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CCNA2 Semester 2, 2010 2Dept. of Information Communications, Dongyang Mirae Univ.

AgendaAgenda

11.1 Introduction to OSPF

Introduction, packet types & message format, OSPFalgorithm & process, AD, authentication

11.2 Basic OSPF Configuration

Topology, router & network command, router ID,examination & verification commands

11.3 The OSPF Metric

Metric & modification

11.4 OSPF and Multiaccess Networks

DR/BDR election and their use in multiaccess networks

11.5 More OSPF Configuration

Redistributing an OSPF default route, fine-tuning

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IntroductionIntroduction

OSPF: a linklink--state, classlessstate, classless routing protocol

Developed as the robust solution to overcome the RIPslimitation in the larger networks

ScalabilityScalability to much larger network using the concept of areasconcept of areas

Fast convergenceFast convergence OSPF metric as an arbitrary value called cost

The Cisco IOS uses bandwidth

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Key concepts1.Introduction to OSPF (11.1)

ScalableScalable, robust link-state routing protocol

Supports hierarchical designhierarchical design using multiple areas

Reduces overhead, speeds up convergence, confines networkinstability to one area, improves performance

Build a complete topological database

Uses Dijkstras SPF algorithmSPF algorithm to build routing table

Uses cost as metric

2.Basic OSPF configurationconfiguration (11.2) ParametersParameters to be configured

Process-id, network address & wildcard-mask, area number

Adjustable OSPF parameters

Hello & dead interval timing, router priority, metric

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3.The OSPF metric (11.3) is costcost Cisco IOS uses accumulated bandwidthaccumulated bandwidth for an entire route as the

cost

Administrator can modify the cost of a link

4.OSPF in multi-access network (11.4) Use DR/BDRDR/BDR to reduce update packet between neighbors

Router priorityRouter priority affects the process of DR/BDR election

Administrator can adjust the router priority

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ObjectivesObjectives

Describe the background and basic featuresbasic features of OSPF

Identify and apply the basic OSPF configuration commandsbasic OSPF configuration commands

Describe, modify and calculate the metric used by OSPFmetric used by OSPF

Describe the Designated Router/Backup Designated Router(DR/BDRDR/BDR) election processelection process in multiaccess networks

Describe the uses ofadditional configuration commandsadditional configuration commands inOSPF

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Introduction to OSPFIntroduction to OSPF

11.1 Introduction to OSPF11.1.1 Background of OSPF

11.1.2 OSPF Message Encapsulation

11.1.3 OSPF Packet Types

11.1.4 Hello Protocol

11.1.5 OSPF Link-State Updates

11.1.6 OSPF Algorithm

11.1.7 Administrative Distance

11.1.8 Authentication

11.2 Basic OSPF Configuration




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Background ofOSPFBackground ofOSPF

Began in 1987 by IETF1989 OSPFv1 released in RFC 1131

This version was experimental & never deployed

1991 OSPFv2 released in RFC 1247

1998 OSPFv2 updatedin RFC 2328

1999 OSPFv3 for IPv6 published in RFC 2740

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OSPF Message EncapsulationOSPF Message Encapsulation

OSPF packet

OSPF packet

HeaderHeader

Data

Header Data

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OSPFPacket TypesOSPFPacket Types

Packet types defined by the type code in the packet header1.Hello packet (type=0x01)

Establishes and maintains adjacency with other OSPF routers

2.The Database Description (DBD, DD) packet (type=0x02)

An abbreviated list of the sending router's link-state database

Used by receiving routers to check against the local link-statedatabase.

3.Link-State Request (LSR) packet (type=0x03)

Receiving routers request more information about specific entry inthe DBD

4.Link-State Update (LSU) packet (type=0x04) Replies to specific LSRs or announces new information

Contains 11 different types of Link-State Advertisements (LSAs)

5.Link-State Acknowledgement (LSAck) packet (type=0x05)

Acknowledges the receipt of other types of OSPF packet

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Hello ProtocolHello Protocol

Purpose of OSPF hello packet Ensure bi-directional communication and maintain keepalive

Advertise parameters on which routers must agree to becomeneighbors

Hello interval, dead interval, network type

Discover OSPF neighbors & establish adjacencies Elect a Designated Router (DR) and a Backup DR (BDR) used in

multi-access networks Type: OSPF Packet Type: Hello (1),

DBD (2), LSR (3), LSU (4), LSACK (5) Router ID: ID of the originating router Area ID: area from which the packet

originated

Network Mask: Subnet maskassociated with the sending interface Hello Interval: number of seconds

between the sending router's hellos Router Priority: Used in DR/BDR

election Designated Router (DR): Router ID

of the DR, if any Backup Designated Router (BDR):

Router ID of the BDR, if any List of Neighbors: lists the OSPF

Router ID of the neighboring router(s)

the highest ip addressconfigured on anyinterface in this router

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Neighbor establishment Any other OSPF neighbors on any of its links?

1. Send and receive hello on all OSPF-enabled interfaces

2. Determine the OSPF router ID of the sending router

3. Establishes adjacency with the neighbor

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OSPF Hello intervals Usually periodically multicast (all SPF routers; 224.0.0.5)

10 sec. (multiaccess & point-to-point segment) or

30 sec. (NBMA segments such as Frame relay, X.25, ATM)

OSPF Dead intervals

Time for the router to wait without receiving hello packet before itthinks the neighbor is down

When expired, link-state update about the down neighbor is

flooded Default value = 4 hello interval


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Electing a DR (designated router) and BDR (backup DR) Used to reduce the amount of OSPF traffic on multicast networks

A router with the (second) highest priorityhighest priority is elected as the (B)DRDR

Without DR & BDR

All OSPF routers flood link-state update packets to all others

With DR & BDR

1.All OSPF routers send link-state update packets to DR & BDR

Multicast to 224.0.0.6

2.DR sends them to all other routers

Multicast to 224.0.0.5

BDR takes over DR when DR fails

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OSPF LinkOSPF Link--State UpdatesState Updates

Link-state update (LSU) packet Used for OSPF routing updates

Deliver link state advertisements (LSAs)

An LSA contains information about neighbors & path costs

An LSU packet can contain 1 LSAs

There are 11 different types of LSAs in a LSU packet

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OSPF AlgorithmOSPF Algorithm

Maintains a link-state database containing the received LSAsUses Dijkstra's shortest path first (SPF) algorithm to create

an SPF tree

The SPF tree is then used to populate the IP routing table

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OSPF process steps summary1.Send hello packets and wait to receive hello packets from

others (packet type=1;downinit state)

2.Using received hello packets, [elect DR/BDR in multi-accessnetwork and] establish adjacencies (2-way state)

In the MA networks, adjacencies are established only with DR

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3. Go through a sequence of status The two neighbors determine a master/slave relationship based

on the ID (highest IP interface address) (type=1; exstart state)

Exchange topology summary (type=2;exchange state)

Called database description packet (DDP)

Link-state request (LSR;type=3) link-state update (LSU;type=4) link-state acknowledge (LSAck;type=5); loading state

LSAs in the LSU packets are multicast

In MA network, other routers multicast LSUs to DR first (224.0.0.6)

Next, DR multicasts them to all other routers (224.0.0.5)

LSUs are flooded over an OSPF area

Record LSAs in a link-state database and reply with a LSA Ack.

Repeat the processes until the databases are complete (in fullstate)

All routers in an OSPF area have the same topological database

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DDP

LSR

LSU

This is detailed information

DDP

LSAckI have received LSU

LSU

LSAck

This is detailed information

LSR

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4.Apply SPF algorithm to calculate the shortest loop free route Routing information is recorded in a routing table

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Administrative DistanceAdministrative Distance

Default Administrative Distance for OSPF is 110 OSPF is preferred over IS-IS and RIP, but not over EIGRP

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AuthenticationAuthentication

OSPF can be configured for authentication Routers will only accept routing information from other routers

configured with the same password or authenticationinformation

RIPv2, EIGRP, OSPF, IS-IS, & BGP all supports encryption

and authentication This is an interface specific configuration

Authentication does not encrypt the router's routing table

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Basic OSPF ConfigurationBasic OSPF Configuration

11.1 Introduction to OSPF11.2 Basic OSPF Configuration

11.2.1 Lab Topology

11.2.2 The router ospf Command

11.2.3

The network Command11.2.4 OSPF Router ID

11.2.5 Verifying OSPF

11.2.6 Examining the Routing Table




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Lab TopologyLab Topology

Discontiguous & VLSM IP addressing schemeSerial links ofvarious bandwidths (not now, yet)

Multiple paths to each remote network

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TheThe router ospfrouter ospf CommandCommand

Enabling OSPF on a router

1 process_id 65,535, chosen by the administrator

A router can have multiple OSPF processes (unusual & expensive)

Used to identify each OSPF process on a router (local to the router) Locally significant

No need to match other OSPF routers in order to establish adjacencies withthose neighbors

Ex: On R1~R3, enable OSPF with the process id of 1

R1(config)#router ospfprocess_id

R1(config)#router ospf 1

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TheThe networknetwork CommandCommand

The function of the network commandAny interfaces matching the network address in the network

command will be enabled to send and receive OSPF packets.

This network (or subnet) will be included in OSPF routingupdates.

Syntax

OSPF always requiresalways requires the wildcard maskconfiguredconfigured

Specify the [range of] interfaces that will be enabled for OSPF

Can be 0.0.0.0 if the single interface is to be specified

area area-idrefers to the OSPF area

An OSPF area is a group of routers that share link-state information

Only single area OSPF is covered in CCNA

R1(config)#network network-address wildcard-maskarea area-id

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TheThe networknetwork CommandCommand

Configure R1~R3 with network command: Use area_idof 0

Apply appropriate wildcard-maskfor each subnet

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OSPF Router IDOSPF Router ID

Determining the router IDAn IP address used to identify a routerused in DR/BDR electionDR/BDR election

Must be unique for each router in an OSPF area

3 rule for deriving the router ID

1. Use IP address configured with OSPF router-idcommand

It takes precedence over loopback and physical interface addresses

It is used at the next reload orat a manual OSPF process restart

2. If not configured, router chooses highest IP address of anyloopback interfaces

3. If no loopback interfaces are configured, then the highest IPaddress on any active interface is used

Ex: Using the criteria described above, determine the router IDs

for R1~R3

and verify them

R1(config-router)#routeridip_address

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Verifying current router IDR1#sh ip protocol

Routing Protocol is "ospf 1"Outgoing update filter list for all interfaces is not setIncoming update filter list for all interfaces is not setRouter ID 192.168.10.5

Number of areas in this router is 0. 0 normal 0 stub 0 nssa

R1#show ip ospfRouting Process "ospf 1" with ID 192.168.10.5

Supports only single TOS(TOS0) routes

R1#show ip ospf interface s0/0/0Serial0/0/0 is up, line protocol is up

Internet address is 192.168.10.1/30, Area 0Process ID 1, Router ID 192.168.10.5, Network Type POINT-TO-

POINT, Cost: 64

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Router ID & loopback addresses Once configured, loopback interface is always up and never fails

It is always preferred over any IP address of real interfaces

Ex: Configure loopback 0 of R1~R3 with 10.1.1.1, 10.2.2.2,10.3.3.3 with the subnet mask of /32 (Are router IDs changed?)

Modifying the router IDModifying the router ID

The router ID is selected when OSPF is configured with its firstOSPF network command

After that moment, modifying the router ID requiresrequires reload or

restarting the OSPF process

Ex: Apply the modifications in the above example and verify them

R1#reload OR,R1#clear ip ospf process

effective only forrouter-idcommand

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Verifying OSPFVerifying OSPF

R3#show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface10.1.1.1 1 FULL/- 00:00:35 192.168.10.5 Serial0/0/010.2.2.2 1 FULL/- 00:00:36 192.168.10.9 Serial0/0/1

show ip ospf neighborcommand Used to verify and troubleshoot OSPF neighbor relationships

Two routers may not form an OSPF adjacency if:

Unmatched subnet masks, causing the routers to be on separatenetworks.

Unmatched Hello or Dead timers.

Unmatched OSPF Network types. Missing or incorrect OSPF

network command.

Ex: Verify the neighbors atR1~R2

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Additional OSPF troubleshooting commands

Ex: Apply above commands on R1~R3 and examine the outputs

Command Description

show ip protocolsDisplays OSPF process ID, router ID,networks router is advertising & administrativedistance

show ip ospfDisplays OSPF process ID, router ID, OSPFarea information & the last time SPF algorithmcalculated

show ip ospf interface Displays hello interval and dead interval

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R1#show ip protocols

Routing Protocol is "ospf 1"Outgoing update filter list for all interfaces is not setIncoming update filter list for all interfaces is not setRouter ID 10.1.1.1Number of areas in this router is 1. 1 normal 0 stub 0 nssa

Maximum path: 4Routing for Networks:172.16.1.16 0.0.0.15 area 0192.168.10.0 0.0.0.3 area 0192.168.10.4 0.0.0.3 area 0

Routing Information Sources:Gateway Distance Last Update

192.168.10.2 110 00:17:06192.168.10.6 110 00:17:06

Distance: (default is 110)

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R2#show ip ospfRouting Process "ospf 1" with ID 10.2.2.2Supports only single TOS(TOS0) routesSupports opaque LSA

SPF schedule delay 5 secs, Hold time between two SPFs 10 secsMinimum LSA interval 5 secs. Minimum LSA arrival 1 secs

Number of external LSA 0. Checksum Sum 0x000000Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 0Number of DoNotAge external and opaque AS LSA 0Number of areas in this router is 1. 1 normal 0 stub 0 nssa

External flood list length 0

Area BACKBONE(0)Number of interfaces in this area is 3Area has no authenticationSPF algorithm executed 3 times

Area ranges areNumber of LSA 6. Checksum Sum 0x04734eNumber of opaque link LSA 0. Checksum Sum 0x000000

Number of DCbitless LSA 0Number of indication LSA 0Number of DoNotAge LSA 0

Flood list length 0

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R3#show ip ospf interface serial 0/0/1Serial0/0/1 is up, line protocol is up

Internet address is 192.168.10.10/30, Area 0Process ID 1, Router ID 10.3.3.3, Network Type POINT-TO-POINT, Cost:

64Transmit Delay is 1 sec, State POINT-TO-POINT,

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Hello due in 00:00:08

Index 3/3, flood queue length 0Next 0x0(0)/0x0(0)Last flood scan length is 1, maximum is 1

Last flood scan time is 0 msec, maximum is 0 msecNeighbor Count is 1 , Adjacent neighbor count is 1

Adjacent with neighbor 192.168.10.9

Suppress hello for 0 neighbor(s)

Must match toform adjacencyMust match toform adjacency

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Examining the Routing TableExamining the Routing Table

R1#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter areaN1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGPi - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area* - candidate default, U - per-user static route, o - ODRP - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masksC 10.1.1.1/32 is directly connected, Loopback0O 10.10.10.0/24 [110/65] via 192.168.10.2, 00:54:26, Serial0/0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

C 172.16.1.16/28 is directly connected, FastEthernet0/0O 172.16.1.32/29 [110/65] via 192.168.10.6, 01:03:15, Serial0/0/1

192.168.10.0/30 is subnetted, 3 subnetsC 192.168.10.0 is directly connected, Serial0/0/0

C 192.168.10.4 is directly connected, Serial0/0/1O 192.168.10.8 [110/128] via 192.168.10.6, 00:54:26, Serial0/0/1

[110/128] via 192.168.10.2, 00:54:26, Serial0/0/0

Not advertised

by OSPF

Not automaticallysummarized

Equal-costload balancing

Examine the routing table of R1~R3

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The OSPF MetricThe OSPF Metric



11.3.1 OSPF Metric

11.3.2 Modifying the Cost of the Link11.4 OSPF and Multiaccess Networks


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OSPF MetricOSPF Metric

The OSPF metric is defined as cost in RFC 2328:A cost is associated with the output side of each router interface

This cost is configurable by the system administrator

The lower the cost, the more likely the interface is to be used toforward data traffic.

Values to be used to determine the cost are not specified by RFC

The cost in Cisco IOS

The cumulativecumulative bandwidthsbandwidths of the outgoing interfaces from therouter to the destination network

In EIGRP, minimum bandwidth is used

Cost =Cost = 101088/ bandwidth/ bandwidth [bps]

108 is the reference bandwidth

Can be modified to accommodate links with the bandwidth > 108

R1(config-router)#auto-cost reference-bandwidth ref-bw

Rate in Mbps (1 ~ 4294967)Default = 100

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Cisco cost values for several types of link

Verifying the cost (calculated bandwidth) of a link

R1#show ip ospf interface s0/0/0Serial0/0/0 is up, line protocol is upInternet address is 192.168.10.1/30, Area 0Process ID 1, Router ID 10.1.1.1, Network Type POINT-TO-POINT, Cost: 64Transmit Delay is 1 sec, State POINT-TO-POINT,Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

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R1#show ip route

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masksC 10.1.1.1/32 is directly connected, Loopback0

O 10.10.10.0/24 [110/65] via 192.168.10.2, 00:54:26, Serial0/0/0

OSPF accumulates costs The accumulated value from a router to the destination network

Ex: Cost from R1 to R2s LAN

= Cost1 + Cost2

= 64+1 = 65

Cost1=64

Cost2=1

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Default bandwidth on serial interfaces Most serial link default to 1.544Mbps

Some are 128kbps

Does not actually affect nor reflect the speed of the link

Need to configure the interface with the actual link speed

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Modifying the Cost of the LinkModifying the Cost of the Link

Router(config-if)#bandwidth bandwidth-kbps

Two alternatives in modifying the cost of a link1. Modifying bandwidth of the interface

Cost is calculated by the formula cost = 108/bandwidth

2. Modifying the cost of the interface itself

1 cost 65,535

Useful in multi-vendor environment, where bandwidth is not usedas the cost

Must be modified on all interfaces of a link

Router(config-if)#ip ospf cost cost

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Modifying the Cost of the LinkModifying the Cost of the Link

Thebandwidth command vs. the ip ospf cost command

Ex: Modify the cost as shown in the topology, using either of

the commands. Then verify changes in the routing table of

each router

1

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OSPF and Multiaccess NetworksOSPF and Multiaccess Networks




11.4.1 Challenges in Multiaccess Networks11.4.2 DR/BDR Election Process

11.4.3 OSPF Interface Priority


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Challenges in MultiaccessChallenges in MultiaccessNetworksNetworks

Point-to-multipoint

Five network types defined in OSPF :1.Broadcast Multiaccess

Ethernet, Token Ring, or FDDI

2.Point-to-point

Point-to-point serial link, HDLC3.Nonbroadcast Multiaccess (NBMA)

Frame Relay, X.25, ATM

4.Point-to-multipoint

5.Virtual links For connecting discontiguous backbone area

(area 0) in multi-area OSPF

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Two challenges in the flooding of LSAs in mulitaccessnetworks:

1.Multiple adjacencies, one adjacency for every pair of routers.

2.Extensive flooding of LSAs

Multiple Adjacencies ForN routers in a multiaccess network, every router make

adjacencies with N-1 routers

Total adjacencies = N(N-1)/2

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Flooding of LSAs N(N-1) LSAs are needed (see the animation in 11.4.1 of the

curriculum)

Solution: Designated Router (DR)

Elect a DR & backup DR (BDR) in the multiaccess network BDR acts as DR when DR fails

All other routers (DROthers) make adjacencies only with DR &BDR

Only 2N

-3

adjacencies are made (including DRBDR adjacency)A routersends LSAs only to DR & BDR (2 LSAs)

Multicast to 224.0.0.6 (ALLDRouters - All DR routers)

Only DR floods received LSAs to all other routers (N-1 LSAs)

Multicast to 224.0.0.5 (AllSPFRouters - All OSPF routers)

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DR/BDR Election ProcessDR/BDR Election Process

Changes in the scenario topology DR/BDR elections do not occur in point-to-point networks

Multiaccess topology scenario is needed

Configure OSPF with process ID 1 & area 0

Fa0/0 192.168.1.4/24D

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DR/BDR Election DR: Router with the highest OSPFhighest OSPF interface priorityinterface priority

BDR: Router with the second highest OSPF interface priority

The default OSPF interface priority is 1

The OSPF interface priority can be modified by:

The interface connected to the MA network in the election process

For equal priority, the highest router IDhighest router ID is used to break the tie.

Adjacencies

DROthers only form FULL adjacencies with the DR and BDR

Still form a neighbor(2way) adjacencies with any DROthers

Still receive Hello packets from all other DROther routers

Router(config-if)#ip ospf priority {0~255}

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Verify the routerstate and DR/BDR

using show ipospf interfacefa0/0 command on

A~D

A#show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface192.168.31.22 1 FULL/BDR 00:00:32 192.168.1.2 FastEthernet0/0192.168.31.33 1 FULL/DR 00:00:35 192.168.1.3 FastEthernet0/0

B#show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface192.168.31.11 1 FULL/DROTHER 00:00:37 192.168.1.1 FastEthernet0/0192.168.31.33 1 FULL/DR 00:00:32 192.168.1.3 FastEthernet0/0

C#show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface192.168.31.11 1 FULL/DROTHER 00:00:32 192.168.1.1 FastEthernet0/0192.168.31.22 1 FULL/BDR 00:00:35 192.168.1.2 FastEthernet0/0

D

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Timing of DR/BDR electionAs soon as the 1st router with an OSPF enabled interface is

active on the multiaccess network

A router with a lower router ID that is configured first willbecome the DR

The router enabled later with higher priority cannot be the DR/BDR

Conditions for the changes in DR/BDR:

Conditions for BDR DR (promotion)

The DR fails.

The OSPF process on the DR fails.

The multiaccess interface on the DR failsDROther with the highest priority (or router ID) among DROthers is

elected as BDR

Conditions for DR/BDR re-election process

Both current DR and BDR must fail at the same time

Failed [B]DR becomes DROther when it is re-enabled

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BDR DR takeover processA#sh ip ospf neigh

Neighbor ID Pri State Dead Time Address Interface192.168.31.22 1 FULL/BDR 00:00:33 192.168.1.2 FastEthernet0/0192.168.31.33 1 FULL/DR 00:00:23 192.168.1.3 FastEthernet0/0A#00:15:30: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.31.33 on FastEthernet0/0 from FULL to DOWN,

Neighbor Down: Dead timer expired00:15:30: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.31.33 on FastEthernet0/0 from FULL to Down:Interface down or detachedA#A#sh ip ospf neigh

Neighbor ID Pri State Dead Time Address Interface192.168.31.22 1 FULL/DR 00:00:39 192.168.1.2 FastEthernet0/0

A#

00:18:15: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.31.33 on FastEthernet0/0 from EXCHANGE toFULL, Exchange DoneA#sh ip ospf neigh

Neighbor ID Pri State Dead Time Address Interface192.168.31.22 1 FULL/DR 00:00:39 192.168.1.2 FastEthernet0/0192.168.31.33 1 FULL/DROTHER 00:00:31 192.168.1.3 FastEthernet0/0A#

RouterCdown

RouterCre-up

BDRDR

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Making sure that the routers you want to be DR and BDR winthe election

Changing the OSPF interface priority can control the elections

OR the router ID if the priorities are identical for all routers in thebroadcast multiple access network

1.Boot up routers in the order of the DR, BDR, and all others, OR

2.Shut down the interface on all routers, then enable DR, BDR, and then all others.

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OSPFInterface PriorityOSPFInterface Priority

Controlling the election priority Set the interface priorityinterface priorityhigher for the preferred router

The larger, the higher priority

Router with interface priority value 0 can never participate DR/BDR

election process Force election

Shutdown and re-enable all interfaces

The order of re-enabling is still important

Ex: Set the interface priority of fa0/0 in A to 250, B to 200, then

restart election process, then verify the priorities and identify theDR & BDR

Shutdown all Fa0/0s, then enable Ds and Bs, wait DR/BDRselection, then enable As and Cs

Examine neighbor tables and identify DR/BDR

Shutdown DRs Fa0/0, wait 40sec., re-enabe and identify DR/BDR

Router(config-if)#ip ospf priority {0~255}

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More OSPF ConfigurationMore OSPF Configuration




11.5 More OSPF Configuration11.5.1 Redistributing an OSPF Default Route

11.5.2 Fine-tuning OSPF

R di t ib ti OSPF D f ltR di t ib ti OSPF D f lt

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Redistributing an OSPF DefaultRedistributing an OSPF DefaultRouteRoute

R1(config)#ip route 0.0.0.0 0.0.0.0 lo11.Configuring a static default route on R1

Exit interface is not configured as part of the OSPF routingdomain

R1 becomes an ASBR (autonomous system boundary router) The router located between an OSPF routing domain and a non-

OSPF network

Redistributing an OSPF DefaultRedistributing an OSPF Default

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Redistributing an OSPF DefaultRedistributing an OSPF DefaultRouteRoute

2.Propagating installed static default route to all the routers ina normal OSPF area

The OSPF router does not, by default, generate a defaultroute into the OSPF domain

Ex: Verify the redistributed default route in R1~R3

R1(config-router)#default-information originate [metric value] [metric-type type]

OSPF external

route type

(Default=2)

Default=1

R2#show ip route

C 192.168.10.8 is directly connected, Serial0/0/1O*E2 0.0.0.0/0 [110/1] via 192.168.10.10, 00:00:10, Serial0/0/1

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FineFine--tuning OSPFtuning OSPF

Modifying reference bandwidth

ref-bw is the new reference bandwidth in Mbps (1 ~ 4294967)

Default = 100

Make sure to configure on all routers in the OSPF routingdomain

Ex: Modify the reference bandwidth to 10Gbps and comparethe changes in the metrics in R1

R1(config-router)#auto-cost reference-bandwidth ref-bw

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Modifying OSPF timers Impact of reducing the timers

Increase in traffic

Faster detection of network failures and quick convergence

Modifying OSPF Hello and Dead intervals

Modifying Hello interval results in the automatic modification ofDead interval

Explicit modification is strongly recommended for the purpose of

documentation The Hello and Dead intervals should match between two adjacent

routers

In EIGRP, it is not the adjacency condition

Router(config-if)#ip ospf hello-interval seconds

Router(config-if)#ip ospf dead-interval seconds

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Ex: Modify the hello & dead intervals of R1R2 to the half ofdefault value

Verify R1 & R2s neighbors and timers

Modify R1s timers

Wait for over 20 seconds

Examine what happens in IOS Examine changes R1s neighbors and timers

Modify R2s timers

Verify R1 & R2s neighbors and timers

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RFC 2

328 describes OSPF link state concepts and operations

OSPF Characteristics

A commonly deployed link state routing protocol

Employs DRs & BDRs on multi-access networks

DRs & BDRs are electedDR & BDRs are used to transmit and receive LSAs

Uses 5 packet types:

1: HELLO

2: DATABASE DESCRIPTION

3: LINK STATE REQUEST

4: LINK STATE UPDATE

5: LINK STATE ACKNOWLEDGEMENT

SummarySummary

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SummarySummary

OSPF Characteristics Metric = cost [ = 108/bandwidth [bps] in Cisco IOS]

Lowest cost = best path

Configuration

Enable OSPF on a router using the following command

R1(config)#router ospfprocess-id

Use the network command to define which interfaces

will participate in a given OSPF process

Router(config-router)#network network-addresswildcard-mask area area-id

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SummarySummary

Verifying OSPF configuration Use the following commands

show ip protocol

show ip route

show ip ospf interfaceshow ip ospf neighbor

Advertising the existing 0.0.0.0/0 into the OSPF domain

defaultinformation originate

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