© synergon informatika rt., 1999 chapter 9 configuring open shortest path first

© Synergon Informatika Rt., 1999

Synergon Informatika Rt.

Chapter 9

Configuring Open Shortest Path First

2

Objectives

Upon completion of this chapter, you will be able to perform the following tasks:

Describe OSPF features and operation

Configure OSPF for proper operation

Use Cisco IOS summarization and stub-area features for OSPF

Verify OSPF operation

3

OSPF Features and Operation

4

OSPF Overview

Standard IP link-state routing protocol

Designed to overcome RIP limitations

5

OSPF Features

Has no hop count limitation

Supports VLSM

Uses multicast addressing for updates

Has faster convergence

Allows for routing authentication

Supports hierarchical routing

6

OSPF Hierarchical Routing

Consist of areas and autonomous systems

Minimizes routing update traffic

Area 0

Area 1 Area 2

Autonomous System

7

OSPF Router Types

Area 1 Backbone Area 0 Area 2

External AS

Area Border Router

AS Boundary Router

Internal Router

Backbone Router

8

The Backbone and Virtual Links

Backbone center of communication

Virtual links provide path to backbone

Avoid configuring virtual links if possible

Virtual LinkArea 0

(Backbone)

Area 1 Area 2

Area 3

9

Another Use for Virtual Links

Link discontinuous backbone

- Merged networks

- Redundancy

Area 3Area 0Area 0

10

The Link-State Database

Represents the network topology

Shared with OSPF routers in same area

Adjacent

Link-State Database Link-State Database

11

Link-State Advertisements (LSAs)

Sourced by router connected to link

Flooded by all other routers in area

Transmitted at each link-state change

LSA

12

Link-State Advertisement Operation

LSA LSA

LSA

LSA received

Add to database

Flood LSA

Run SPF to calculate new table

Reset timer for entryYes

No

Is entry in topological database?

13

Types of Link-State Packets

O — OSPF Derived Intra-Area (Router LSA)IA — Inter-Area (Summary LSA)E1 — Type 1 External RouteE2 — Type 2 External Route

Routing Table ExternalAS

ASBR

DRArea 1 Area 0

ABR

Network

Router

External

Summary

14

Types of External Routes

Type-1 (E1) metric — external + internal

Type-2 (E2) metric — external cost only

5 5

Area 1 Area 0

R1R2R3

N1 cost = 30N2 cost = 20

N1 cost = 25N2 cost = 20

Cost = 20

20N2(E2)

N1(E1)

15

OSPF Network Types

Point-to-Point

BroadcastMultiaccess

X.25Frame Relay

NonbroadcastMultiaccess

16

OSPF Router ID

Number by which the router is known to OSPF

- Default: The highest IP address on an active interface

- Can be overridden by a loopback interface

Network131.108.0.0

Bogus Loopback AddressEx: 1.1.1.1• Not in OSPF table• Saves address space

Bogus Loopback AddressEx: 1.1.1.1• Not in OSPF table• Saves address space

Real Loopback AddressEx: 131.108.17.5• In OSPF table• Uses address space

Real Loopback AddressEx: 131.108.17.5• In OSPF table• Uses address space

17

Neighbors

Form using the Hello protocol

Hello

18

Adjacencies

Hellos elect designated router (DR) and backup designated

router (BRD)

Each router forms adjacency with DR and BDR

DR BDR

19

Designated Router Election

Hello packets exchanged via IP multicast

Router with highest OSPF priority elected

Hello

DR BDR

P=3 P=2

P=1 P=1 P=0

20

Multiaccess Network

New router announces presence

DR and BDR respond

Adding a Router

NewRouter

DR

BDR224.0.0.5 Hello

21

Multiaccess Network (cont.)

New router sends LSAs to DR and BDR

BDR waits for DR to respond

Adding a Router

New Router

DR

BDR224.0.0.6 LSA

22


DR transmits LSAs to other routers

DR must receive ACK from all routers

Adding a Router

New Router

DR

BDR

224.0.0.5 LSA

23


Source router tells DR on 224.0.0.6

DR tells others on 224.0.0.5

Other routers flood LSAs

Link-State Change

New Router

DR

BDR

XX

LSA

LSA

LSA

24

Configuring OSPF

25

OSPF Basic Configuration Commands

Router (config) #

router ospf process-idrouter ospf process-id

Enables on OSPF routing process

Router (config-router ) #

network address wildcard-mask area area-idnetwork address wildcard-mask area area-id

• Selects participating interfaces

26

OSPF Basic Configuration Example

TokenRing

router ospf 63network 172.16.5.3 0.0.0.0 area 1network 172.16.0.0 0.0.255.255 area 0network 192.168.10.5 0.0.0.0 area 1

router ospf 63network 172.16.5.3 0.0.0.0 area 1network 172.16.0.0 0.0.255.255 area 0network 192.168.10.5 0.0.0.0 area 1

172.16.5.3

192.168.10.5172.16.1.1

172.16.3.1

T0

E0E1

E2

Area 1

Area 0

27

OSPF Virtual Link Command

Router (config-router) #

Area area-id virtual-link router-idArea area-id virtual-link router-id

Creates a virtual link

28

OSPF Virtual Link Example

TokenRing

R2:router ospf 63area 1 virtual-link 192.168.10.5


Area 1

Area 0

Area 3

Router ID 192.168.10.5

Router ID 192.168.20.123



29

Vendor Interoperability

TokenRing

• Assigns a cost to an outgoing interface

• May be required for interoperability

• Use default cost between Cisco devices

Traffic

Non-CiscoCisco

Router (config-if ) #

Ip ospf cost costIp ospf cost cost

30

Using Route Summarization, Stub Areas, and Redistribution

31

OSPF Route Summarization

X

• Minimizes routing table entries

• Localizes impact of a topology change

Area 1

ABRs

Summarization

Area 0 Backbone

32

Route Summarization (cont.)

• Interarea (IA) summary link carries mask

• One entry can represent several subnet

Area 1 Area 0ABR

Summarization

O 131.108.4.0 255.255.255.0O 131.108.8.0 255.255.255.0O 131.108.12.0 255.255.255.0O 131.108.16.0 255.255.255.0O 131.108.20.0 255.255.255.0O 131.108.24.0 255.255.255.0O 131.108.28.0 255.255.255.0

IA 131.108.16.0 255.255.240.0

Routing Table for C

Routing Table for B

A B C

33

Route Summarization Issue

Existing Subnet

Mask = 252 1111 11 00 Valid

131.108.12.0 255.255.255.0 Address = 12 0000 11 00 3rd Subnet

Summary Route

Mask = 240 1111 0000 Invalid Subnet

131.108.12.0 255.255.240.0 Address = 12 0000 1100 zero

• Some addresses may need reallocating

34

Route Summarization Commands


Area area-id range address maskArea area-id range address mask

Consolidates IA router on an ABR


Summary-address address maskSummary-address address mask

• Consolidates external routes on an ASBR

35

Route Summarization ExampleArea 0

Area 1 Area 2

172.16.96.0 - 172.16.127.0 255.255.255.0

172.16.64.0 - 172.16.95.0 255.255.255.0

172.16.32.0 - 172.16.63.0 255.255.255.0

172.16.32.1 172.16.64.1

172.16.127.1172.16.96.1

Interface Addresses

(255.255.255.0 mask)Interface Addresses

(255.255.255.0 mask)

R1#router ospf 100network 172.16.32.0 0.0.31.255 area 1network 172.16.96.0 0.0.31.255 area 0area 0 range 172.16.96.0 255.255.224.0area 1 range 172.16.32.0 255.255.224.0




R1 R2

36

Stub Areas

• Hide external routes, reduce database

• Consolidate external links - 0.0.0.0

Area 1 Area 0Area 2

0.0.0.0

BGPStub Area172.20.64.0 - 172.20.95.0

255.255.255.0

External AS

37

Stub Area Restrictions

X

External AS

0.0.0.0

Area 2

• single exit point or, if multiple exit points,

suboptimal path acceptable

• Transit area for virtual links disallowed

• An ASBR cannot be internal to stube

Single Exit Point

38

Totally Stubby Areas

• Block external and summary routes

• Know only intra-area and default routes

Area 1 Area 0Area 2

0.0.0.0

BGPStub Area172.20.64.0 - 172.20.95.0

255.255.255.0

External AS

Summary (IA) Route

39

OSPF Stub Area Commands


Area area-id stub [no-summary]Area area-id stub [no-summary]

Creates a stub network


Area area-id default-cost costArea area-id default-cost cost

• Specifies cost for default route sent into stub area

40

OSPF Stub Area Example

External AS

Area 0

Stub Area 2

192.168.15.2192.168.15.1

192.168.14.1

E0 S0

S0

R3

R4

R3#interface Ethernet 0ip address 192.168.14.1 255.255.255.0interface Serial 0ip address 192.168.15.1 255.255.255.252

router ospf 100network 192.168.14.0 0.0.0.255 area 0network 192.168.15.0 0.0.0.255 area 2area 2 stub

R3#interface Ethernet 0ip address 192.168.14.1 255.255.255.0interface Serial 0ip address 192.168.15.1 255.255.255.252

router ospf 100network 192.168.14.0 0.0.0.255 area 0network 192.168.15.0 0.0.0.255 area 2area 2 stub

R4#interface Serial 0ip address 192.168.15.2 255.255.255.252

router ospf 15network 192.168.15.0 0.0.0.255 area 2area 2 stub

R4#interface Serial 0ip address 192.168.15.2 255.255.255.252


41

OSPF Totally Stubby Example

External AS

Area 0

Stub Area 2

192.168.15.2192.168.15.1

192.168.14.1

E0 S0

S0

R3

R4

R3#

router ospf 100network 192.168.14.0 0.0.0.255 area 0network 192.168.15.0 0.0.0.255 area 2area 2 stub no-summaryarea 2 stub default-cost 20

R3#

router ospf 100network 192.168.14.0 0.0.0.255 area 0network 192.168.15.0 0.0.0.255 area 2area 2 stub no-summaryarea 2 stub default-cost 20

R4#


R4#


42

OSPF Route Redistribution

RIP

IGRP

Enhanced IGRP

IS-IS

OSPF

BGP EGP

• Allows routing-information exchange between OSPF and other routing protocols

43

OSPF redistribution Command


Redistribute protocol [ process-id ] [ metric value ] [ metric-type value ] [ subnet ]

Redistribute protocol [ process-id ] [ metric value ] [ metric-type value ] [ subnet ]

Redistributes routes from OSPF into other routing protocols

(and vice versa)

44

OSPF Redistribution Example 1

RIP OSPF Area 0

172.16.62.1172.16.9.1 S0

S1R1

172.16.8.1 172.16.63.1

Redistribution between RIP and OSPF

router ospf 109network 172.16.62.0 0.0.0.255 area 0network 172.16.63.0 0.0.0.255 area 0redistribute rip subnets metric-type 1 metric 20

router ripnetwork 172.16.0.0passive-interface serial 0passive-interface serial 1default-metric 10redistribute ospf match internal external 1 external 2

router ospf 109network 172.16.62.0 0.0.0.255 area 0network 172.16.63.0 0.0.0.255 area 0redistribute rip subnets metric-type 1 metric 20

router ripnetwork 172.16.0.0passive-interface serial 0passive-interface serial 1default-metric 10redistribute ospf match internal external 1 external 2

45

Redistribution Example 2

“Back door” creates potential loop

RIP RIP

RIP

OSPF Area 0

R1

R2

R3

172.16.9.1

172.16.8.1

router ospf 109network 172.16.62.0 0.0.0.255 area 0network 172.16.63.0 0.0.0.255 area 0redistribute rip subnets metric-type 1 metric 20distribute-list 11 out rip

access-list 11 permit 172.16.8.0 0.0.7.255

router ospf 109network 172.16.62.0 0.0.0.255 area 0network 172.16.63.0 0.0.0.255 area 0redistribute rip subnets metric-type 1 metric 20distribute-list 11 out rip

access-list 11 permit 172.16.8.0 0.0.7.255

46

Verifying OSPF Operation

47

Show ip ospf interface Command

• Verifies interfaces are in correct areas

Router# show ip ospf interface e 0 Ethernet 0 is up, line protocol is up Internet Address 203.250.14.1 255.255.255.0, Area 0.0.0.0 Process ID 10, Router ID 203.250.13.41, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State BDR, Priority l Designated Router (ID) 203.250.15.1, Interfece address 203.250.14.2 Backup Designated router (ID) 203.250.13.41, interface address 203.250.14.1 Timer intervale configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 0:00:02 Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor 203.250.15.1 (Designated Router) Loopback0 is up, line protocol is up Internet address 203.250.13.41 255.255.255.255, Area 1 Process ID 10, Router ID 203.250.13.41, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host

Router# show ip ospf interface e 0 Ethernet 0 is up, line protocol is up Internet Address 203.250.14.1 255.255.255.0, Area 0.0.0.0 Process ID 10, Router ID 203.250.13.41, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State BDR, Priority l Designated Router (ID) 203.250.15.1, Interfece address 203.250.14.2 Backup Designated router (ID) 203.250.13.41, interface address 203.250.14.1 Timer intervale configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 0:00:02 Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor 203.250.15.1 (Designated Router) Loopback0 is up, line protocol is up Internet address 203.250.13.41 255.255.255.255, Area 1 Process ID 10, Router ID 203.250.13.41, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host

48

Show ip ospf Command

Router # show ip ospf Routing Process "ospf 1" with ID 2.2.2.2 Supports only single TOS (TOS0) routes SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Number of areas in this router is 1 Area 23

Number of interfaces in this area is 3 Area has no authenticationSPF algorithm executed 19 times Area ranges areLink State Update Interval is 0:30:00 and due in 0:04:55 Link State Age Interval is 0:20:00 and due in 0:04:55

Router # show ip ospf Routing Process "ospf 1" with ID 2.2.2.2 Supports only single TOS (TOS0) routes SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Number of areas in this router is 1 Area 23

Number of interfaces in this area is 3 Area has no authenticationSPF algorithm executed 19 times Area ranges areLink State Update Interval is 0:30:00 and due in 0:04:55 Link State Age Interval is 0:20:00 and due in 0:04:55

• Displays general information about the OSPF routing process

49

show ip ospf database Command

Router# show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 1)

Router Link States (Area 23)Link ID ADV Age Seq# Checksum Link

Router count3.3.3.3 3.3.3.3 78 0x80000032 0x8086 54.4.4.4 4.4.4.4 1691 0x80000028 0xEllC 12.2.2.2 2.2.2.2 1693 0x80000030 0x835E 51.1.1.1 1.1.1.1 1696 0x80000026 0x80A1 1

Net Link States (Area 23)

Link ID ADV Router Age Seg# Checksum150.100.4.2 4.4.4.4 1691 0x80000030 0x2FCE150.100.1.2 2.2.2.2 1693 0x80000024 0xFB29

Router# show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 1)

Router Link States (Area 23)Link ID ADV Age Seq# Checksum Link

Router count3.3.3.3 3.3.3.3 78 0x80000032 0x8086 54.4.4.4 4.4.4.4 1691 0x80000028 0xEllC 12.2.2.2 2.2.2.2 1693 0x80000030 0x835E 51.1.1.1 1.1.1.1 1696 0x80000026 0x80A1 1

Net Link States (Area 23)

Link ID ADV Router Age Seg# Checksum150.100.4.2 4.4.4.4 1691 0x80000030 0x2FCE150.100.1.2 2.2.2.2 1693 0x80000024 0xFB29

50

show ip protocol CommandRouter> show ip protocol Routing Protocol is "ospf 300" Sendinq updates every 0 seconds Invalid after 0 seconds, hold down 0, flushed after 0 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Redistributing: ospf 300Routing for Networks: 183.8.0.0/0.0.255.255 144.253.100.0/0.0.0.255Routing Information Sources: Gateway Distance Last Update 14d.253.100.0 110 6d21 183.8.128.12 110 0:17:32 192.3.63.192 110 0:17:33 153.50.193.1 110 0:17:33 183.8.6d.130 110 6d19 183.8.64.128 110 0:17:33 133.3.4.0 110 0:17:33 131.108.100.3 110 0:17:33 Distance: (default is 1l0) - - More - -

Router> show ip protocol Routing Protocol is "ospf 300" Sendinq updates every 0 seconds Invalid after 0 seconds, hold down 0, flushed after 0 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Redistributing: ospf 300Routing for Networks: 183.8.0.0/0.0.255.255 144.253.100.0/0.0.0.255Routing Information Sources: Gateway Distance Last Update 14d.253.100.0 110 6d21 183.8.128.12 110 0:17:32 192.3.63.192 110 0:17:33 153.50.193.1 110 0:17:33 183.8.6d.130 110 6d19 183.8.64.128 110 0:17:33 133.3.4.0 110 0:17:33 131.108.100.3 110 0:17:33 Distance: (default is 1l0) - - More - -

51

Other OSPF show CommandsRouter#

show ip ospf virtual-linksshow ip ospf virtual-links

• Displays parameters about OSPF virtual links

Router#

show ip ospf neighbor detailshow ip ospf neighbor detail

• Displays neighbor information per interface

Router#

show ip ospf border-routersshow ip ospf border-routers

• Displays routes to the ABR and ASBR

52

Summary

OSPF is a scalable, standards-based link-state routing protocol

OSPF features:

Hierarchical design

VLSM support

Vendor interoperability

Route summarization

Route redistribution

© synergon informatika rt., 1999 chapter 9 configuring open shortest path first

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