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1© 2003, Cisco Systems, Inc. All rights reserved.

RST-10018118_05_2003_c2

OSPF(Open Shortest Path First

Protocol)

Kevin Chi ��

[email protected]

Cisco Systems.


RST-10018118_05_2003_c2

Agenda

• OSPF Overview

• OSPF Terminology

• OSPF Operation

• Multiple OSPF Area



RST-10018118_05_2003_c2

A day in a life of a router

- Find path

- Forward packet, forward packet, forward packet, forward packet...

- Find alternate path

- Forward packet, forward packet, forward packet, forward packet…

- Repeat until powered off


RST-10018118_05_2003_c2

Routing versus Forwarding

• Routing = building maps and giving directions

• Forwarding = moving packets between interfaces according to the “ directions”



RST-10018118_05_2003_c2

IP Routing - finding the path

• Path derived from information received from a routing protocol

• Several alternative paths may exist

best next hop 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.)


RST-10018118_05_2003_c2

• Static routes

• Default routes

• Dynamic routing

IP Routing Learns Destinations



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Static Routes

• Routes configured manually

• Useful when few or just one route exist

• Can be administrative burden

• Frequently used for default route


RST-10018118_05_2003_c2

Default Routes

• Route used if no match is found inforwarding table

• Can be carried by routing protocols

• Two modelsSpecial network number:

0.0.0.0 (IP)

Flagged in routing protocolRIP, RIPv2 : network 0.0.0.0

IGRP, EIGRP : ip default-network

OSPF,ISIS : default originate



RST-10018118_05_2003_c2

Dynamic Routing - Routing Protocol Goals

• Optimal path selection

• Loop-free routing

• Fast convergence

• Limited design administration

• Minimize update traffic

• Handle address limitations

• Support hierarchical topology

• Incorporate rapid convergence

• Easy to configure

• Adapts to changes easily and quickly

• Does not create a lot of traffic

• Scales to a large size

• Compatible with existing hosts and routers

• Supports variable length subnet masks and discontiguous subnets

• Supports policy routing


RST-10018118_05_2003_c2

NameName

RIPRIP

RIPv2RIPv2

IGRPIGRP

EIGRPEIGRP

OSPFOSPF

IS-ISIS-IS

BGPBGP

TypeType

DVDV

DVDV

DVDV

Adv DVAdv DV

LSLS

LSLS

Path VecPath Vec

ProprietaryProprietary

NoNo

NoNo

YesYes

YesYes

NoNo

NoNo

NoNo

FunctionFunction

InteriorInterior

InteriorInterior

InteriorInterior

InteriorInterior

InteriorInterior

InteriorInterior

ExteriorExterior

UpdatesUpdates

30 Sec30 Sec

30 Sec30 Sec

90 Sec90 Sec

TrigTrig

TrigTrig

TrigTrig

IncrIncr

MetricMetric

HopsHops

HopsHops

CompComp

CompComp

CostCost

CostCost

N/AN/A

VLSMVLSM

NoNo

YesYes

NoNo

YesYes

YesYes

YesYes

SummSumm

AutoAuto

AutoAuto

AutoAuto

BothBoth

ManMan

AutoAuto

AutoAuto

• IP routing protocols are characterized as

YesYes

Internet Routing Protocols



RST-10018118_05_2003_c2

IP RIP

• Routing Information Protocol

• Widely available

• Hop count metric

• Periodic update

• Easy to implement

• One of the first available

• RFC 1058

• Simple = limited

• Slow convergence

• No VLSM

• No discontiguous subnets

• Max 15 Hops


RST-10018118_05_2003_c2

• Scalability concerns:ConvergenceUpdate trafficMetric limitations

Distance Vector

B DCA

Update Interval

RoutingTable

RoutingTable

Update Interval

RoutingTable

Update Interval

RoutingTable

B RoutingUpdate

DCAA CB D



RST-10018118_05_2003_c2

R1

R2

R3

T1

56k

T1

1 Hop Hops

RIP Metric

0 Hops

Path I

Path IIHost A

Host B

Host A sends traffic to Host B


RST-10018118_05_2003_c2

.6

.13

.9

.10.14

.5

A

C

B172.16172.16.40.1

255.255.255.0

172172.16.50.1255.255.255.0

172.16.60.1255.255.255.0

192.168.1.8255.255.255.252

192.168.1.4255.255.255.252

192.168.1.12255.255.255.252

Where Is 172.16.0.0?

Discontiguous IP Subnet

Routing Protocols will by DefaultSummarize Major Networks



RST-10018118_05_2003_c2

.6

.13

.9

.10.14

.5

A

C

B172.16.40.1

255.255.255.0

172.16.50.1255.255.255.0

172.16.60.1255.255.255.0

172.16.1.8255.255.255.252

172.16.1.4255.255.255.252

172.16.1.12255.255.255.252

VLSM : Variable Length Subnet Mask

Conserve IP Addresses

172.16.1.X With a 255.255.255.252 maskOr /30 the 1 subnet

my be broken into 64 Subnets


RST-10018118_05_2003_c2

Administrative Distance• The router treats different routing protocols with a different preference

Route SourceRoute Source Default DistanceDefault Distance

Connected InterfaceConnected Interface

Static RouteStatic Route

Enhanced IGRP Summary RouteEnhanced IGRP Summary Route

External BGPExternal BGP

Internal Enhanced IGRPInternal Enhanced IGRP

IGRPIGRP

OSPFOSPF

IS-ISIS-IS

RIPRIP

EGPEGP

External Enhanced IGRPExternal Enhanced IGRP

Internal BGPInternal BGP

Unknown, Discard RouteUnknown, Discard Route

00

11

55

2020

9090

100100

110110

115115

120120

140140

170170

200200

255255



RST-10018118_05_2003_c2

OSPF


RST-10018118_05_2003_c2

OSPF

• Open ShortestPath First

• Link state or SPF technology

• Developed by OSPF working group of IETF (RFC 1247)

• Designed for TCP/IP Internet environment

• Fast convergence

• Variable-length subnet masks

• Discontiguoussubnets

• No periodic updates

• Route authentication

• Delivered two years after IGRP

• OSPF standard described in RFC2328



RST-10018118_05_2003_c2

Link State

Topology Information Is Kept in a Database Separate from the

Routing Table

AABBCC

2213131313

QQZZXX

X’s Link State

ZZ

XX

YYQQ

Z’s Link State

Q’s Link State


RST-10018118_05_2003_c2

Link-State

TopologicalDatabase

2 SPF

Algorithm

3 Shortest Path First Tree

4

Routing Table

5

Link-State Advertisements C

1 DA

B

C

D



RST-10018118_05_2003_c2

Why Is It Called a Link State Protocol?

• Traditional Distance Vector Routing Protocols (DVRP) relay information regarding their relative distance to a destination

• Link State Protocols relay specific link characteristics and state information

• Only changes or updates are sent across the network

• Each router uses that information to build a routing table on it’s own


RST-10018118_05_2003_c2

Link-State vs. Distance Vector Protocols

• Link-State router tells ALL other routers about ONLY its neighbors and links

• Distance Vector router tells ONLY neighbors about ALL routes



RST-10018118_05_2003_c2

What Is a Link State Protocol ?

• The network can be viewed as a “ jigsaw puzzle”

• Each piece of the puzzle holds one router

• Each router creates a Link State Packet (LSP) which represents its own jigsaw piece

• LSPs are flooded reliably within the network

• The LSPs are collected by each router to form a Link State Database (LSDB) or complete “ picture” of the network

• Use Shortest Path First (SPF) algorithm to put the pieces together


RST-10018118_05_2003_c2

LSP ALSP B

LSP C

LSP D

to B

to EtoDto C

to A

to Dto C

to B

LSP E

to A to B

to A

to E

Link State Protocols

to C to D



RST-10018118_05_2003_c2

All Routers Have the Same View

• All routers exchange all LSPs

Via a reliable flooding mechanism

• All routers store all LSPs in a link-state database (LSDB)

Separate from the routing table (RIB)

All routers should have exactly the same LSDB, but different RIBs


RST-10018118_05_2003_c2

Router A - LSDBRouter B - LSDB

Router D - LSDBRouter C - LSDB

Router E - LSDBLSPE

LSPE

LSPELSPE

LSPE

LSPA

LSPA

LSPALSPA

LSPALSPC

LSPC

LSPCLSPC

LSPC

LSPB

LSPB

LSPBLSPB

LSPBLSPD

LSPD

LSPDLSPD

LSPD

All Routers Have the Same LSDB



RST-10018118_05_2003_c2

Link State Routing

• Neighbour discovery

• Constructing an LSP

• Distribute LSP

• Compute routes

• On network failureNew LSPs flooded

All routers recompute routing tables


RST-10018118_05_2003_c2

What To Do With LSPs?

• Each router calculates a topology map by executing Dijkstra’s Shortest Path First algorithm (SPF)

the topology is calculated as a Shortest Path Tree (SPT), with itself as root

each router computes a different Shortest Path Tree (SPT)

• From the SPT the RIBs are calculated

RIB : Routing Information Base



RST-10018118_05_2003_c2

• Dijkstra is a path finding algorithm

• Will find the shortest path from A to B given intermediate path and cost information

• One of many path finding algorithms:

Dijkstra, best path, A*, etc

Dijkstra

Shortest Path First (SPF) Algorithm


RST-10018118_05_2003_c2

Dijkstra’s algorithm

• �� ! �� ! �� ! �� ! � � � � � �� " # � � �� " # � � �� " # � � �� " # � � �� $ % �& ' ! (� $ % �& ' ! (� $ % �& ' ! (� $ % �& ' ! (

• ��) �* + $ % & ' ! �, - . / ! * 0�1 � ") �* + $ % & ' ! �, - . / ! * 0�1 � ") �* + $ % & ' ! �, - . / ! * 0�1 � ") �* + $ % & ' ! �, - . / ! * 0�1 � "2 3 4 5 62 3 4 5 62 3 4 5 62 3 4 5 6 � � � � �� 07 8 9 * + ! �: ; (07 8 9 * + ! �: ; (07 8 9 * + ! �: ; (07 8 9 * + ! �: ; (

• �< �6 $ % = > ? ��1 @ 5 ��A �(�< �6 $ % = > ? ��1 @ 5 ��A �(�< �6 $ % = > ? ��1 @ 5 ��A �(�< �6 $ % = > ? ��1 @ 5 ��A �(B � ��C D ! EB � ��C D ! EB � ��C D ! EB � ��C D ! E F �F �F �F � � G � ��H I 6� G � ��H I 6� G � ��H I 6� G � ��H I 6 J �J �J �J � 0000F �F �F �F � ��( , � K L M ��1 N O C D E��( , � K L M ��1 N O C D E��( , � K L M ��1 N O C D E��( , � K L M ��1 N O C D E F �F �F �F � ��! � P Q R @ (! � P Q R @ (! � P Q R @ (! � P Q R @ (

• �S ��S ��S ��S �� F �F �F �F � � � $ % �& ' ! � T U V W ) X < X� � $ % �& ' ! � T U V W ) X < X� � $ % �& ' ! � T U V W ) X < X� � $ % �& ' ! � T U V W ) X < XS ( Y Z [ 0= \ @ 5 ��! ] P Q # �S ( Y Z [ 0= \ @ 5 ��! ] P Q # �S ( Y Z [ 0= \ @ 5 ��! ] P Q # �S ( Y Z [ 0= \ @ 5 ��! ] P Q # �! R * + �1 (! R * + �1 (! R * + �1 (! R * + �1 (



RST-10018118_05_2003_c2

Dijkstra

• Link state databaseCreated with link state packets (LSPs) from each router

• TENT databaseTentative triples (ID, path cost, direction)

Shortest Path First (SPF) Algorithm


RST-10018118_05_2003_c2

Dijkstra (SPF) Overview

• PATH database

Best path triples (ID, path cost, direction)

• Forwarding database

Aka the routing table



RST-10018118_05_2003_c2

Dijkstra (SPF) Overview (Cont.)

• All routers exchange Link State Packets (LSPs)

• Each starts with itself as root

• Tent is built from LSPs

• Path is created by examining and comparing tent triples

• Once path is final the forwarding table is populated


RST-10018118_05_2003_c2

Dijkstra Basics

• Router IDs are alphabetic

• Costs are numeric

• Lowest cost best

B B A A C C

D D

E E F F G G

4

2

2 2

2

1 4

1



RST-10018118_05_2003_c2

LSP Data

B B A A C C

D D

E E F F G G

4

2

2 2

2

1 4

1

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2


RST-10018118_05_2003_c2

Dijkstra Example—1/7

• As an example start with B

• A and C costs are tent

B B

A A C C

(0)

(1) (4)

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 B/1B/1

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2



RST-10018118_05_2003_c2


• Now fill in C• D,E are in tent• BC is now in path

B B

A A C C

(0)

(1) (4)

D D

E E (5)

(3)

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2


RST-10018118_05_2003_c2

• Now fill in C• D,E are in tent• BC is now in path

A A (4)


B B

C C

(0)

(1)

D D

E E (5),(4)

(3) F F (5)

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2



RST-10018118_05_2003_c2

A A (4)

• Now fill in A• G is in tent• BA is now in path


B B

C C

(0)

(1)

D D

E E (5),(4)

(3) G G

(6) F F (5)

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2


RST-10018118_05_2003_c2

A A (4)


• CD is removed• ED is placed in path

B B

C C

(0)

(1)

D D

E E (4)

(3) G G

(6) F F (5)

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2



RST-10018118_05_2003_c2

A A (4)


• Now fill in F• G is Tent• GF does not

provide better path • EF is in path

B B

C C

(0)

(1)

D D

E E (4)

(3) G G

(6) F F (5),(8)

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2


RST-10018118_05_2003_c2

A A (4)


• Now fill in G• FG is removed• AG is in path

B B

C C

(0)

(1)

D D

E E (4)

(3) G G

(6) F F (5)

BB CC DD EE FF

B/4B/4

GG

A/4A/4

AA

B/1B/1 C/4C/4 C/2C/2 E/2E/2 A/2A/2

G/2G/2 C/1C/1 D/4D/4

E/2E/2

E/1E/1 D/1D/1

F/2F/2

G/2G/2 F/2F/2



RST-10018118_05_2003_c2

Autonomous System

OSPF Terminology

RoutingTable

Lists Best Routes

Topology Database

Lists All Routes

Neighborship Database

Lists Neighbors

Cost = 10

Cost = 1785 Cost = 6

Neighbors

TokenRing

Interfaces

Area 1Area 0


RST-10018118_05_2003_c2

OSPF Topologies

Point-to-Point

NBMA

Broadcast Multiaccess

X.25Frame Relay



RST-10018118_05_2003_c2

FDDI Dual Ring

FDDI Dual Ring

Optimal Path Utilisation

N1

N2 N3

N4

N5R1

R2

R3

R4

Cost = 1 Cost = 1

Cost = 10

Cost = 10

The optimal path is determined by thesum of the interface costs: Cost = 10^8/BW


RST-10018118_05_2003_c2

Fast Convergence

• Detection Plus LSA/SPF

XR1 R3

R2

N2

Primary Path

N1

Alternate Path



RST-10018118_05_2003_c2

Fast Convergence

• Finding a new route

LSA flooded throughout area

Acknowledgement based

Topology database synchronised

Each router derives routing table to destination networks

LSA

XR1

N1


RST-10018118_05_2003_c2

FDDIDual Ring

Low Bandwidth Utilisation

• Only changes propagated

• Multicast on multi-access broadcast networks

R1

LSA

XLSA

LSA : Link State Advertisement



RST-10018118_05_2003_c2

Utilises IP Multicast for Sending/Receiving Updates

• Broadcast networksDR and BDR —> AllSPFRouters (224.0.0.5)

All other routers —> AllDRRouters (224.0.0.6)

• Hello packets sent to AllSPFRouters(Unicast on point-to-point and virtual links)


RST-10018118_05_2003_c2

OSPF Areas

• Group of contiguous hosts and networks

• Per area topological database

Invisible outside the area

Reduction in routing traffic

• Backbone area contiguous

All other areas must be connected to the backbone

• Virtual Links Area 1Area 4

Area 0Backbone Area

Area 2 Area 3



RST-10018118_05_2003_c2

Classification of Routers

• Internal Router (IR)

• Area Border Router (ABR)

• Backbone Router (BR)

• Autonomous System Border Router (ASBR)

Area 1

IRArea 0

Area 2 Area 3

IR

ABR

To other AS

ASBR

/BR

/BR


RST-10018118_05_2003_c2

OSPF Route Types

Intra-area Route “ O”all routes inside an area

Inter-area Route “ O IA”routes advertised from one area to another by an Area Border Router

External Route “ O E1” or “ O E2”routes imported into OSPF from other protocol or static routes

Area 0Area 2 Area 3

ABR

To other AS

ASBR



RST-10018118_05_2003_c2

Inter-Area Route Summarisation

• Prefix or all subnets

• Prefix or all networks

• ‘Area range’ command

1.A 1.B 1.C

FDDIDual Ring

R1 (ABR)

R2

Network1

Next HopR1

Network1.A1.B1.C

Next HopR1R1R1

With summarisation

Withoutsummarisation

BackboneArea 0

Area 1


RST-10018118_05_2003_c2

External Routes

• Redistributed into OSPF

• Flooded unaltered throughout the AS

• OSPF supports two types of external metricsType 1 external metrics

Type 2 external metrics (Default)

RIPIGRPEIGRPBGPetc.

OSPF

Redistribute



RST-10018118_05_2003_c2

External Routes

• Type 1 external metric: metrics are added to the summarised internal link cost

NetworkN1N1

Type 11110

Next HopR2R3

Cost = 10

to N1External Cost = 1

to N1External Cost = 2R2

R3

R1

Cost = 8

Selected Route


RST-10018118_05_2003_c2

External Routes

• Type 2 external metric: metrics are compared without adding to the internal link cost

NetworkN1N1

Type 212

Next HopR2R3

Cost = 10

to N1External Cost = 1

to N1External Cost = 2R2

R3

R1

Cost = 8

Selected Route



RST-10018118_05_2003_c2

Topology/Link State Database

• A router has a separate LS database for each area to which it belongs

• All routers belonging to the same area have identical database

• SPF calculation is performed separately for each area

• LSA flooding is bounded by area


RST-10018118_05_2003_c2

Protocol Functionality

• Bringing up adjacencies

• LSA types

• Area classification



RST-10018118_05_2003_c2

The Hello Protocol

• Responsible for establishing and maintaining neighbour relationships

• Elects designated router on multi-access networks

FDDIDual Ring

Hello

HelloHello


RST-10018118_05_2003_c2

Neighborship

Hello

afadjfjorqpoeru39547439070713

Router IDHello/dead intervalsNeighborsArea-IDRouter priorityDR IP addressBDR IP addressAuthentication passwordStub area flag

* *

**

* Entry must match on neighboring routers

Hello

AA

DD EE

CCBB



RST-10018118_05_2003_c2

Neighborship (cont.)

Hello

afadjfjorqpoeru39547439070713

Router IDHello/dead intervalsNeighborsArea-IDRouter priorityDR IP addressBDR IP addressAuthentication passwordStub area flag

**

**

* Entry must match on neighboring routers

Hello

AA

DD EE

CCBB


RST-10018118_05_2003_c2

DR and BDR

DRDR BDRBDR

Hellos elect DR and BDR to represent segment

Each router then forms adjacency with DR and BDR



RST-10018118_05_2003_c2

Designated Router

• One per multi-access networkGenerates network links advertisements

Assists in database synchronization

Designated Router

Designated Router

BackupDesignated Router

BackupDesignated

Router


RST-10018118_05_2003_c2

Designated Router by Priority

• Configured priority (per interface)

• Else determined by highest router ID

Router ID is the loopback interface address, if configured, otherwise the highest IP address

144.254.3.5

R2 Router ID = 131.108.3.3

131.108.3.2 131.108.3.3

R1 Router ID = 144.254.3.5

DR



RST-10018118_05_2003_c2

When to Become Adjacent

• Underlying network is point to point

• Underlying network type is virtual link

• The router itself is the designated router

• The router itself is the backup designated router

• The neighbouring router is the designated router

• The neighbouring router is the backup designated router


RST-10018118_05_2003_c2

Building the Adjacency

1- Down: No information has been received from anybody on the segment.2- Init: The interface has detected a Hello packet coming from a neighbor but bi-directional communication has not yet been established.3- Two-way: There is bi-directional communication with a neighbor. The router has seen itself in the Hello packets coming from a neighbor. At the end of this stage the DR and BDR election would have been done. At the end of the 2way stage, routers will decide whether to proceed inbuilding an adjacency or not. The decision is based on whether one of the routers is a DR or BDR or the link is a point-to-point or a virtual link.4- Exstart: Routers are trying to establish the initial sequence number that is going to be used in the information exchange packets. The sequence number insures that routers always get the most recent information. One router will become the primary and the other will become secondary. The primary router will poll the secondary for information.5- Exchange: Routers will describe their entire link-state database by sending database description packets. At this state, packets could be flooded to other interfaces on the router.6- Loading: At this state, routers are finalizing the information exchange. Routers have built a link-state request list and a link-state retransmission list. Any information that looks incompleteor outdated will be put on the request list. Any update that is sent will be put on the retransmission list until it gets acknowledged.7- Full: At this state, the adjacency is complete. The neighboring routers are fully adjacent. Adjacent routers will have a similar link-state database.



RST-10018118_05_2003_c2

Neighbor and Adjacency

RTD# show ip ospf interface e 0Ethernet0 is up, line protocol is upInternet Address 203.250.14.4 255.255.255.0, Area 0.0.0.0Process ID 10, Router ID 192.208.10.174, Network Type BROADCAST, Cost: 10Transmit Delay is 1 sec, State DROTHER, Priority 1Designated Router (ID) 203.250.15.1, Interface address 203.250.14.2Backup Designated router (ID) 203.250.13.41, Interface address 203.250.14.1Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 0:00:03Neighbor Count is 3, Adjacent neighbor count is 2Adjacent with neighbor 203.250.15.1 (Designated Router)Adjacent with neighbor 203.250.13.41 (Backup Designated Router)

RTD# show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface203.250.12.1 1 2WAY/DROTHER 0:00:37 203.250.14.3 Ethernet0203.250.15.1 1 FULL/DR 0:00:36 203.250.14.2 Ethernet0203.250.13.41 1 FULL/BDR 0:00:34 203.250.14.1 Ethernet0


RST-10018118_05_2003_c2

Routing Protocol Packets

• Share a common protocol header

• Routing protocol packets are sent with type of service (TOS) of 0

• Five types of OSPF routing protocol packets

Hello - packet type 1

Database description - packet type 2

Link-state request - packet type 3

Link-state update - packet type 4

Link-state acknowledgement - packet type 5



RST-10018118_05_2003_c2

Different Types of LSAs

• Five distinct type of LSAs

Type 1 : Router LSA

Type 2 : Network LSA

Type 3 and 4: Summary LSA

Type 5 and 7: External LSA


RST-10018118_05_2003_c2

Router LSA (Type 1)

• Describes the state and cost of the router’s links to the area

• All of the router’s links in an area must be described in a single LSA

• Flooded throughout the particular area and no more

• Router indicates whether it is an ASBR, ABR, or end point of virtual link



RST-10018118_05_2003_c2

Network LSA (Type 2)

• Generated for every transit broadcast and NBMA network

• Describes all the routers attached to the network

• Only the designated router originates this LSA

• Flooded throughout the area and no more

NBMA : Non-Broadcast Multi-Access


RST-10018118_05_2003_c2

Summary LSA (Type 3 and 4)

• Describes the destination outside the area but still in the AS

• Flooded throughout a single area

• Originated by an ABR

• Only intra-area routes are advertised into the backbone

• Type 4 is the information about the ASBR



RST-10018118_05_2003_c2

External LSA (Type 5)

• Defines routes to destination external to the AS

• Default route is also sent as external

• Two types of external LSA:E1: Consider the total cost up to the external destination

E2: Considers only the cost of the outgoing interface to the external destination


RST-10018118_05_2003_c2

Not Summarised: Specific Links

BackboneArea #0

External links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.D

3.A

3.C

3.B

1.A1.B1.C1.D

3.A3.B3.C3.D

2.A2.B2.C

2.A

2.C

2.B

TokenRing

TokenRing

• Specific link LSA advertised out• Link state changes propagate out

ASBR



RST-10018118_05_2003_c2

BackboneArea #0

External links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.D

3.A

3.C

3.B

2.A

2.C

2.B

TokenRing

TokenRing

ASBR

Not Summarised: Specific Links

2.A2.B2.C3.A3.B3.C3.D

1.A1.B1.C1.D3.A3.B3.C3.D

1.A1.B1.C1.D2.A2.B2.C

• Specific link LSA advertised in• Link state changes propagate in


RST-10018118_05_2003_c2

Summarised: Summary Links

BackboneArea #0

ASBR

External links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.D

3.A

3.C

3.B

2.A

2.B

TokenRing

TokenRing

• Only summary LSA advertised out• Link state changes do not propagate

1 3

2



RST-10018118_05_2003_c2

Summarised: Summary Links

BackboneArea #0

3.D

3.A

2.B

• Only summary LSA advertised in• Link state changes do not propagate

ASBR

External links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.C

3.B

2.A

TokenRing

TokenRing

2,3

1,3

1,2


RST-10018118_05_2003_c2

Addressing

Area 1network 131.108.0.0subnets 17-31range 255.255.240.0



Area 0network 192.117.49.0range 255.255.255.0

Assign contiguous ranges of subnets per area to facilitate summarisation



RST-10018118_05_2003_c2

TokenRing

TokenRing

TokenRing

TokenRing

Regular Area

From area 1’s viewpoint • Summary networks from other areas injected• External networks injected, for example network X.1

ASBR

External Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

2,3

1,3

1,2X.1

X.1

X.1

X.1

2.D2.C

2.B

3.A

3.D


RST-10018118_05_2003_c2

TokenRing

TokenRing

TokenRing

TokenRing

Normal Stub Area

From area 1’s viewpoint

• Summary networks from other areas injected

• Default network injected into the area - represents external links

• Default path to closest area border router

• Define all routers in the area as stub

area x stub command

ASBR

External Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

& Default

1,3

1,2X.1

X.1

X.1

X.1

2.D2.C

2.B

3.A

3.D

2,3



RST-10018118_05_2003_c2

TokenRing

TokenRing

TokenRing

TokenRing

Totally Stubby Area

From area 1’s viewpoint

• Only a default network is injected into the areaRepresents external networks and all inter-area routes

• Default path to closest area border router

• Define all routers in the area as totally stubby area x stub no-summary command

ASBR

External Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

2, 3 & Default

1,3

1,2X.1

X.1

X.1

X.1

2.D2.C

2.B

3.A

3.D

Default


RST-10018118_05_2003_c2

TokenRing

TokenRing

TokenRing

TokenRing

Not-So-Stubby Area

• Capable of importing external routes in a limited fashion

• Type-7 LSA’s carry external information within an NSSA

• NSSA Border routers translate selected type-7 LSAs into type-5 external network LSAs

ASBR

External Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

Default

1,3

1,2X.1

X.1

X.1X.1

2.D2.C

2.B

3.A

3.DExternal Networks

X.2

X1, X2

X1, X2



RST-10018118_05_2003_c2

Area 0.0.0.0

Area 0.0.0.1

Area 0.0.0.4

Virtual Link

Area 0.0.0.3 Area 0.0.0.5

VirtualLink

Virtual Links

Area 0.0.0.6


RST-10018118_05_2003_c2

Area 0

Area 1

Area 2

Area3

Virtual Links

XXXXVirtual Link

Virtual LinkRouter A (RID 171.0.1.7)

router ospf 100area 2 virtual-link 171.0.1.5

Router B (RID 171.0.1.5)router ospf 100

area 2 virtual-link 171.0.1.7

Router A

Router B



RST-10018118_05_2003_c2

Summary

• Scalable OSPF Network Design

Area hierarchy

Stub areas

Contiguous addressing

Route summarisation


RST-10018118_05_2003_c2

Reference Commands



RST-10018118_05_2003_c2

OSPF - Adding Networks(Method One)

• redistribute connected subnets

Works for all connected interfaces on the router but sends networks as external type-2s -which are not summarizedrouter ospf 100

redistribute connected subnets

• Not recommended


RST-10018118_05_2003_c2

OSPF - Adding Networks

• Specific network statements

Every interface needs a OSPF network statement. Interface that should not be broadcasting OSPF Hello packets needspassive-interface.router ospf 100

network 192.168.1.4 0.0.0.3 area 51

network 192.168.1.6 0.0.0.3 area 51

passive interface Serial 1/0



RST-10018118_05_2003_c2


• Network statements - wildcard mask

Every interface covered by wildcard mask used in OSPF network statement. Interfaces that should not be broadcasting OSPF Hello packets need passive-interface or default passive-interface. router ospf 100

network 192.168.1.0 0.0.0.255 area 51

default passive-interface default

no passive interface POS 4/0


RST-10018118_05_2003_c2


• Key Theme when selecting a technique: Keep the Link State Database Lean

Increases Stability

Reduces the amount of information in the Link State Advertisements (LSAs)

Speeds Convergence Time



RST-10018118_05_2003_c2

OSPF Logging Neighbour Changes

• The router will generate a log messagewhenever an OSPF neighbour changes state

• Syntax:[no] ospf log[no] ospf log--adjacencyadjacency--changeschanges

• Example of a typical log message:%OSPF%OSPF--55--ADJCHG: Process 1, ADJCHG: Process 1, NbrNbr 223.127.255.223 on 223.127.255.223 on Ethernet0 from LOADING to FULL, Loading DoneEthernet0 from LOADING to FULL, Loading Done


RST-10018118_05_2003_c2

Number of State Changes

• The number of state transitions is available via SNMP (ospfNbrEvents) and the CLI:

show ip ospf neighbor [type number] show ip ospf neighbor [type number] [neighbor[neighbor--id] [detail]id] [detail]

Detail—(Optional) Displays all neighbours given in detail (list all neighbours). When specified, neighbour state transition counters are displayed per interface or neighbour ID



RST-10018118_05_2003_c2

State Changes (Continued)

• To reset OSPF-related statistics, use the clear ip ospf countersclear ip ospf counters EXEC command. At this point neighborneighbor is the only available option; it will reset neighbour state transition counters per interface or neighbour id

clear ip ospf counters [neighbor [<type clear ip ospf counters [neighbor [<type number>] [neighbornumber>] [neighbor--id]]id]]


RST-10018118_05_2003_c2

OSPF Router ID

• If the loopback interface exists and has an IP address, that is used as the router ID in routing protocols - stability!

• If the loopback interface does not exist, or has no IP address, the router ID is the highest IP address configured - danger!

• New sub command to manually set the OSPF Router ID:

router-id <ip address>



RST-10018118_05_2003_c2

OSPF Clear/Restart

• clear ip ospf [pid] redistributionThis command can now clear redistribution based on OSPF routing process ID. If no pid is given, it assumes all OSPF processes.

• clear ip ospf [pid] countersThis command can now clear counters based on OSPF routing process ID. If no pid is given, it assumes all OSPF processes.

• clear ip ospf [pid] processThis command will restart the specified OSPF process. If no pid is given, it assumes all OSPF processes. It attempts to keep the old router-id, except in cases, where a new router-id was configured, or an old user configured router-id was removed. Since this command can potentially cause a network churn, a user confirmation is required before performing any action.


RST-10018118_05_2003_c2

Redistributing Routes into OSPF

ROUTER OSPF <pid#x>

REDISTRIBUTE {protocol} <as#y>

<metric>

<metric-type (1 or 2)

<tag>

<subnets>



RST-10018118_05_2003_c2

Router Sub-commands

• NETWORK <n.n.n.n> <mask> AREA <area-id>

• AREA <area-id> STUB {no-summary}

• AREA <area-id> AUTHENTICATION

• AREA <area-id> VIRTUAL-LINK <router-id>...

• AREA <area-id> RANGE <address mask>


RST-10018118_05_2003_c2

Interface Subcommands

• IP OSPF COST <cost>

• IP OSPF PRIORITY <8-bit-number>

• IP OSPF HELLO-INTERVAL <number-of-seconds>

• IP OSPF DEAD-INTERVAL <number-of-seconds>

• IP OSPF AUTHENTICATION-KEY <8-bytes-of-password>



RST-10018118_05_2003_c2

Router#

show ip ospf interface

Verifying OSPF Operation

• Displays area ID and adjacency information

Router#

show ip protocols

• Verifies that OSPF is configuredRouter#

show ip route

• Displays all the routes learned by the router


RST-10018118_05_2003_c2

• Displays OSPF timers and statistics

• Displays information about DR, BDR, and neighbors

• Displays the link-state database

Verifying OSPF Operation (cont.)

Router#

show ip ospf neighbor detail

Router#

show ip ospf database

Router#

show ip ospf



RST-10018118_05_2003_c2

• Allows you to clear the IP routing table

Router#

clear ip route *

Router#

debug ip ospf option

• Displays router interaction during the hello, exchange, and flooding processes

Verifying OSPF Operation (cont.)


RST-10018118_05_2003_c2

Configuring OSPF ABRs

E010.64.0.1

10.64.0.2E0

S010.2.1.2

10. 2.1.1S1AA BB CC

ABR

<Output Omitted>

interface Ethernet0

ip address 10.64.0.1 255.255.255.0

!

<Output Omitted>

router ospf 77

network 10.0.0.0 0.255.255.255 area 0

Area 1Area 0

<Output Omitted>

interface Ethernet0

ip address 10.64.0.2 255.255.255.0

!

interface Serial0

ip address 10.2.1.2 255.255.255.0

<Output Omitted>

router ospf 50

network 10.2.1.2 0.0.0.0 area 1

network 10.64.0.2 0.0.0.0 area 0



RST-10018118_05_2003_c2

OSPF Stub Area Configuration Example

192.168.15.2

Area 0

Stub Area 2

192.168.14.1 192.168.15.1S0

S0E0

R3#

interface Ethernet 0ip address 192.168.14.1 255.255.255.0interface Serial 0 ip 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 0 ip address 192.168.15.2 255.255.255.252

router ospf 15network 192.168.15.0 0.0.0.255 area 2area 2 stub

ExternalAS

R4R4

R3R3


RST-10018118_05_2003_c2 104

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