Passive Interface Configure Single-area OSPFv2Configuring Passive InterfacesOSPF Interface Configuration(optional)OSPF Interface ConfigurationOSPF Verification/Debug

Documents

Introduction to LANs - JustAnswer(1).pdf · ITE PC v4.0 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public. 3. Link-State Routing Link state routing protocols-Also