Designing and Troubleshooting OSPFNetworksledvina/vyuka/PDS/PDS_2005/14_OSPF.pdf · Designing and Troubleshooting OSPF Networks November 1997, Paris Networkers 97 1 Cisco Systems

Designing and Troubleshooting OSPF Networks November 1997, Paris

Networkers 97 1

Cisco Systems Confidential

Designing and TroubleshootingOSPF Networks

1NW97_EMEA_112

2NW97_EMEA_112

Agenda

•• IntroductionIntroduction

• OSPF Technical Overview

• Designing OSPF Networks

• OSPF Troubleshooting Basics

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What Is Routing?

Step 2—Switch packets basedon routing table information User

Traffic

??

Step 1—Build Routing Table

AABBCC

2213131313

QQZZXX

RoutingInformation

AABBCC

2213131313

QQZZXX

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Routing Protocols

• Should:Select “optimal” pathsPrevent loopsConverge quicklyHave as little impact on user data as possible




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Which Protocol(s) to Choose?• Selection criteria and considerations:

Routed Protocol: IP, IPX, AT, …Interoperability Structure/Topology of the networkConvergence time and robustnessBandwidth, Memory and CPU utilizationFeatures: VLSM, Load Balancing,Summarization, Filtering Capabilities ...Ease of configuration, migration, maintenance,troubleshooting and debugging

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Link State TechnologyCompare it to a jigsaw puzzle:

• Each router represents a piece of a puzzle• The piece contains information about all

links of this router: To which networks they connect, the mask of these networks and eventual neighbors on that link, ...




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Link State Technology (cont.)AABBCC

2213131313

QQZZXX

Flooding

• Each piece of the puzzle is flooded through the entire routing area

• Every router stores a copy of each of thesepieces in a database, which is kept separate from the routing table

• New LSA only when Link State changes;additionally periodic refreshs (30 minutes)

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Link State Technology (cont.)AABBCC

2213131313

QQZZXX

Shortest Path Calculation

• Each router ‘composes the jigsaw puzzle’: By executing the Dijkstra Algorithm the topology is calculated in form of a tree

• The root of the tree is the calculating router itself

• All routers in area must have an identicalarea database

SPF Tree




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Link State Technology (cont.)Splitting into areas

Backbone Area

Non-Backbone Areas

• Reduced Size of areas- reduces size of area database- reduces number of Dijkstra executions- reduced complexity of Dijkstra (faster convergence)- filtering and summarization capabilities

(less memory needed)

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OSPF Background

• Open Shortest Path First

• Link state technology

• Developed by OSPF working group of IETF,specified in a number of RFCs

• Interior Gateway Protocol (IGP)

• For TCP/IP Internet environment only, runs directly over IP (protocol-id 89)

• VLSM support




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Router ID• Each router must have a unique Router ID

• The router ID is selected during boot sequence:

- highest IP address of a loopback- if no loopback, then highest IP of a LAN - not pre-emptive: doesn’t change through configuration after boot up

- if no IP address configured then error message: ‘could not allocate router id’

• In the future will be configurable

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OSPF Packet • 5 packet types

- Hello- Database Descriptor- Link State Request- Link State Update- Link State Ack.

• Neighbors must match on

- Area ID- Autype, eventual Auth.- Some options: E, DC, ...

version type packet length

Router ID

Area ID

Checksum Autype

Designated Router

Authentication

0 8 16 32

OSPF PACKET HEADEROSPF PACKET HEADER




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The Hello Protocol

• Responsible for establishing and maintaining neighbor relationships

• Elects designated router onmulti-access networks

Hello

HelloHello

FDDI

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The Hello Packet

• Router priority• Hello interval

• Router dead interval• Network mask• Options: T-bit, E-bit, ...

• List of neighbors

FDDI

Hello

HelloHello




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Neighboring States

• 2-way

Router sees itself in the received hello packets

Adds reliability to the protocol

Transient state before becoming adjacent

2-Way

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Adjacency

• Two neighboring routers, which have synchronized their link state database areadjacent (FULL) neighbors

• They maintain their database synchronized.Whenever their link state database changes,they inform their adjacent neighbors.

• Not every pair of neighbors must become adjacent

FULL




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Designated RouterIf each pair of neighborson LAN would be adjacent: N * (N - 1) / 2 adjacencies

Solution:

Adjacencies only to DRand BDR

BDR DR

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Designated Router

• One DR per multi-access network

- Generates Net LSA

- Assists in database synchronization

BDR

BDR DR

DR




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Designated Router Election

• Election done during neighbor discovery (Hellos)

• Priority can be configured (per interface)• Otherwise determined by highest RID• Not pre-emptive

R2 Router ID = 131.108.3.3

131.108.3.2 131.108.3.3

R1 Router ID = 144.254.3.5

DR

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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 neighboring router is the designated router

• The neighboring router is backup designated router




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Adjacencies in Further Detail

• Initial Synchronization- Master/Slave negotiation

- Exchange of Database Descriptors

- Requesting individual LSAs from neighbor

- Synchronization

• Flooding of LSAs along adjacencies

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Benefit of Database Descriptors => Link Failures

R2R1

- R1 looses neighbor and deletes all routes to cloud- R1 keeps LSAs from routers in cloud in database !!

(until maxage = 60 minutes)- Link comes back, they exchange database descriptors,

compare both databases and request only changed or new LSAs => Low Bandwidth utilization




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LSAs Propagate Along Adjacencies

• IP multicast used to send LSAs to the OSPFDesignated Router Group (224.0.0.6) and to the all OSPF router group (224.0.0.5)

• LSAs acknowledged along adjacencies

• Acknowledgements are unicasts

R2

DR BDR

Full

R1

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Classifications of Routers and Areas

• An Area is a collection of routers. Area 0 is the backbone area

• Area Border Router (ABR) has interfaces in different areas (one to backbone)

• Autonomous System Border Router (ASBR) injects routes from outside into

the AS• An Interior Router IR has all

interfaces in the same area

Area 0

Area 2

To Other AS

Area 1

IR

ASBR

ABR




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Understanding OSPF =Understanding the Database

• Which router generates which kind of LSA ?

• Which routers should receive and install theLSA in the database ?

• What is the information in LSA ?

• How is a route selected with this LSA ?

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Router LSA (LSA type 1)

• Every router generates a LSA type 1

• LSA type 1 contains all the links and types of links (transit-, stub network) to which this router connects

• Flooded through the entire area

• In general: all types of LSAs carry a sequence-number, the age, the LS-ID and the advertising router ID




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Network LSA (LSA type 2)

• Generated by the Designated Router• LSA type 2 contains IP address and mask of

the multi-access network and a list of all attached routers (by RID)

• Flooded through the entire area

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Summary LSA (type 3 and 4)

• Generated by ABR

• Reports networks,located in anotherarea

• Flooded through all areas, which don’tcontain that network

• Generated by ABR

• Reports its best pathto the ASBR an area,which don’t contain the ASBR

LSA type 3 LSA type 4




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External LSA (LSA type 5)

• Generated by ASBR- through redistribution

• Flooded through all areas

• Two types- external type 1 - external type 2 (default)

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External Routes redistribution ofnetwork N into OSPFwith External Cost = 1

Cost = 10 ASBR 2

ASBR 3

R1

Selected Route

redistribution ofnetwork N into OSPFwith External Cost = 2Type 1Type 1

costcost 1111costcost 1010

Next HopNext HopR1R1R3R3

Cost = 8

NetworkNetworkN1N1N1N1

• Type 1 external metric- accumulates costs




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redistribution ofnetwork N into OSPFwith External Cost = 1

Cost = 10 ASBR 2

ASBR 3

R1

Selected RouteType 2Type 2costcost 11costcost 22

Next HopNext HopR2R2R3R3

Cost = 8

NetworkNetworkN1N1N1N1

• Type 2 external metric- non accumulative costs

redistribution ofnetwork N into OSPFwith External Cost = 2

External Routes

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Forwarding Address

• Forwarding address is set if next hop address belongs to a network within the OSPF AS

(Field in AS external links advertisement)Router R2, only installs external route, if next hop is

reachable via an intra or inter area route

AS#2R1

R2

R3

AS#1

N1

N2N3OSPF

BGP, RIP or else

FDDITokenRing

NetworkNetworkN3N3

Next HopNext HopR3R3




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LSA OverviewArea 2

IR

RL Area 0

To Other AS

Area 1

ASBREL

ABR

SL 3, 4

SL 3

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Topology/ Link State Database

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

• All routers belonging to the samearea have identical database

• SPF calculation is performed separatelyfor each area

• Each router calculates its own tree

• LSA flooding is bounded by area




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Cost of a Route

10

1562

1562

10R1

DestinationNetwork k

CostCost31443144

..

Next HopNext HopR2R2..

NetworkNetworkkk..

R2

Routing Table

• Sum of all outgoing interface costs along the calculated shortest path (Dijkstra)

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Cost of a Route (cont.)• Outgoing interface cost (default):

Cost = constant / BW , const. 100 Mb/sExample: Ethernet = 10, FDDI = 1

• Not scaling for high speed links =>configurable (two options)

a) interface subcommand- ospf cost

b) router subcommand- ospf auto-cost reference-bandwidth

(default 100 Mb/s)




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Route Selection• longest match (as every protocol)• type of route, preference in following order:

- intra area route- inter area route- external type 1- external type 2

• cost of route- calculated by SPF algorithm

• equal cost load balancing- default 4 paths- ospf subcommand ‘maximum-path <1-6> ‘

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Designing OSPF Networks

•• Hierarchical StructureHierarchical Structure

• Addressing, Summarization and Filtering

• Stub Areas

• On Demand Circuits

• NBMA Networks




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Hierarchical Structure

BackboneArea 0

Area 1 Area 2 Area 3

• Structure must exist or be created

• Explicit topology has precedence over addressing

• Two level hierarchy

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Basic Rules for Areas

BackboneArea 0


• Rules

- Backbone area (= area 0) must be present, if multiple areas exist

- All areas must have connection to backbone

- Backbone must be physically contiguous




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Area 2Network 131.108.0.0Subnets 33–47Range 255.255.240.0

Area 0Network 192.117.49.0Range 255.255.255.0



VLSM and Discontiguous Subnets

• Subnets can be discontiguous- they can belong to the same area- they can be spread over different areas

• Intelligent grouping of subnets is vital to take advantage of summarization capabilities

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Why Areas ?

• Is it correct,that the topology from one area is invisible from outside of the area ?

that routes (destination) in one area areinvisible from outside of the area ?

that splitting into area this reduces routing traffic and the size of the database ?that this adds stability ?




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Area Topology

• Can non-backbone areas communicate directly toeach other ?

• No !

• But connecting routers can belong to backbone:virtual links

BackboneArea 0

Area 1 Area 2

BackboneArea 0

Area 1

Area 2

?

?

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Virtual Links

• Virtual links are possible between any two routers that have an interface to a common non-backbone area.

• Through virtual link router becomes a backbone router• Can introduce backbone stability through redundancy.

Can keep the backbone together. (some topologies)• Heavy usage is not recommended and bad practice.

Indicates bad design

BackboneBackboneArea 0Area 0

BackboneBackboneArea 0Area 0

Area 3

Area 1

Area 2 Area 4

Virtual Links




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OSPF Link SummarizationBackbone

Area 0


• ABR create summary link advertisement

• Summary Links = Auto Summarization

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Not Summarized: Specific Links

BackboneArea #0

1.A

1.C

1.B

1.D 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

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

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing




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Not Summarized: Specific Links

BackboneArea #0

1.A

1.C

1.B

1.D 3.D

3.A

3.C

3.B

2.A2.B2.C2.D3.A3.B3.C3.D

1.A1.B1.C1.D2.A2.B2.C2.D

2.A

2.C

2.BTokenRing

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing

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

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

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External Route Information

• External routes are flooded everywhere• Link state changes are seen everywhere

External links

1.A

1.C

1.B

3.D

3.A

3.C

3.B

2.A2.C

2.BTokenRing

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing

4a4b

5a5b

ASBR

5a5b

BackboneArea 04a

4b

ASBR

5a5b

5a5b

1.D

4a4b

4a4b

External links




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Regular Area

• Summary LSA from other areas injected

• External LSA are flooded everywhere

• Specific links from outside AS and from other areas known

Can cause routing trouble—link flaps !

• Do we need to know all specific links everywhere ?

Solutions: Summarization, Stub-Areas and Filtering

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Summarization at ABR, ASBR• Summarization of Summary Links at ABR

with area-range command• Summarization of externals at ASBR with

summary-address command• Summaries are only advertised if at least one

route within the range is known• Cost of the summary is minimum of all more

specific (in future it will be maximum)• Summarization useable for routing policies,

based on longest match routing




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Summarization Example

Area 0Area 1ASBR

ABR

} optional

router ospf 100area 0 range 136.0.0.0 255.0.0.0!int loopbackIP address 136.96.1.1 IP route 136.0.0.0 int null 0

router ospf 100summary-address 138.0.0.0 255.0.0.0redistribute rip

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Summarization, Some Tricks• Stabilize through use of loopback

- optional

- gives always a more specific route- Be careful it could change RID during reload

• Avoid loops and drop quickly, in absence ofmore specific routes with static route to null

- optional

• Use of summarization to influence routing

- redundant ABRs: Summaries through one and more specific through the other




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Summarized: Summary Links

• Only summaries advertised out• Specific link changes do not propagate

BackboneArea #0

External links

1.A

1.C

1.B

1.D 3.D

3.A

3.C

3.B

13

2

2.A

2.C

2.B

ASBR

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing

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Summarized: Summary Links

• Only summaries advertised out• Specific link changes do not propagate

BackboneArea #0

1.A

1.C

1.B

1.D 3.D

3.A

3.C

3.B

2,3 1,2

1,3

2.A

2.C

2.BTokenRing

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing




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External Route Information

External links

External links

1.A

1.C

1.B

3.D

3.A

3.C

3.B

2.A2.C

2.BTokenRing

TokenRing

TokenRing

TokenRing

TokenRing

TokenRing

4

5

ASBR

5BackboneArea #0

4

4

ASBR

55

1.D

4

• Less external routes• Specific link changes do not propagate

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Additional Control of Flooding• Use of default-information instead of specific

link informationdefault-information originate

default-information originate [always]

• Stub areas

• Totally stub areas

• Not So Stubby Area NSSA

• Filtering of LSAs

• Route Tagging




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Normal Stub Area

• Configuration: all routers within stub areamust be configured as stub

router ospf 1area 1 stub

• Summary LSAs from other areas injected• Default LSA injected into stub area• Default path to closest area border router• External LSAs not injected => No external

route flaps

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Totally Stubby Area• External and Summary LSA are not advertised

into totally stub area• Only default LSA injected into area (always)• Default path to closest ABR• Configuration:

On ABRs, connecting to area x: area x stub no-summary

All other routers in area x: area x stub

• Ext. and summary link flaps not visible




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• New type of area, supported as of 11.2

• Creates transit areas, capable of importing external routes in a limited fashion

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

• NSSA border routers translate type-7 LSAs into type-5 LSAs

• Filtering and Summarization Capabilities atNSSA-ABRs

Not So Stubby Area

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NSSA Example

RIP NSSA 1 BackboneArea 0

RIPType 7

Type 7 Type 5

This NSSA ASBR redistributesRIP routes into type 7 LSAs

This NSSA ABR translatesType 7 into Type 5 LSAs• Filtering Possibility• Summarization Possible

router ospf 1 area 1 nssa




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LSA Filtering• Routers within an area must have identical area

database. Filtering of LSAs within an area isnot possible:

· distribution-list out: ignored

· distribution-list in: LSA in db, not in routing table

• Filtering of Summary LSA at ABR· Theoretical possible, not supported

• Filtering possibilities:· External LSAs at ASBR· External LSAs at NSSA borders

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Enhancements to SupportDemand Circuits

• Allows efficient operation over demand circuits

ISDN circuits, X25 SVCs, dialup lines

• Hellos and periodic refresh of OSPF routing suppressed

• Opens on demand circuits when new LSA has to be sent. Limited use in big areas

• Allows the underlying connection to be closed in the absence of application traffic




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Demand Circuits (Cont.)• All OSPF packets carry DC option bit

• High bit of LS age field is used as “do not age bit”

• All routers in area must support DC option, ifDC is located in Stub or NSSA area

• All routers in domain must support DC option,if DC is located in regular area

• If mismatch in the option field, no neighborship possible => IOS must be 11.2 or higher,everywhere within area

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Route Tagging

A

BC

D

• Autonomous system Bwants to

Propagate routes fromA —> D, but notnot propagate routes from C —> D

• OSPF tags routes with 32-bit value

-default is 0

-can be set during redistributionfrom A into OSPF and from Cinto OSPF

-can be used when redistributingOSPF into D (route-map with match on tag)




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NBMA Networks

• Different Options- on main interface (multipoint)- on subinterfaces

- point-to-point- point-to-multipoint

branch office

central sitecentral site

branch office

publicframe-relay

network

regional site

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NBMA on Subinterfaces

• Point-to-Point Subinterface- network needed for each p2p link

- use mask x.x.x.252

- no DR

- net/mask reported through type-1 LSA

- PVC failure takes subinterface interface down-> resilient




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on subinterfaces (cont.)

• Point-to-Multipoint Subinterface

- one network for cloud

- PVC failures don’t take interface down

- 3 ways for OSPF to model the cloud

- OSPF Network Type Broadcast

- OSPF Network Type NBMA- OSPF Network Type Multipoint

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• OSPF Network Type Broadcast(don’t do it)

- broadcast keyword in mapping required

- full mesh required for DR election- creation of Net-LSA

- fails when full mesh broken




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•OSPF Network Type NBMA(doable, but not the preferred way)

- OSPF neighbour statement required

- mappings don’t need broadcast statement- mapping statements for endpoints required

- no full mesh required- DR election (tip: set ospf priority)- creation of NET-LSA

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• OSPF network type multipoint(recommended way)

- require broadcast keyword in mappings- no DR, no Net-LSA

- host route for each end-point of cloud- more routes -> flooding -> memory

- no full mesh requirement- resilient through host-routes -> rerouting



Documents

Designing and Troubleshooting OSPFNetworksledvina/vyuka/PDS/PDS_2005/14_OSPF.pdf · Designing and Troubleshooting OSPF Networks November 1997, Paris Networkers 97 1 Cisco Systems