multiarea ospf slides

8/8/2019 MultiArea OSPF Slides

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Ch.6 OSPFPart 2 of 2: Multi-Area OSPF

CCNP version 3.0

Rick Graziani

Cabrillo College

* Updated by HoonJae Lee (Dongseo Univ.)

2006. 3. 3

2

Note

Optional: This lab coincides with my Multi-area OSPF Labhandout, Rick's OSPF Scenarios (Word doc)

Most of the slides in this presentation do not come directlyfrom CCNP 1 version 3.0, OSPF, but slides which containa little more detail and explanation of OSPF.


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Optional: Ricks OSPF Scenarios

Optional: We will be using the following handout for this presentation:

Ricks OSPF Handout:

1. OSPF Multi-Area - All Normal Areas

2. OSPF Multi-Area - Stub Area

3. OSPF Multi-Area - Totally Stubby Area

This handouts can be downloaded from (Word doc):

http://www.cabrillo.cc.ca.us/ciscoacad/curriculum/presentations/semester5/OSPF_Scenario_Handout.doc

4

Part I - LSAs using all normal areas

Multi Area OSPF

Normal Areas

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Suggestion to Instructors:Draw this network on thewhite-board as it will be usedfor discussion throughoutthese slides.


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Topics

Areas LSAs show ip ospf database (summary of link state database) show ip route Stub Areas Totally Stubby Areas E1 and E2 routes Default Routes Route Summarization NSSA (Not So Stubby Areas) Multiple ABR Scenario

Multiple ASBR Scenario Virtual Links Load Balancing show commands

6

Issues with large OSPF nets

Large link-state table

Each router maintains a LSDB for all links in the area

The LSDB requires the use of memory

Frequent SPF calculations

A topology change in an area causes each router to re-run SPF to

rebuild the SPF tree and the routing table.

A flapping link will affect an entire area.

SPF re-calculations are done only for changes within that area.

Large routing table

Typically, the larger the area the larger the routing table.

A larger routing table requires more memory and takes more time toperform the route look-ups.

Solution: Divide the network into multiple areas


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OSPF uses Areas

Hierarchical routing enables you to separate large internetworks (autonomoussystems) into smaller internetworks that are called areas.

With this technique, routing still occurs between the areas (called inter-arearouting), but many of the smaller internal routing operations, such asrecalculating the database re-running the SPF algorithm, are restricted withinan area.

8

Question: I understand the routing table is recalculated every time the routerreceives an new version of an LSA. Does OSPF recalculate its routing tablewhen their is a topology change in another area? show ip ospf displays nochange in SPF execution, but show ip ospf database shows a change in thetopology?

Answer: Good question! OSPF areas are designed to keep issues like flappinglinks within an area. SPF is not recalculated if the topology change is in

another area. The interesting thing is that OSPF distributes inter-area (betweenareas) topology information using a distance-vector method. OSPF uses link-state principles only within an area. ABRs do not announce topologicalinformation between areas, instead, only routing information is injected intoother areas. ABRs relay routing information between areas via distance vectortechnique similar to RIP or IGRP. This is why show ip ospf does not show achange in the number of times SPF has been executed when the topologychange is in another area.

Note: It is still a good idea to perform route summarization between areas,announcing multiple routes as a single inter-area route. This will hide anychanges in one area from affecting routing tables in other areas.

An advantage of Multiple Areas


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OSPF uses Areas

10

OSPF Router Types


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OSPF Router Types

InternalInternal: Routers with all their interfaces within the same area

BackboneBackbone: Routers with at least one interface connected to area 0

ASBRASBR(Autonomous System Boundary Router): Routers that have at leastone interface connected to an external internetwork (anotherautonomous system)

ABRABR(Area Border Router): Routers with interfaces attached to multipleareas.

12

OSPF Packet Types

Last week we discussed various OSPF packets, used for: Means for dynamic neighbor discovery

Detect unreachable neighbors within a finite period of time

Ensure two-way communications between neighbors

Ensure correctness of basic interace parameters between neighbors

Provide necessary information for the election of the Designated andBackup Designated routers on a LAN segement

Request link state information from another router

Sharing data base summary and detailed information

Acknowledge the receipt of an OSPF packet

OSPF packet typesOSPF Type-2 (DBD)

OSPF Type-3 (LSR)

OSPF Type-4 (LSU)

OSPF Type-5 (LSAck)

OSPF Type-1 (Hello)


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OSPF Type 4 - Link State Advertisements

This week we will look at OSPF Type 4 packets more closely

OSPF packet types

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OSPF packet types (Extra)

OSPF Type-4 packets have 7 LSA packets (later)


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LSAs used for discovering routes and reaching Full State, along withMaintain Routes

LSA Types

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LSA Types

LSA Types 1 through 5

We will look at these in detail as we discuss areas in this chapter.

LSA Type 6 MOSPF (Multicast OSPF)

Not supported by Cisco. MOSPF enhances OSPF by letting routers use their link-state

databases to build multicast distribution trees for the forwarding ofmulticast traffic.

LSA Type 7 NSSA External Link Entry

Originated by an ASBR connected to an NSSA. Type 7 messages can be flooded throughout NSSAs and translated

into LSA Type 5 messages by ABRs.

Routes learned via Type-7 LSAs are denoted by either a N1 or andN2 in the routing table. (Compare to E1 and E2).

We will discuss this more later when we look at NSSA areas.


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Area Types

Standard or Normal AreasBackbone

Non-Backbone

StubStub Area

Totally Stubby Area (TSA)

Not-so-stubby-area (NSSA)

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Area Types

(Standard area):(Backbone area):, 0., OSPFOSPF.(Stubby area): OSPF OSPF,ASdefault route(0.0.0.0/0).(TSA, Totally Stubby area):ASAS., 0.0.0.0/0.. NSSA(Not-So-Stubby area): 7 LSA, 7 LSA 5 LSA.


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Multi Area OSPF

Normal Areas

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Routes Received on all OSPF Routers

Overview of Normal Areas This will all be explained!

Receives all routes from within A.S.:

Within the local area LSA 1 and LSA 2

From other areas (Inter-Area) LSA 3, LSA 4, LSA 5

Receives all routes from External A.S.s (External AS means routes not from thisOSPF routing domain):

From external ASs LSA 5 As long as routes are being redistributed by the ASBR (more later)

Default Routes

Received only if default-information-originate command was used (later) If default-information-originate command is not used, then the default

route is not received



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Your Turn - In groups, examine running-configs

Look at the running-configs for 1. OSPF Multi-Areas- All Normal Areas

Look at the OSPF network statements!


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1. OSPF Multi-Areas - All Normal Areas

ASBRrouter ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

!

ip classless

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0ip route 13.0.0.0 255.0.0.0 Null0

ABR-1interface FastEthernet0/0

ip address 172.16.1.2 255.255.255.0

ip ospf priority 200

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.51.0 0.0.0.255 area 51

ABR-2interface FastEthernet0

ip address 172.16.1.3 255.255.255.0

ip ospf priority 100

!

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.10.4 0.0.0.3 area 1

Internalrouter ospf 1

network 172.16.0.0 0.0.255.255 area 1

ABR contains networkstatements for each areait belongs to, using theproper area value.


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Understanding LSAs

show ip ospf database This is not the link state database, only a summary.

It is a tool to help determine what routes are included in the routing table.

We will look at this output to learn the tool as well as become familiar withthe different types of LSAs.

To view the link state database use: show ip ospf database[router|network|]

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | LS type |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Advertising Router |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS sequence number |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS checksum | length |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LSA Header

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LSA 1 Router LSA Generated by each router for each area it belongs to. Describes the states of the links in the area to which this router belongs.

Tells the other routers in the area about itself and its links to adjacent OSPFrouters, and leaf networks.

Flooded only within the area. On multi-access networks, sent to the DR. Denoted by just an O in the routing table or C if the network is directly

connected.

ABR will include a set of LSA 1s for each area it belongs to.

A C

D

2

5

B

15 Router As LSA 1swhich are flooded to all

other routers in thisarea.

Leaf network

LSA 1 - Router Link States


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When a new LSA 1 is received and installed in the LSDB, the routerforwards that LSA, using hop-by-hop or asynchronous flooding.

The LSA is sent out all OSPF interfaces that are in the Exchange Stateor a higher state.

For interfaces in Exstart or lesser state, the router will wait until it is outof Exstart.

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0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | 1 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| 0 |V|E|B| 0 | # links |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link Data |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Type | # TOS | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| TOS | 0 | TOS metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link Data |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |



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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID

192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1s being sentwithin Area 0

LSA 1LSA 1LSA 1


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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID

192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1s being sentwithin other areas

LSA 1

LSA 1

LSA 1

LSA 1



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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1Originated

LSA 1s are flooded outother interfaces withinthe same area.

LSA 1flooded


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For Router Links:

The Link State ID is always the same as the Advertising Router Advertising Router is the Router ID of the router that created this LSA 1

Internal#show ip ospf data

OSPF Router with ID (192.168.4.1) (Process ID 1)

Router Link States (Area 1) my one area


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ABR-2#show ip ospf data


Router Link States (Area 1) my one area


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ABR-2 - show ip route

172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks

O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0

O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0

C 172.16.10.4/30 is directly connected, Serial0

C 172.16.1.0/24 is directly connected, FastEthernet0O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0

O E2 12.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0


192.168.3.0/32 is subnetted, 1 subnets

C 192.168.3.1 is directly connected, Loopback1


Denoted by just an O in the routing table, or a C Why is there only just anO for this network and not the other

networks?

Directly connected or via another area.


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Your Turn -Discuss in groups (LSA 1s)

Using the Multi-area OSPF Lab Handout: Lab 1. OSPFNormal Areas verify these results.

Look at the link state database summary (show ip ospfdatabase) commands and the Router Links States (LSA1s) foreach router.

Look at the routing tables (show ip route) and notice the routeswithin that routers area.

Why do some routers have more than one set of Router LinksStates?

Where does show ip ospf database tell you the RouterID. Where does show ip ospf database tell you the Area.

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LSA 2 Network LSA Generated by the DR on every multi-access network Denoted by just an O in the routing table or C if the network is

directly connected.

Flooded only within the originating area. LSA 2s are in link state database for all routers within area, even those

routers on not on multi-access networks or DRs on other multi-accessnetworks in the same area.

ABR may include a set of LSA 2s for each area it belongs to.

LSA 2 - Network Link States


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0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | 2 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Network Mask |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Attached Router |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |


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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID

192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 2

No LSA 2s for ABR-1in Area 51, or forInternal because noother routers on multi-access segment.

flooded

DR

DR

LSA 2

LSA 2flooded

LSA 2

LSA 2



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ASBR#show ip ospf data


Net Link States (Area 0)

(LSA 2 - Generated by the DR)

Link ID ADV Router Age Seq# Checksum

172.16.1.2 192.168.2.1 201 0x8000000D 0xCFE8


Link ID 172.16.1.2 = IP address of DR on MultiAccess Network ADV Router 192.168.2.1 = Router ID of DR Bottom line: Net Link States (LSA2s) should display the RouterIDs of

the DRs on all multi-access networks in the area and their IPaddresses.

Ricks reminder: LSA 2 -> Ethernet = Layer 2 or D R1 2

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Look at the link state database summary (show ip ospfdatabase) commands and the Net Links States (LSA2s) foreach router.

Look at the routing tables (show ip route) and notice the multi-access routes within that routers area.

Could a router have more than one entry in its listing of NetLinks States?

Could an area with a broadcast segment, still have no LSA 2s?



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LSA 3 Summary LSA Originated by the ABR. Describes links between ABR and Internal Routers of the Local Area ABR will include a set of LSA 3s for each area it belongs to. LSA 3s are flooded throughout the backbone (Area 0) and to other

ABRs.

Routes learned via LSA type 3s are denoted by an IA (Inter-area) inthe routing table.

LSA 3 Summary Net Link States

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0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | 3 or 4 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Network Mask |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| 0 | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| TOS | TOS metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |



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42

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID

192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1s are sent asLSA 3s into otherareas by the ABRs.

LSA 1LSA 1

LSA 3LSA 3

LSA 1



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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas


LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1LSA 3

LSA 3


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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1

LSA 3

LSA 3

Area 1

LSA 1s are sent as LSA 3sinto other areas by theABRs.

Routers only see the topology of the area they belong to. When a link in one area changes, the adjacent routers originate in LSA 1s and

flood them within the area, causing intra-area (internal) routers to re-run theSPF and recalculating the routing table.

ABRs do not announce topological information between areas. ABRs only inject routing information into other areas, which is basically a

distance-vector technique.


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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1s are sent as LSA3s into other areas by the

ABRs.

LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1

LSA 3

LSA 3

Area 1

ABRs calculate intra-area routes for directly attached areas and announcethem to all other areas as inter-area routes, using LSA 3s.

OSPF ABRs will only announce inter-area routes that were learned from thebackbone area, area 0.

The backbone area serves as a repository for inter-area routes. This keeps OSPF safe from routing loops.

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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID

192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Dont forget about the LSA1s from Area 0.

Multi Area OSPF

Normal Areas

LSA 1LSA 1LSA 3

LSA 3

LSA 1



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Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 3 LSA 3

Normal AreasLSA 1s

Not ABR

ABRs calculate intra-area routes for directly attached areas andannounce them to all other areas as inter-area routes, using LSA 3s.

In normal operation, OSPF ABRs will only announce inter-arearoutes that were learned from the backbone area, area 0. RTC does not forward LSA 3s from Area 1 to Area 51, and does not

forward LSA 3s from Area 51 to Area 1.

The backbone area serves as a repository for inter-area routes. This keeps OSPF safe from routing loops.

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Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 3

LSA 3

Normal Areas

LSA 1s

Not ABR

Example of an LSA 1 originated in Area 1, sent to Area 0 as an LSA 3,and the sent to Area 51 as an LSA 3.

RTC does not forward the LSA 3s back into Area 1, or routingloops may develop.

Again, in normal operations, OSPF ABRs will only announce inter-arearoutes that were learned from the backbone area, area 0.

Note: RTC will create LSA 1s and flood them within the appropriatearea.


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Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

Normal Areas

LSA 1s LSA 1s

LSA 3 LSA 3

LSA 3LSA 3

Not ABR

RTC does not forward LSA 1s from Area 1 as LSA 3s into Area 51.

RTC does not forward LSA 1s from Area 51 as LSA 3s into Area 1. Any LSA 3s from RTC are not forwarded into Area 0 by RTA or RTB. OSPF specification states that ABRs are restricted to considering LSA

3s only from the backbone area to avoid routing information loops.

50

Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

Normal Areas

LSA 1s

LSA 3

LSA 3

X

Area 1 routers re-runSPF, creates new SPF

tree and updatesrouting table.

Update is sent to Area 0 and Area51 routers using a distancevector update technique. SPF notre-run, but routers update routingtable.

Topology Change: Down Link

When a router detects a topology change it immediately sends out LSA1s (Router LSAs) with the change.

In the case of a down link, the age of the LSA is set to MaxAge (3,600seconds) Routers that receive LSAs with the age equal to MaxAgeremove this entry from their LSDB (Link State Data Base).

Routers that receive the LSA 1s, within the area of the change, re-runtheir SPF algorithm, to build a new SPF tree and then make thechanges to their IP routing tables. (Continued next slide)


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51

Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

Normal Areas

LSA 1s

LSA 3

LSA 3

X

Area 1 routers re-runSPF, creates new SPFtree and updatesrouting table.

Update is sent to Area 0 and Area51 routers using a distance

vector update technique. SPF notre-run, but routers update routingtable.

Topology Change: Down Link

ABR RTA receives the LSA 1 and recalculate their SPF for that area,Area 1.

RTA floods the change as a LSA 3 within its other area, Area 0. RTB receives the LSA 3 and floods it within Area 51. Area 0 and Area 51 routers do not recalculate their SPFs, but inject the

change into their routing tables.

Note: LSA 3s (and other Inter-Area routes) are viewed as leaf nodes inthe SPF tree.

52

Multi Area OSPF

Normal Areas

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID

192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32



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ASBRASBR# show ip ospf database

Summary Net Link States (Area 0)


(Area 1 networks - Advertising Router ABR-2)

172.16.10.4 192.168.3.1 278 0x80000001 0xD126

172.16.20.0 192.168.3.1 278 0x80000001 0xA746


172.16.51.1 192.168.2.1 206 0x80000005 0xA832


Link ID = IP network addresses of networks in other areas ADV Router = ABR Router ID sending the LSA-3 Divided by ABR Bottom line: Should see networks in other areas and the ABR advertising that

route.

Ricks reminder: LSA 3 -> networks sent by the A B R1 2 3

54

ASBRASBR# show ip route


O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:54, FastEthernet0/0



C 172.16.1.0/24 is directly connected, FastEthernet0/0


C 10.1.0.0 is directly connected, FastEthernet0/1

S 11.0.0.0/8 is directly connected, Null0






Routes learned via LSA type 3s are denoted by an IA (Inter-AreaRoutes) in the routing table.


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Another example: non-area 0 router, Internal

Internal# show ip ospf database

LSA 3 - Generated by the ABR. Describes links between ABR and

Internal Routers of the Local Area




172.16.1.0 192.168.3.1 848 0x80000005 0xD339

172.16.51.1 192.168.3.1 843 0x80000001 0xB329

Internal# show ip route


O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0

C 172.16.20.0/24 is directly connected, FastEthernet0


O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0192.168.4.0/32 is subnetted, 1 subnets


O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0

56

Using the Multi-area OSPF Lab Handout: Lab 1. OSPF Normal Areas verify these results.

Look at the link state database summary (show ipospf database) commands and the Summary NetLinks States (LSA3s) for each router.

Look at the routing tables (show ip route) and noticethe Interarea (IA) routes.

Why do some routers have more than one set ofSummary Net Links States?



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LSA 4 ASBR Summary LSA Originated by the ABR. Flooded throughout the backbone area to the other

ABRs.

Describes the reachability to the ASBRs Advertises an ASBR (Router ID) not a network Included in routing table as an IA route. Same format as a LSA 3 - Summary LSA, except LSA 4

ASBR Summary LSA the Network Mask field is always 0

Exceptions

Not flooded to Stub and Totally Stubby networks. More on this later

LSA 4 ASBR Summary Link States

58

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | 3 or 4 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Network Mask |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| 0 | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| TOS | TOS metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |



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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Normal Areas

LSA 4

LSA 4

LSA 4

LSA 5s flooded

LSA 4

Area 1

Flooded throughout the backbone area to the other ABRs. Describes the reachability to the ASBRs How do the ABRs know about the ASBR? I am still researching this,

but I believe when routers receive an LSA 5 (AS External LSA) withexternal route information, the routers denote the Router ID being theASBR.

60

ABR-2

ABR-2# show ip ospf database

Summary ASB Link States (Area 1)

LSA 4 - Reachability to ASBR.

Not flooded to Stub and Totally Stubby networks.


192.168.1.1 192.168.3.1 801 0x80000003 0x93CC


Link ID 192.168.1.1 = Router ID of ASBR ADV Router 192.168.3.1 = Router ID ABR advertising route

Bottom line: Routers in non-area 0, should see Router ID of ASBRand its ABR to get there .

Ricks reminder: LSA 4 -> Reachability to the A S B R1 2 3 4


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Internal


Summary ASB Link States (Area 1)

LSA 4 - Reachability to ASBR.

Not flooded to Stub and Totally Stubby networks.


192.168.1.1 192.168.3.1 912 0x80000003 0x93CC


Link ID 192.168.1.1 = Router ID of ASBR

ADV Router 192.168.3.1 = Router ID ABR advertising route Note: No LSA 4s for Area 0 on Router B

62


Look at the link state database summary (show ip ospfdatabase) commands and the Summary Net Links States(LSA4s) for each router.

Why do some routers have more than one set of Summary ASBLinks States and others may not (like RouterA and ASBR)?

Which Area 0 routers have LSA 4s in their LSDB? Why dont some Area 0 routers have LSA 4s in their LSDB?



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65

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

Normal Areas

66

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Normal Areas

LSA 5

LSA 5

LSA 5LSA 5

LSA 5s floodedLSA 5

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

Redistribute command creates an ASBR router. Originated by the ASBR. Describes destination networks external to the OSPF Routing Domain Flooded throughout the OSPF AS except to stub and totally stubby

areas


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LSA 5 - AS External Link States

ABR-2

ABR-2# show ip ospf databaseAS External Link States O T H E R networks1 2 3 4 5

68

ABR-2ABR-2# show ip route


O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0

O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0








Designated by E2 Notice that the cost is 20 for all three routes, we will see why later. It has to do with E2 routes and where the default cost is 20.

Redistribute command (Route Optimization chapter): If a value is notspecified for the metric option, and no value is specified using the default-metric command, the default metric value is 0, except for OSPF wherethe default cost is 20.



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69


Type-5 AS External Link States

LSA 5 - External Networks originated by the ASBR,

Flooded throughout A.S. except to Stub and Totally StubbyLink ID ADV Router Age Seq# Checksum Tag

11.0.0.0 192.168.1.1 1191 0x80000001 0x3FEA 0

12.0.0.0 192.168.1.1 1191 0x80000001 0x32F6 0

13.0.0.0 192.168.1.1 1191 0x80000001 0x2503 0




O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0



O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0



O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0

70

E1 vs. E2 External Routes

External routes fall under two categories:

external type 1

external type 2 (default)

The difference between the two is in the way the cost (metric) of theroute is being calculated.

The cost of a type 2 route is always the external cost, irrespective ofthe interior cost to reach that route.

A type 1 cost is the addition of the external cost and the internal costused to reach that route.

A type 1 route is always preferred over a type 2 route for the samedestination.

More later



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Look at the link state database summary (show ip ospfdatabase) commands and the AS External Links States (LSA5s)for each router.

Also, look at the routing tables for each router.

How many sets of LSA 5s does the ABRs have in their link statesummary database? Notice the ASBRs entries.


72

Stub Areas

Considerations for both Stub and Totally Stubby Areas

An area could be qualified a stub when:

There is a single exit point (a single ABR) from that area. Morethan one ABR can be used, but be ready to accept non-optimalrouting paths.

If routing to outside of the area does not have to take an optimalpath.

The area is not needed as a transit area for virtual links (later).

The ASBR is not within the stub area

The area is not the backbone area (area 0)

Stub areas will result in memory and processing savings dependingupon the size of the network.


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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID

192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Stub Area

74

Receives all routes from within A.S.:

Within the local area - LSA 1s and LSA 2s (if appropriate) From other areas (Inter-Area) - LSA 3s

Does not receive routes from External A.S. (External Routes).

ABR: ABR blocks all LSA 4s and LSA 5s. If LSA 5s are not known inside an area, LSA 4s are not necessary. LSA 3s are propagated by the ABR.

Note: Default route is automatically injected into stub area by ABR

External Routes: Once the ABR gets a packet headed to a default route, itmust have a default route, either static or propagated by the ASBR viadefault information originate (coming!)

Configuration:

All routers in the area must be configured as stub

Stub Areas


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75

ABR-2

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.10.4 0.0.0.3 area 1

area 1 stub


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77

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Stub Area

LSA 4

LSA 4

X BlockedLSA 5

Blocked XLSA 5

LSA 3

LSA 3

Defaultroute toABRinjected

Changes in External routes no longer affect Stub Area routing tables.

78

Internal



LSA 3 - Generated by the ABR.

Describes links between ABR and Internal Routers of the Local Area

Link ID ADV Router Age Seq# ChecksumDefault Route Advertised by ABR-1

0.0.0.0 192.168.3.1 243 0x80000001 0x8A46

Area 0 networks - Advertised by ABR-1

172.16.1.0 192.168.3.1 243 0x80000006 0xEF1E

Area 51 networks - Advertised by ABR-1

172.16.51.1 192.168.3.1 243 0x80000002 0xCF0E

Notice that there are no LSA 4s or LSA 5s for stub area routers. Default Route injected by ABR (LSA 3)

Stub Areas


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Internal


Gateway of last resort is 172.16.10.5 to network 0.0.0.0


O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:03:08, Serial0



O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:03:08, Serial0



O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:08, Serial0

Stub Areas

NOTE on default route:

ABR will advertise a default route with a cost of 1 cost of 65 = 1 + 64 (serial link) Using bandwidth of 128K, not 64K: 782 = (100,000,000/128,000) + 1

80

ABR-2

ABR-2# show ip route

Gateway of last resort is not set


O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:01:59,FastEthernet0

O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:01:59, Serial0

C 172.16.10.4/30 is directly connected, Serial0C 172.16.1.0/24 is directly connected, FastEthernet0






Stub Areas

Notice, there is no automatic default route on ABR, as there are with theinternal stub routers.


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81

Using the Multi-area OSPF Lab Handout: Lab 2. OSPF StubArea.

Look at the link state database summary (show ip ospfdatabase) commands and the Summary Net Links States (LSA3s).

Note: A Stub area may have more than one ABR, but because ofthe default route, the internal routers will not be able todetermine which router is the optimal gateway outside the ASand end up load balancing between the multiple ABRs.

Your Turn -Discuss in groups (Stub)

82

Totally Stubby Areas

Cisco proprietary, however the RFC does make some provisions for thisas an optional feature..

Same considerations as with Stub areas:

An area could be qualified a stub when there is a single exit point (asingle ABR) from that area or if routing to outside of the area does not

have to take an optimal path. The area is not needed as a transit area for virtual links (later).



Stub areas will result in memory and processing savings depending

upon the size of the network. - This is even more true with Totally

Stubby areas


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83

ASBR

ABR-1 ABR-2

Internal

Area 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Totally Stubby Area

84

Receives routes from within A.S.:

Only from within the local area - LSA 1s and LSA 2s (if appropriate) Does not receive routes from other areas (Inter-Area) - LSA 3s

Does not receive routes from External A.S. (External Routes)

ABR:

ABR blocks all LSA 4s and LSA 5s. ABR blocks all LSA 3s, except propagating a default route. Default route is injected into totally stubby area by ABR.

Configuring:

All routers must be configured as stub ABR must be configured as stub no-summary



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85

ABR-2

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.10.4 0.0.0.3 area 1

area 1 stub no-summary

^^ Command: area area stub no-summary

Internal

router ospf 1

network 172.16.0.0 0.0.255.255 area 1

area 1 stub

^^ Command: area area stub


86

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X BlockedLSA 5

Blocked XLSA 5

LSA 3

LSA 3

Defaultroute toABRinjected Area 1

X

LSA 3s (Inter-Area routes) are blocked by the ABR. ABR blocks all LSA 4s (reachability to ASBR) and LSA 5s (External routes) The ABR injects a default route (LSA 3) into the stub area, pointing to the ABR.

(This does not mean the ABR has a default route of its own.)

Essentially, internal routers in a Totally Stubby Area only see the default routeto the ABR.


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87

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X BlockedLSA 5

Blocked XLSA 5

LSA 3

LSA 3


X

Changes in any networks outside the Totally Stubby Area no longeraffects the Totally Stubby routing tables.

88

Internal



LSA 3 - Generated by the ABR.

Describes links between ABR and Internal Routers of the Local Area

Link ID ADV Router Age Seq# ChecksumDefault Route Advertised by ABR-2

0.0.0.0 192.168.3.1 205 0x80000003 0x8648

Default Route injected by ABR (LSA 3)


Default route is injected into totally stubby area by ABR for all other networks(inter-area and external routes)

Does not receive routes from other areas (Inter-Area) Does not receive routes from External A.S. (External Routes)


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89

Internal


Gateway of last resort is 172.16.10.5 to network 0.0.0.0






O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:09, Serial0

Default route is injected into totally stubby area by ABR for all other networks(inter-area and external routes)

Does not receive routes from other areas (Inter-Area) Does not receive routes from External A.S. (External Routes)


90

ABR-2

ABR-2# show ip route

Gateway of last resort is not set


O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:35,FastEthernet0

O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:02:35, Serial0

C 172.16.10.4/30 is directly connected, Serial0C 172.16.1.0/24 is directly connected, FastEthernet0







ABR will forward Intra-Area routes (to other areas within AS) Notice, there is not an automatic default route in the ABRs routing

table like there is with the internal Totally Stubby routers.


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91

OSPF Design Tips

Different people have different approaches to designing OSPFnetworks.

The important thing to remember is that any protocol can failunder pressure.

The idea is not to challenge the protocol but rather to work withit in order to get the best behavior. CCO

OSPF design considerations

92

Number of Routers per Area

The maximum number of routers per area depends on several factors,including the following:

What kind of area do you have?

What kind of CPU power do you have in that area?

What kind of media?

Will you be running OSPF in NBMA mode?

Is your NBMA network meshed?

Do you have a lot of external LSAs in the network?

Are other areas well summarized?

For this reason, it's difficult to specify a maximum number of routers perarea.



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93

Stub and Totally Stubby Areas:

An area could be qualified a stub when there is a single exit point (asingle ABR) from that area or if routing to outside of the area does nothave to take an optimal path.

The area is not needed as a transit area for virtual links (later).



Stub areas will result in memory and processing savings depending

upon the size of the network. - This is even more true with Totally

Stubby areas Totally Stubby areas is a Cisco enhancement.


94

Quick Review

Areas LSAs Stub Area Totally Stubby Area


48/94

48

95

LSA-1 - Router LSA

96

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID

192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1s being sentwithin Area 0

LSA 1LSA 1 LSA 1



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49

97

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1s being sentwithin other areas

LSA 1

LSA 1

LSA 1

LSA 1


98

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID

192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1Originated

LSA 1s are flooded outother interfaces withinthe same area.

LSA 1flooded



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50

99

LSA-2 - Network LSA

100

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID

192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 2

No LSA 2s for ABR-1in Area 51, or forInternal because noother routers on multi-access segment.

flooded

DR

DR

LSA 2

LSA 2flooded

LSA 2

LSA 2



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51

101

LSA-3 - Summary LSA

102

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID

192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas


LSA 1LSA 1

LSA 3LSA 3

LSA 1



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52

103

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Multi Area OSPF

Normal Areas


LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1LSA 3

LSA 3


104

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID

192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Dont forget about the LSA1s from Area 0.

Multi Area OSPF

Normal Areas

LSA 1LSA 1LSA 3

LSA 3

LSA 1



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53

105

LSA-4 ASBR Summary LSA

106

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Normal Areas

LSA 4

LSA 4

LSA 4

LSA 5s flooded

LSA 4

Area 1

Flooded throughout the backbone area to the other ABRs. Describes the reachability to the ASBRs


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54

107

LSA-5 - External LSA

108

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

Normal Areas


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55

109

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Normal Areas

LSA 5

LSA 5

LSA 5LSA 5

LSA 5s floodedLSA 5

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

Redistribute command creates an ASBR router. Originated by the ASBR. Describes destination networks external to the OSPF Routing Domain Flooded throughout the OSPF AS except to stub and totally stubby

areas

110

Stub Area


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56

111

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Stub Area

LSA 4

LSA 4

X BlockedLSA 5

Blocked XLSA 5

LSA 3

LSA 3

Defaultroute toABRinjected

112

Totally Stubby Area


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57

113

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X BlockedLSA 5

Blocked XLSA 5

LSA 3

LSA 3


X

114

Next

E1 and E2 routes Default Routes Route Summarization NSSA (Not So Stubby Areas)

Multiple ABR Scenario Multiple ASBR Scenario Virtual Links Load Balancing show commands


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58

115

NSSA (Not So Stubby Area)

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH

116

Relatively new, standards based OSPF enhancement, RFC 1587. NSSA allows an area to remain a stub area, but carry external routing

information (Type 7 LSAs) from its stubby end back towards the OSPFbackbone.

ASBR in NSSA injects external routing information into the backbone

and the NSSA area, but rejects external routing information comingfrom the ABR.

The ABR does not inject a default route into the NSSA. This is true for a NSSA Stub, but a default route is injected for a

NSSA Totally Stubby area.

Note: RFC 1587, A default route must not be injected into the NSSAas a summary (type-3) LSA as in the stub area case.

What??? Following scenario is only example of how NSSA works. For the

purposes of learning about NSSAs, dont get hung up on the whys andwhat ifs.



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59

117

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH

Default route via RTG

NSSA Stub Area

Area 2 would like to be a stub network. RTH only supports RIP, so RTG will run RIP and redistribute those routes in

OSPF.

Unfortunately, this makes the area 2 router, RTG, an ASBR and thereforearea 2 can no longer be a stub area.

RTH does not need to learn routes from OSPF, a default route to RTG is all itneeds.

But all OSPF routers must know about the networks attached to the RIP router,RTH, to route packets to them.

118

NSSA Stub Area (cont.) NSSA allow external routes to be advertised into the OSPF AS while retaining

the characteristics of a stub area to the rest of the OSPF AS.

ASBR RTG will originate Type-7 LSAs to advertise the external destinations. These LSA 7s are flooded through the NSSA but are blocked by the NSSA ABR. The NSSA ABR translates LSA 7s into LSA 5s and flood other areas.

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7sBlocked


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60

119

Type 7 LSA NSSA External Link Entry

Originated by an ASBR connected to an NSSA. Type 7 messages can be flooded throughout NSSAs and translated

into LSA Type 5 messages by ABRs.

Routes learned via Type-7 LSAs are denoted by either a defaultN1 or an N2 in the routing table. (Relative to E1 and E2).

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7sBlocked

120

Configuring NSSA Stub Area

Configured for all routers in Area 2:

router ospf 1

network 172.16.2.0 0.0.0.255 area 2

area 2 nssa

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7

LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7sBlocked

NSSA Generic


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121

NSSA Stub and NSSA Totally Stubby

There are two flavors in NSSA:

Stub

Totally Stubby

Area 2 routers may or may not receive Inter-area routes from RTA, dependingupon NSSA configuration

NSSA areas have take on the same characteristics as stub and totally stubbyareas, along with the characteristics of NSSA areas.


122

NSSA Stub

NSSA stub areas:

NSSAs that block type 4 and 5, but allow type 3.

To make a stub area into an NSSA, use the following command underthe OSPF configuration.

This command must be configured on all routers in area 2.

router ospf 1

area 2 nssa


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123

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH

LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7sBlocked

NSSA Stub Areas

0.0.0.0/0

LSA 3s

XRTHroutes:E1/E2

RTH routes:N1/N2LSA 4s & LSA 5s

X


NSSA Stub Area Routing Tables:

RTG: Area 2 routes, Area 0 routes (IA), RTH RIP routes No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation

Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), Area 0 routes (IA)

No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes if redistributed from

RTA ASBR (E1/E2)

RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes if redistributed fromRTA (E1/E2)

Note: Area 2 routers may or may not receive E1/E2 routes from RTA, depending upon NSSAconfiguration (next).

124

Area 2 routers:

router ospf 1

network 172.16.2.0 0.0.0.255 area 2

area 2 nssa

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7

LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7sBlocked

NSSA Stub Areas

0.0.0.0/0

LSA 3s

XRTHroutes:

E1/E2

RTH routes:N1/N2LSA 4s & LSA 5s

X


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125

NSSA Totally Stubby Area

NSSA totally stub areas: Allow only summary default routes and filters everythingelse.

To configure an NSSA totally stub area, use the following command under theOSPF configuration on the NSSA ABR:

router ospf 1

area 2 nssa no-summary

Configure this command on NSSA ABRs only.

All other routers in area 2 (internal area 2 routers):router ospf 1

area 2 nssa

After defining the NSSA totally stub area, area 2 has the following characteristics (in

addition to the above NSSA characteristics):

No type 3 or 4 summary LSAs are allowed in area 2. This means nointer-area routes are allowed in area 2.

A default route is injected into the NSSA totally stub area as a type 3summary LSA by the ABR.

NSSA Totally Stubby

126

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7

LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7sBlocked

NSSA Totally Stubby Areas

0.0.0.0/0

LSA 3s

XRTHroutes:

E1/E2

RTH routes: N1/N2LSA 4s & LSA 5s

X

RTB (ABR):

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.2.0 0.0.0.255 area 2 ...

area 2 nssa no-summary

Area 2 routers:

router ospf 1

network 172.16.2.0 0.0.0.255 area 2

area 2 nssa


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64

127

NSSA

Area 2

Backbone Area

Area 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7sBlocked

NSSA Totally Stubby Areas

0.0.0.0/0

LSA 3s

XRTHroutes:

E1/E2

RTH routes: N1/N2LSA 4s & LSA 5s

X

NSSA Totally Stubby Area Routing Tables:

RTG: Area 2 routes, RTH RIP routes, 0.0.0.0/0 (IA) route from RTB (ABR)

Totally Stubby: No Area 0 routes or external routes from RTA Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), 0.0.0.0/0 (IA) route from RTB

(ABR)

Totally Stubby: No Area 0 routes or external routes from RTA

RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes from RTA ASBR(E1/E2) if redistributed by ASBR

RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes (E1/E2)

128

Area 2NSSA Area 0

RTARTBRTCRTD

RTE

RIP

Default Route

172.16.3 .0/24 172.16.2.0/24 172.16.1.0 /24

10.0.0.0/8

200.200.200.0/24

222.222.222.0/24

For More on NSSA

For more examples

See NSSA document on my web site for more info.


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129

Virtual Links

130

All areas in an OSPF autonomous system must be physicallyconnected to the backbone area (area 0).

In some cases where this is not possible, you can use a virtual link toconnect to the backbone through a non-backbone area.

As mentioned above, you can also use virtual links to connect twoparts of a partitioned backbone through a non-backbone area.

The area through which you configure the virtual link, known as atransit area, must have full routing information.

Must be configured between two ABRs.

The transit area cannot be a stub area.

Virtual Links


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131

A virtual link has the following two requirements: It must be established between two routers that share a common

area and are both ABRs.

One of these two routers must be connected to the backbone.

Doyle, should be used only as a temporary fix to an unavoidabletopology problem.

Virtual Links

132

The command to configure a virtual link is as follows:

area virtual-link RTA(config)#router ospf 1

RTA(config-router)#network 192.168.0.0 0.0.0.255 area 51

RTA(config-router)#network 192.168.1.0 0.0.0.255 area 3

RTA(config-router)#area 3 virtual-link 10.0.0.1

...

RTB(config)#router ospf 1

RTB(config-router)#network 192.168.1.0 0.0.0.255 area 3

RTB(config-router)#network 192.168.2.0 0.0.0.255 area 0

RTB(config-router)#area 3 virtual-link 10.0.0.2

Virtual Links


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133

OSPF allows for linking discontinuous parts of the backbone using avirtual link.

In some cases, different area 0s need to be linked together . This canoccur if, for example, a company is trying to merge two separateOSPF networks into one network with a common area 0.

In other instances, virtual-links are added for redundancy in casesome router failure causes the backbone to be split into two. (CCO)

Whatever the reason may be, a virtual link can be configured betweenseparate ABRs that touch area 0 from each side and having acommon area.

Virtual Links

134

Virtual Links Another Example


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135

Route Summarization

Inter-Area Route Summarization - Area Range

By default ABRs do not summarize routes between areas.

Route summarization is the consolidation of advertised addresses.

This feature causes a single summary route to be advertised to other areas byan ABR.

In OSPF, an ABR will advertise networks in one area into another area.

If the network numbers in an area are assigned in a way such that they arecontiguous, you can configure the ABR to advertise a summary route thatcovers all the individual networks within the area that fall into the specifiedrange.

On the ABR (Summarizes routes before injecting them into different area)

Router(config-router)# area area-id range network-

address subnet-mask

area-id- Identifier of the area about which routes are tobe summarized. (From area)

136

RTB is summarizing the range of subnets from 128.213.64.0 to 128.213.95.0into one range: 128.213.64.0 255.255.224.0.

This is achieved by masking the first three left most bits of 64 using a mask of255.255.224.0.

128.213.64.0/24 - 010 00000

128.213.95.0/24 010 11111

-----------------------------------------

128.213.64.0/19 - 01000000

Route Summarization


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137

Route Summarization

In the same way, RTC is generating the summary address 128.213.96.0255.255.224.0 into the backbone.

Note that this summarization was successful because we have two distinctranges of subnets, 64-95 and 96-127.

128.213.96.0/24 - 011 00000

128.213.127.0/24 011 11111

-----------------------------------------

128.213.96.0/19 - 01100000

138

128.213.64.0/24 - 010 00000

128.213.95.0/24 010 11111

-----------------------------------------

128.213.64.0/19 - 01000000

RTB

router ospf 100

area 1 range 128.213.64.0 255.255.224.0

Route Summarization


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139

128.213.96.0/24 - 011 00000

128.213.127.0/24 011 11111

-----------------------------------------

128.213.96.0/19 - 01100000RTC

router ospf 100

area 2 range 128.213.96.0 255.255.224.0

Route Summarization

140

External Route Summarization - summary-address

When redistributing routes from other protocols into OSPF (later), each route isadvertised individually in an external l ink state advertisement (LSA).

However, you can configure the Cisco IOS software to advertise a single routefor all the redistributed routes that are covered by a specified network address

and mask. Doing so helps decrease the size of the OSPF link state database.

On the ASBR only (Summarizes external routes before injecting them into theOSPF domain.)

Router(config-router)# summary-address network-address

subnet-mask

Route Summarization


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141

RTA

router ospf 100

summary-address 128.213.64.0 255.255.224.0

redistribute bgp 50 metric 1000 subnets (later)

RTD

router ospf 100

summary-address 128.213.96.0 255.255.224.0

redistribute bgp 20 metric 1000 subnets (later)

Route Summarization

142

Injecting Default Routes into OSPF

By default, 0.0.0.0/0 route is not propagated from the ASBR to otherrouters.

An autonomous system boundary router (ASBR) can be forced togenerate a default route into the OSPF domain.

As discussed earlier, a router becomes an ASBR whenever routes areredistributed into an OSPF domain.

However, an ASBR does not, by default, generate a default route intothe OSPF routing domain.


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The way that OSPF generates default routes (0.0.0.0) varies dependingon the type of area the default route is being injected into.

Stub and Totally Stubby Areas

For stub and totally stubby areas, the area border router (ABR) to thestub area generates a summary link-state advertisement (LSA) with thelink-state ID 0.0.0.0.

This is true even if the ABR doesn't have a default route.

In this scenario, you don't need to use the default-information

originate command.


144

All routers in the area must be configured as stub including the ABR:

router ospf 1

area 1 stub

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Stub Area

LSA 4

LSA 4

X BlockedLSA 5

Blocked XLSA 5

LSA 3

LSA 3


Stub Area


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All routers in the area must be configured as stub except the ABR stub no summary:

ABR: router ospf 1 Other: router ospf 1

area 1 stub no-summary area 1 stub

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID

192.168.3.1/32Pri 100Pri 200

Lo - RouterID

192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X BlockedLSA 5

Blocked XLSA 5

LSA 3

LSA 3


X

Totally Stubby Area

146

Normal Areas

By default, in normal areas routers don't generate default routes.

To have an OSPF router generate a default route, use the default-information originate command.

This generates an external type-2 link with link-state ID 0.0.0.0 andnetwork mask 0.0.0.0.

This command should only be used on the ASBR.

Some documentation states this command works only on anASBR while other documentation states this command turns arouter into an ASBR.



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To have OSPF generate a default route use the following:

router ospf 10

default-information originate [always] [metric metric-

value] [metric-type type-value] [route-map map-name]


148

There are two ways to generate a default.

1) default-information originate

If the ASBR already has the default route (ip route 0.0.0.0 0.0.0.0),you can advertise 0.0.0.0 into the area.

2) default-information originate always

If the ASBR doesn't have the route (ip route 0.0.0.0 0.0.0.0), you canadd the keyword always to the default-information originate

command, and then advertise 0.0.0.0.

You should be careful when using the always keyword. If your routeradvertises a default (0.0.0.0) inside the domain and does not have adefault itself or a path to reach the destinations, routing will bebroken.



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149

ASBRrouter ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

default-information originate

ip route 0.0.0.0 0.0.0.0 10.0.0.2

ASBR

ABR-1 ABR-2

Internal

Area 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

0.0.0.0/00.0.0.0/0

0.0.0.0/0

0.0.0.0/0 0.0.0.0/0

0.0.0.0/0

0.0.0.0/0


150

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

default-information originate always

ip route 0.0.0.0 0.0.0.0 10.0.0.2

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24

172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

0.0.0.0/00.0.0.0/0

0.0.0.0/0

0.0.0.0/0 0.0.0.0/0

0.0.0.0/0

No 0.0.0.0/0 route, butpropagated anyway oralways



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151

E1 vs. E2 External Routes

External routes fall under two categories, external type 1 andexternal type 2.

The difference between the two is in the way the cost (metric) of theroute is being calculated.

A type 1 (E1) cost is the addition of the external cost and the internalcost used to reach that route.

The cost of a type 2 (E2) route is always the external cost,irrespective of the interior cost to reach that route.

Type 2 (E2) is the default!

Redistributing External Routes

152

router ospf 1

redistribute routing-protocolmetric-type [1|2]

metric-type 1 - A type 1 cost is the addition of the external cost andthe internal cost used to reach that route.

redistribute rip [metric value] metric-type 1

metric-type 2 - The cost of a type 2 route is always the external cost,irrespective of the interior cost to reach that route.

redistribute rip [metric value] metric-type 2

We will look at this command, along with internal/external costs, laterin the chapter discussion route redistribution.



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metric-type 2 - The cost of a type 2 route is always the external cost,irrespective of the interior cost to reach that route.

redistribute rip [metric value]metric-type 2

More later, but here is a taste of the metric valueoption

If a value is not specified for the metric valueoption, and no value isspecified using th

multiarea ospf slides

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