cis185 route lecture3 ospf part1

8/8/2019 Cis185 ROUTE Lecture3 OSPF Part1

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Configuring OSPF Part 1 of 2

CIS 185 CCNP ROUTE

Rick GrazianiCabrillo College

[email protected]

Last Updated: Fall 2010


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2

Topics

Review of OSPF

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


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Single Area OSPF - Review


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Introduction to OSPF

OSPF is:

Classless

Link-state routing protocol

Uses the concept of areas for scalability

RFC 2328 defines the OSPF metric as an arbitrary value called cost.

Cisco IOS software uses bandwidth to calculate the OSPF cost metric.


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5

The network Command

The area area-idrefers to the OSPF area.

A group of OSPF routers that share link-state information.

All OSPF routers in the same area must have the same link-state

information in their link-state databases.This is accomplished by routers flooding their individual link

states to all other routers in the area.

Router(config-router)# networknetwork-address wildcard-mask a

rea area-id


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6

1 Flooding of link-state

information

2 Building a

Topological

Database

3 SPF Algorithm

4 SPF Tree

5 Routing Table

Link State Concepts


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Before two routers can form an OSPF neighbor adjacency, they

must agree on three values:

Hello intervalDead interval

Both the interfaces must be part of the same network, including

having the same subnet mask.

IP MTU must match

Neighbors and

Adjacencies


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Hello Intervals

By default, OSPF Hello packets are sent:

10 seconds on multiaccess and point-to-point segments

30 seconds on nonbroadcast multiaccess (NBMA) segments (Frame

Relay, X.25, ATM).

In most cases, use multicast addressALLSPFRouters at 224.0.0.5.


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Dead Intervals

Cisco uses a default of four times the Hello interval.

40 seconds - Multiaccess and point-to-point segments.

120 seconds - NBMA networks.

Dead interval expires

OSPF removes that neighbor from its link-state database.

Floods the link-state information about the down neighbor out all

OSPF-enabled interfaces.


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Modifying OSPF Intervals

Dead time is counting down from 40 seconds. Refreshed every 10 seconds when R1 receives a Hello from the neighbor.

R1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

10.3.3.3 0 FULL/ - 00:00:35 192.168.10.6 Serial0/0/1

10.2.2.2 0 FULL/ - 00:00:36 192.168.10.2 Serial0/0/0


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Modifying OSPF Intervals

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

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


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Basic OSPF Configuration Lab Topology

The router ospf command

The network command

OSPF Router ID

Verifying OSPF

Examining the Routing Table


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OSPF Router ID is an IP address used to uniquely identify an OSPF router.

Also used in the DR and BDR process.

1.Use the IP address configured with the OSPF router-idcommand.

2.Highest IP address of any of its loopback interfaces.

3.Highest active IP address of any of its physical interfaces.

OSPF Router ID

Router ID?

Router ID?

Router ID?


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Verifying New Router IDs (Loopbacks)R1# show ip protocols

Routing Protocol is ospf 1Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 10.1.1.1

R2# show ip protocols

Routing Protocol is ospf 1

Outgoing update filter list for all interfaces is not set


Router ID 10.2.2.2

R3# show ip protocols

Routing Protocol is ospf 1



Router ID 10.3.3.3


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Verifying OSPF

Neighbor ID: The router ID of the neighboring router.

Pri: The OSPF priority of the interface.

State: The OSPF state of the interface.

Dead Time:

Address: The IP address of the neighbors interface

Interface: Local interface

R1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

10.3.3.3 1 FULL/ - 00:00:30 192.168.10.6 Serial0/0/1

10.2.2.2 1 FULL/ - 00:00:33 192.168.10.2 Serial0/0/0


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R1# show ip ospf interface serial 0/0/0

Serial0/0/0 is up, line protocol is up

Internet Address 192.168.10.1/30, Area 0

Process ID 1, Router ID 10.1.1.1,Network Type POINT_TO_POINT, Cost: 64

Transmit Delay is 1 sec, State POINT_TO_POINT,

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

Verifying OSPF


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Verifying OSPF

R1# show ip protocolsRouting Protocol is ospf 1



Router ID 10.1.1.1

Number of areas in this router is 1. 1 normal 0 stub 0 nssa

Maximum path: 4Routing for Networks:

172.16.1.16 0.0.0.15 area 0

192.168.10.0 0.0.0.3 area 0

192.168.10.4 0.0.0.3 area 0

Reference bandwidth unit is 100 mbps

Routing Information Sources:

Gateway Distance Last Update

10.2.2.2 110 11:29:29

10.3.3.3 110 11:29:29

Distance: (default is 110)

OSPF Process ID

OSPF Router ID

Networks OSPF is

advertising that are

originatingfrom this router

OSPF Neighbors

Administrative Distance


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Verifying OSPFR1# show ip ospf

Routing Process ospf 1 with ID 10.1.1.1

Start time: 00:00:19.540, Time elapsed: 11:31:15.776

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capability

Router is not originating router-LSAs with maximum metric

Initial SPF schedule delay 5000 msecs

Minimumhold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs

Incremental-SPF disabled

Minimum LSA interval 5 secs

Minimum LSA arrival 1000 msecs

Area BACKBONE(0)

Number of interfaces in thisarea is 3

Areahas no authentication

SPFalgorit

hm l

ast executed 11:30:31.628

ago

SPF algorithm executed 5 times


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Verifying OSPF

Any time a router receives new information about the topology (addition,deletion, or modification of a link), the router must:

Rerun the SPF algorithm

Create a new SPF tree

Update the routing table

The SPF algorithm is CPU intensive, and the time it takes for calculationdepends on the size of the area.

R1# show ip ospf



Maximum wait time between two consecutive SPFs 10000 msecs


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Verifying OSPF

A flapping link can cause OSPF routers in an area to constantly recalculatethe SPF algorithm, preventing proper convergence.

If there is a route in the routing table the router will continue to forwardthe packet.

SPF schedule delay.

To minimize this problem, the router waits 5 seconds (5000 msec) afterreceiving an LSU before running the SPF algorithm.

Minimum hold time:

To prevent a router from constantly running the SPF algorithm, there isan additional hold time of 10 seconds (10,000 ms).

The router waits 10 seconds after running the SPF algorithm beforererunning the algorithm.

R1# show ip ospf




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Verifying OSPF




Process ID 1, Router ID 10.1.1.1,Network Type POINT_TO_POINT, Cost: 64




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Examining the Routing Table

Unlike RIPv2 and EIGRP, OSPF does not automatically summarize at major

network boundaries.

R1# show ip route

Codes: D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

192.168.10.0/30 is subnetted, 3 subnets

C 192.168.10.0 is directly connected, Serial0/0/0

C 192.168.10.4 is directly connected, Serial0/0/1

O 192.168.10.8 [110/128] via 192.168.10.2, 14:27:57, Serial0/0/0

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

O 172.16.1.32/29 [110/65] via 192.168.10.6, 14:27:57, Serial0/0/1

C 172.16.1.16/28 is directly connected, FastEthernet0/0


O 10.10.10.0/24 [110/65] via 192.168.10.2, 14:27:57, Serial0/0/0

C 10.1.1.1/32 is directly connected, Loopback0


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The OSPF Metric OSPF Metric

Modifying the Cost of the Link


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OSPF Metric

The OSPF metric is called cost. The following passage is from RFC 2328:

A cost is associated with the output side of each router interface. This

cost is configurable bythe system administrator. The lower the cost, the

more likelythe interface is tobe used to forward data traffic.

RFC 2328 does not specify which values should be used to determine the

cost.


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OSPF Metric

Cisco IOS software uses the cumulative bandwidths of the outgoing

interfaces from the router to the destination network as the cost value.

108 is known as the reference bandwidth

Cisco IOS Cost for OSPF = 108

/bandwidth in bps


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Reference Bandwidth

When this command is necessary, it is recommended that it is usedon all

routers so the OSPF routingmetric remains consistent.

R1(config-router)#auto-co

st reference-

bandwidt

h?

1-4294967 The reference bandwidth in terms of Mbits per second.

R1(config-router)# auto-cost reference-bandwidth 10000

To increase it to 10GigE(10 Gbps Ethernet) speeds, you need to change the reference

bandwidth to 10,000.


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T1 cost 64 + Fast Ethernet cost 1 = 65

The Cost = 64 refers to the default cost of the serial interface,

108/1,544,000 bps = 64, and not to the actual 64-Kbps speed of the link.

R1# show ip route

O 10.10.10.0/24 [110/65] via 192.168.10.2, 14:27:57, Serial0/0/0

OSPF

Accumulates Cost

Serial interfaces bandwidth value

defaults to T1 or 1544 Kbps.


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Default Bandwidth on Serial Interfaces

On Cisco routers, the bandwidth value on many serial interfaces

defaults to T1 (1.544 Mbps).

R1#show interf

ace

seri

al 0/0/0


Hardware is GT96K Serial

Description: Link to R2

Internet address is 192.168.10.1/30

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255


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Modifying the Cost of the Link

The bandwidth command is used to modify the bandwidth value

used by the Cisco IOS software in calculating the OSPF cost metric.

Same as with EIGRP

Router(config-if)#ba

ndwidth

bandwidth-k

bps

R1(config)# inter serial 0/0/0

R1(config-if)# bandwidth 64

R1(config-if)# inter serial 0/0/1

R1(config-if)# bandwidth 256

R1(config-if)# end


Serial0/0 is up, line protocol is up


Process ID 1, Router ID 10.1.1.1, Network Type POINT_TO_POINT, Cost: 1562


100,000,000/64,000 = 1562


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The ip ospf cost Command

An alternative method to using the bandwidth command is to usethe ip ospf cost command, which allows you to directly specifythe cost of an interface.

This will not change the output of the show ip ospf interfacecommand,

R1(config)# interface serial 0/0/0

R1(config-if)# ip ospf cost 1562

R1(config)# inter serial 0/0/0

R1(config-if)# bandwidth 64R1(config-if)# end


Serial0/0 is up, line protocol is up


Process ID 1, Router ID 10.1.1.1, Network Type POINT_TO_POINT, Cost: 1562

100,000,000/64,000 = 1562


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OSPF and Multiaccess

Networks Challenges in Multiaccess Networks

DR/BDR Election Process

OSPF Interface Priority


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Solution: Designated Router

OSPF elects a Designated Router (DR) to be the collection and distribution

point for LSAs sent and received. A Backup Designated Router (BDR) is also elected in case the DR fails.

All other routers become DROthers.


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DR/BDR Election

The following criteria are applied:

1. DR: Router with the highest OSPF interface priority.

2.BDR: Router with the second highest OSPF interface priority.

3.If OSPF interface priorities are equal, the highest router ID is used to

break the tie.

Default OSPF interface priority is 1.

Current configuration, the OSPF router ID is used to elect the DR and BDR.

DR

BDR

DROther


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RouterA# show ip ospf interface fastethernet 0/0

FastEthernet0/0 is up, line protocol is up


Process ID 1, Router ID 192.168.31.11, Network Type BROADCAST, Cost: 1

Transmit Delay is 1 sec, State DROTHER, Priority 1

DesignatedRouter (ID) 192.168.31.33, Interface address 192.168.1.3

Backup Designated router (ID) 192.168.31.22, Interface address192.168.1.2


Verifying Router States


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Timing of DR/BDR Election

If I booted first and started

the election before the

others were ready, I would

be the DR!


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When the DR is elected, it remains the DR until one of the following

conditions occurs:

The DR fails.The OSPF process on the DR fails.

The multiaccess interface on the DR fails.

If the DR fails, the BDR assumes the role of DR, and an election is held to

choose a new BDR.

DR failed! I am now the

DR! Elections will now

happened for BDR

I am now

the BDR!

DR

BDR


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If a new router enters the network after the DR and BDR have been

elected, it willnotbecome the DRor the BDR even if it has a higher

OSPF interface priorityor router ID than the current DRor BDR.

DR

BDR

Timing of

DR/BDR

Election

DROther

I am a new router with the highest

Router ID. I cannot force a new

DR or BDR election, so I am aDROther.


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A previous DR does not regain DR status if it returns to the network.

DR

BDR

Timing of

DR/BDR

Election

DROther

Im back but I dont

get to become DR

again. I am now just a

DROther.

DROther


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If the BDR fails, an election is held among the DROthers to see which router

will be the new BDR.

DR

BDR

Timing of

DR/BDR

Election

BDR

Amongst the

DROthers I have the

highest Router ID, soI am the new BDR!

DROther


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RouterB fails.

Because RouterD is the current BDR, it is promoted to DR.

RouterC becomes the BDR.

DR

BDR

Timing of

DR/BDR

Election

BDR

I am now the new DR!

DROther

I am now the new

BDR!


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We can change the OSPF interface priority to better control our DR/BDRelections.

How can we make sureRouterB is the DR and

RouterA is the BDR,

regarless of RouterID

values?

Want tobe DR

Want tobe

BDR

Highest Router ID

To simplifyour discussion, we

removed RouterD from the topology.


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OSPF Interface Priority

Control the election of these routers with the ip ospf priority interfacecommand.

Priority (Highest priority wins):

0 = Cannot become DR or BDR

1 = Default

Therefore, the router ID determines the DR and BDR.

Priorities are an interface-specific value, they provide better control of the

OSPF multiaccess networks.

They also allow a router to be the DR in one network and a DROther inanother.

Router(config-if)# ip os

pf priority {0 - 255}


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After doing a shutdown and a no shutdown on the Fast Ethernet0/0 interfaces of all three routers, we see the result of the change of

OSPF interface priorities.

RouterA(config)# interface fastethernet 0/0

RouterA(config-if)# ip ospf priority 200

RouterB(config)# interface fastethernet 0/0

RouterB(config-if)# ip os

pf priority 100

Pri = 200

Pri = 100

Highest prioritywins


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Clarifications regarding DR/BDR

Hello packets are still exchanged between all routers on a multi-

access segment (DR, BDR, DROthers,.) to maintain neighbor

adjacencies.

OSPF LSA packets (coming) are packets which are sent from the

BDR/DROthers to the DR, and then from the DR to the

BDR/DROthers. (The reason for a DR/BDR.)

Normal routing ofIP packets still takes the lowest cost route, which

might be between two DROthers.


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More OSPF Configuration Redistributing an OSPF Default Route

Fine-tuning OSPF


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Redistributing

an OSPF

Default Route

If the default-information originate command is not used, thedefault quad zero route will not be propagated to other routers in the

OSPF area.

R1(config)# interface loopback 1

R1(config-if)# ip add 172.30.1.1 255.255.255.252

R1(config-if)# exit

R1(config)# ip route 0.0.0.0 0.0.0.0 loopback 1

R1(config)# router ospf 1

R1(config-router)# default-information originate

The static default route is usingthe

loopback as an exit interface

because the ISP router in this

topologydoes not physicallyexist.


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R3s Routing Table

R3# show ip route

Gateway of last resort is 192.168.10.5 to network 0.0.0.0


O 192.168.10.0 [110/1952] via 192.168.10.5, 00:00:38, S0/0/0

C 192.168.10.4 is directly connected, Serial0/0/0C 192.168.10.8 is directly connected, Serial0/0/1



O 172.16.1.16/28 [110/391] via 192.168.10.5, 00:00:38, S0/0/0


C 10.3.3.3/32 is directly connected, Loopback0O 10.10.10.0/24 [110/782] via 192.168.10.9, 00:00:38, S0/0/1

O*E2 0.0.0.0/0 [110/1] via 192.168.10.5, 00:00:27, Serial0/0/0


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External Type 2 Route

E2 denotes that this route is an OSPF External Type 2 route.

OSPF external routes fall in one of two categories:

External Type 1 (E1)

External Type 2 (E2)

OSPF accumulates cost for an E1 route as the route is being propagated

throughout the OSPF area.

This process is identical to cost calculations for normal OSPF internal routes.

E2 route is always the external cost, irrespective of the interior cost to reach thatroute.

In this topology, because the default route has an external cost of 1 on the

R1 router, R2 and R3 also show a cost of 1 for the default E2 route.

E2 routes at a cost of 1 are the default OSPF configuration.

More later

R3# show ip route

O*E2 0.0.0.0/0 [110/1] via 192.168.10.5, 00:00:27, Serial0/0/0


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Steps to OSPF Operation with States

1. Establishing router adjacencies (Routers are adjacent)

Down State No Hello received

Init State Hello received, but not with this routers Router ID

Hi, my name is Carlos. Hi, my name is Maria.

Two-way State Hello received, and with this routers Router ID

Hi, Maria, my name is Carlos. Hi, Carlos, my name is Maria.

2. Electing DR and BDR Multi-access (broadcast) segments only

ExStart State with DR and BDR

Two-way State with all other routers

3. Discovering Routes

ExStart State

Exchange State

Loading State

Full State (Routers are fully adjacent)

4. Calculating the Routing Table

5. Maintaining the LSDB and Routing Table


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Hello 10.6.0.1

Hello 10.5.0.1

Hello 10.6.0.110.5.0.1

Hello 10.5.0.110.6.0.1

DownInit DownInit2-way 2-way

Down State - Init State Two Way State

Down State - OSPF routers send Hello packets at regular intervals (10 sec.) to establish

neighbors.

When a router (sends or) receives its first Hello packet, it enters the init state.Hello packet contains a list of known neighbors.

When the router sends a Hello packet (unicast reply) to the neighbor with its RouterID and

the neighbor sends a Hello packet packet back with that Router ID, the routers interface

will transition to the two-way state.

Now, the router is ready to take the relationship to the next level.

1. Establishing Adjacencies


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Steps to OSPF Operation with States (cont)

Explanations in Notes Section


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Couple of notes on link state flooding

OSPF is a link state routing protocol and does not send periodic updates

like RIP.

OSPF only floods link state state advertisements when there is a changein topology (this includes when a routers are first booted).

OSPF uses hop-by-hop flooding of LSAs; an LSA received on oneinterface are flooded out other OSPF enabled interfaces.

If a link state entry in the LSDB (Link State DataBase) reaches an age of60minutes (MaxAge) without being updated, it is removed and SPF isrecalculated.

Every 30 minutes (LSRefreshTime), OSPF routers flood only their linkstates to all other routers (in the area).

This is known as a paranoid update

These do not trigger SPF recalculations. Special note: When a link goes down and a router wants to send a LSA to

tell other routers to remove this link state, it sends this link state with avalue of60 minutes (MAXAGE).


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Single Area OSPF

End of Review

CIS 185 Advanced Routing

Rick GrazianiCabrillo College

[email protected]


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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 rebuildthe 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(autonomous systems) into smaller internetworks that are called areas.

With this technique, routing still occurs between the areas (called inter-arearouting).

Some operations are restricted within an area:

Flooding of LSAs

Recalculating the database

Re-running the SPF algorithm


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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

least one interface connected to an external internetwork (another

autonomous system)

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

multiple areas.


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Question: I understand the routing table is recalculated every time the router receives

an new version of an LSA. Does OSPF recalculate its routing table when their is atopology change in another area? show ip ospf displays no change in SPF execution, but

show ip ospf database shows a change in the topology?

Answer: Good question! OSPF areas are designed to keep issues like flapping links

within an area.

SPF is not recalculated if the topology change is in another area.The interesting thing is that OSPF distributes inter-area (between areas) topology

information using a distance-vector method.

OSPF uses link-state principles only within an area.

ABRs do not announce topological information between areas, instead, only routing

information is injected into other areas.

ABRs relay routing information between areas via distance vector technique similar

to RIP or EIGRP.

This is why show ip ospf does not show a change in the number of times SPF has

been executed when the topology change 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 any changes in one area from

affecting routing tables in other areas.

An advantage of Multiple Areas


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OSPF Packet Types

In CCNA we discussed various OSPF packets

OSPF packet types


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OSPF packet types

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


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LSAs used for discovering routes and reaching Full State, along with

Maintain 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 tobuild multicast distribution trees for the forwarding of multicast traffic.

LSA Type 7 NSSA External Link Entry

Next presentation!

LSA Type 8 External attributes LSA for BGP

Not supported by Cisco N/A

LSA Type 9, 10, or 11 Opaque LSAs

Future upgrades


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Area Types

Standard or Normal Areas

Backbone

Non-Backbone

Stub Areas

Stub Area

Totally Stubby Area

Not-so-stubby-area (NSSA)


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Area Types


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Part I - LSAs using all normal areas

Multi Area OSPF

Normal Areas

ASBR

ABR ABRInternal

Internal

Internal

Internal

Backbone

Area

What are the router

types?


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Routes Received on all OSPF Routers

Overview ofNormal 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, LSA4, LSA 5

Receives all routes from External A.S.s (External AS means routes notfrom this OSPF routing domain):

From external ASs LSA 5

As long as routes are being redistributed by the ASBR (more later)

Default Route

Received only ifdefault-information-originate command was used(later)

Ifdefault-information-originate command is not used, then the

default route is not received



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1. OSPF Multi-Areas - All Normal Areas

R33

router ospf 1network 172.16.1.0 0.0.0.255 area 1

network 172.30.1.0 0.0.0.255 area 1

R22router ospf 1

network 172.16.1.0 0.0.0.255 area 1

network 172.30.2.0 0.0.0.255 area 1

R1router ospf 1

network 10.0.0.0 0.0.0.3 area 0

network 9.0.0.0 0.0.0.3 area 0

network 172.16.1.0 0.0.0.255 area 1

network 172.16.2.0 0.0.0.255 area 1

R2router ospf 1

network 192.168.2.0 0.0.0.255 area 0

network 10.0.0.0 0.0.0.3 area 0

network 11.0.0.0 0.0.0.3 area 0

default-information originate

ip route 0.0.0.0 0.0.0.0 serial 0/2

R3router ospf 1

network 11.0.0.0 0.0.0.3 area 0

network 9.0.0.0 0.0.0.3 area 0

network 172.16.10.0 0.0.0.255 area 51

network 172.16.11.0 0.0.0.255 area 51

network 99.0.0.0 0.0.0.3 area 51

R100router ospf 1

network 99.0.0.0 0.0.0.3 area 51

network 99.1.0.0 0.0.255.255 area 51

network 99.0.0.4 0.0.0.3 area 51

R200router ospf 1

network 99.0.0.4 0.0.0.3 area 51network 99.0.0.0 0.0.255.255 area 51

ABR contains network statements for

each area it belongs to, using the

proper area value.


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Multi Area OSPF

Normal Areas

ASBR

ABR ABRInternal

Internal

Internal

Internal

Backbone

Area

What are the routertypes?


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Multi Area OSPF

Normal Areas

ASBR

ABR ABRInternal

Internal

Internal

Internal

Backbone

Area



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Multi Area OSPF

Normal Areas

ASBR

ABR ABRInternal

Internal

Internal

Internal

Backbone

Area



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Understanding LSAs (FYI ONLY) 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.

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 directlyconnected.

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

When a new LSA 1 is received and installed in the LSDB, the router forwards

that LSA, using hop-by-hop or asynchronous flooding.

A C

D

2

5

B

15 RouterAs LSA1s

which are flooded to all

other routers in thisarea.

Leaf network

LSA 1 - Router 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 | 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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LSA 1 Router Link States

LSA 1sLSA 1s

LSA 1s

Each router floods their LSA 1s ONLY within their own area.

LSA 1s only announce the links (networks) within the area.

Router receives LSA 1s from neighbor, floods those LSA 1s to other

neighbors within the same area.


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For Router Links:

Link State ID: Advertising Router ID Advertising Router: Router ID of the router that created this LSA 1

Bottom line: Router Link States (LSA1s) should display all the RouterIDs of

routers in that area, including its own.

Ricks reminder: LSA 1 -> my one area

R100# show ip ospf database

OSPF Router with ID (100.100.100.100) (Process ID 1)

Router Link States (Area 51)


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R100# show ip route


O 172.16.10.0 [110/65] via 99.0.0.1, 00:08:30, Serial0/0

O 172.16.11.0 [110/65] via 99.0.0.1, 00:08:30, Serial0/0


Denoted by just an O in the routing table, or a C

Note: Only partial routing tables will be shown


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LSA 1sLSA 1s

LSA 1s



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81

LSA 2 Network LSA

Generated by the DR on every multi-access network Denoted by just an O in the routing table orC if the network is

directly connected.

Flooded only within the originating area.

LSA 2s are in link state database forall routers within area, even

those routers on not on multi-access networks or DRs on other multi-access networks 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 30 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 |

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


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| LS sequence number |

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


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

| Network Mask |

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

| AttachedRouter |

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

| ... |



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LSA 2s

LSA 2s flooded within area by DR.

LSA 2s

LSA 2s


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85

LSA 2s

LSA 2s



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86

LSA 3 Summary LSA

Originated by the ABR.

Describes links between ABR and Internal Routers of the Local Area


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) in the

routing table.

LSA 3 Summary Net Link States


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LSA 3 Summary LSAs

LSA 3 Summary LSA Originated by the ABR.

Describes links between ABR and Internal Routers of the Local Area


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) in the

routing table.

LSA 3sLSA 1s

LSA 3s

ABRABR


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LSA 3 Summary LSAs

LSA 1sLSA 3s

LSA 3s

ABR ABR


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89

LSA 3 Summary LSAs

LSA 3sLSA 3s

LSA 1s


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90

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 |

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


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| LS sequence number |

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


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

| Network Mask |

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

| 0 | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| TOS | TOS metric |

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

| ... |


LSA 3 S N t Li k St t


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91

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.

LSA 3sLSA 1s

LSA 3s

Process

using DV

technique

not LSA 1

Link States.

New or

change, do

not run SPF

algorithm.


X


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Area 0

Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 3 LSA 3

LSA 1s

Not ABR

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 notforward 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.

Normal Areas


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Area 0

Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 3

LSA 3

Normal Areas

LSA 1s

Not ABR

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

Note: RTC will create LSA 1s and flood them within the appropriatearea. OSPF specification states that ABRs are restricted to considering LSA 3s only

from the backbone area to avoid routing information loops.

Normal AreasUpdate is sent to Area 0 and Area


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Area 0

Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 1s

LSA 3

LSA 3

X

Area 1 routers re-run

SPF, creates new SPF

tree and updatesrouting table.

Update is sent to Area 0 and Area

51 routers using a distance

vector update technique. SPF not

re-run, but routers update routing

table.

TopologyChange: Down Link

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

Age of the LSA is set to MaxAge (3,600 seconds) Routers removethis entry from their LSDB (Link State Data Base).

Routers that receive the LSA 1s, within the area of the change: Re-run their SPF algorithm Build a new SPF tree

Update IP routing tables. (Continued next slide)

Normal AreasUpdate is sent to Area 0 and Area


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Area 0

Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 1s

LSA 3

LSA 3

X

Area 1 routers re-run

SPF, creates new SPF

tree and updates

routing table.

Update s se t to ea 0 a d ea

51 routers using a distance

vector update technique. SPF not

re-run, but routers update routing

table.

TopologyChange: 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.


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Summary Net Link States (Area 1)Link ID ADV Router Age Seq# Checksum

10.0.0.0 1.1.1.1 130 0x8000000c 0x00ec09

9.0.0.0 1.1.1.1 130 0x8000000d 0x00ec09

192.168.2.0 1.1.1.1 130 0x8000000e 0x00ec09

11.0.0.0 1.1.1.1 130 0x8000000f 0x00ec09

172.16.10.0 1.1.1.1 130 0x80000010 0x00ec09

172.16.11.0 1.1.1.1 130 0x80000011 0x00ec09

99.0.0.0 1.1.1.1 130 0x80000012 0x00ec09

99.0.0.4 1.1.1.1 130 0x80000013 0x00ec09

99.1.0.0 1.1.1.1 130 0x80000014 0x00ec09

LSA 3 Summary Net Link States (INTERNAL)

Link ID = IP network addresses of networks in other areas ADV Router = ABR Router ID sending the LSA-3

Bottom line: Should see networks in other areas and the ABR advertising thatroute.

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

ABR

S 3 S S ( )


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98


Summary Net Link States (Area 1) networks sent by the A B R

1 2 3



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99

R2# show ip route


O IA 99.0.0.0/30 [110/1626] via 11.0.0.2, 00:43:01, Serial0/1

O IA 99.0.0.4/30 [110/1627] via 11.0.0.2, 00:43:01, Serial0/1

O IA 99.1.0.0/16 [110/1627] via 11.0.0.2, 00:43:01, Serial0/1


O IA 172.16.1.0 [110/65] via 10.0.0.1, 00:42:21, Serial0/0

O IA 172.16.2.0 [110/65] via 10.0.0.1, 00:42:51, Serial0/0O IA 172.16.10.0 [110/1563] via 11.0.0.2, 00:43:01, Serial0/1

O IA 172.16.11.0 [110/1563] via 11.0.0.2, 00:43:01, Serial0/1


O IA 172.30.1.0 [110/66] via 10.0.0.1, 00:42:21, Serial0/0

O IA 172.30.2.0 [110/66] via 10.0.0.1, 00:42:21, Serial0/0


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


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LSA 4 ASBR Summary LSA

Originated by the ABR. Flooded throughout the area.

Describes the reachability to the ASBRs

Advertises an ASBR (Router ID) not a network

Included in routing table as an IA route.

Exceptions

Not flooded to Stub and Totally Stubby networks.

More on this later

LSA 4 ASBR

Summary 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 |

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


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


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| LS checksum | length |

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

| Network Mask |

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

| 0 | metric |

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

| TOS | TOS metric |

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

| ... |

LSA 4 ASBR Summary Link States

LSA 4 ASBR S Li k St t


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How does the ABRs know about the ASBR?

ASBR sends a type 1 Router LSA with a bit (external bit e bit) that

is set to identify itself as the ASBR.

LSA 1s

(e bit)

LSA 4

LSA 4

LSA 4 ASBR S Li k St t (ABR)


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104


Summary ASB Link States (Area 1)

Link IDADV

Router

Age Seq#

Check

sum

2.2.2.2 1.1.1.1 1482 0x8000000b 0x00ec09

LSA 4 ASBR Summary Link States (ABR)

Link ID - Router ID of ASBR ADV Router - 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 R

ASBR (This) ABR

ABR

LSA 4 ASBR Summary Link States (INTERNAL)


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Summary ASB Link States (Area 1)

Link ID ADVRouter Age Seq# Checksum

2.2.2.2 1.1.1.1 130 0x8000000b 0x00ec09

LSA 4 ASBR Summary Link States (INTERNAL)

Link ID - Router ID of ASBR ADV Router - 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

ASBR (Advertising) ABR

ABR



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LSA 1s

e bitLSA 4

LSA 4


LSA 5 - AS External


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LSA 5 AS External LSA

Originated by the ASBR. Describes destination networks external to the Autonomous System (This

OSPF Routing Domain)

Flooded throughout the OSPF AS except to stub and totally stubby areas

Denoted in routing table as E1 or E2 (default) route (soon)

ASBR Router which redistributes routes into the OSPF domain.

Exceptions

Not flooded to Stub and Totally Stubby networks.

More on this later

LSA 5 AS External

Link States

LSA 5 AS External 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 | 5 |

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

| Link State ID |

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


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


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


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

| Network Mask |

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

|E| 0 | metric |

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

| Forwarding address |

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

| External Route Tag |

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

|E| TOS | TOS metric |

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

| Forwarding address |

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

| External Route Tag |

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

| ... |

LSA 5 - AS External Link States


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R2 (ASBR)

router ospf 1

redistribute static


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110

Redistribute command creates an ASBR router.

LSA 5sOriginated by the ASBR.

Describes destination networks external to the OSPF Routing Domain

Flooded throughout the OSPF AS except to stub and totally stubbyareas

ip route 57.0.0.0 255.0.0.0 ser 0/3

LSA 5sLSA 5

LSA 5

ASBR


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Type-5 AS External Link States O T HE R networks1 2 3 4 5

Note: Packet Tracer does not support LSA 5s

for redistributed routes

R2 (ASBR)router ospf 1

redistribute static

default-information originate

ip route 0.0.0.0 0.0.0.0 ser 0/2

ip route 57.0.0.0 255.0.0.0 ser 0/3



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R1# show ip route

O E2 57.0.0.0/8 [110/20] via 10.0.0.2, 00:16:02, Serial0/0

O*E2 0.0.0.0/0 [110/1] via 10.0.0.2, 00:16:02, Serial0/0

Designated by E2 Notice that the cost is 20 for all redistributed 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 forOSPF where thedefault cost is 20.

Cost of 1 for the redistributed route.

LSA 5 AS External Link States



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Type-5 AS External Link States


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114

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 the route

is being calculated.

The cost of a type 2 route is always the external cost, irrespective of theinterior 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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Stub Areas

Stub Areas


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116

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. More than

one ABR can be used, but be ready to accept non-optimal routing

paths.

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).

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 depending upon

the size of the network.


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Stub Area

Stub Areas


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118

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 ABRExternal Routes: Once the ABR gets a packet headed to a default route, it must havea default route, either static or propagated by the ASBR via default informationoriginate (coming!)

Configuration:

All routers in the area must be configured as stub

Stub Areas

Stub Areas

Additi l C d


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R3 (ABR)

router ospf 1

area 51 stub


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Stub Area

LSA 4 LSA 4 Blocked

LSA 5 BlockedLSA 5

LSA 3LSA 3

Defaultroute to

ABR

injected

Sent by ABR: LSA 3s (Inter-Area routes)

Blocked: LSA 4s (reachability to ASBR) LSA 5s (External routes)

The ABR injects a default route into the stub area, pointing to the ABR. This does not mean the ABR has a default route of its own.

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

We only see routes in

our area, other areas,

and a default route.

No external routes.

each area.

Stub Areas


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Summary Net Link States (Area 51)

Link ID ADV Router Age Seq# Checksum

9.0.0.0 3.3.3.3 1752 0x80000037 0x00ba22

0.0.0.0 3.3.3.3 1612 0x80000038 0x00ca5011.0.0.0 3.3.3.3 625 0x80000039 0x00db11

192.168.2.0 3.3.3.3 614 0x8000003a 0x00dd10

10.0.0.0 3.3.3.3 614 0x8000003b 0x00dd10

172.16.2.0 3.3.3.3 614 0x8000003c 0x00dd10

172.16.1.0 3.3.3.3 614 0x8000003d 0x00dd10

172.30.2.0 3.3.3.3 614 0x8000003e 0x00dc11

172.30.1.0 3.3.3.3 614 0x8000003f 0x00dc11

No LSA 4s orLSA 5s for stub area routers. Default Route injected by ABR (LSA 3)

Stub Areas

Stub Areas


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R200# show ip route

9.0.0.0/30 issubnetted, 1 subnets

O IA 9.0.0.0 [110/129] via 99.0.0.5, 00:25:52, FastEthernet0/0






O 99.0.0.0/30 [110/65] via 99.0.0.5, 00:25:52, FastEthernet0/0


O 99.1.0.0/16 [110/2] via 99.0.0.5, 00:25:52, FastEthernet0/0





O 172.16.10.0 [110/66] via 99.0.0.5, 00:25:52, FastEthernet0/0


172.30.0.0/24 issubnetted, 2 subnetsO IA 172.30.1.0 [110/1693] via 99.0.0.5, 00:25:52, FastEthernet0/0


O IA 192.168.2.0/24 [110/1628] via 99.0.0.5, 00:25:52, FastEthernet0/0


C 200.200.200.200 is directly connected, Loopback0

O*IA 0.0.0.0/0 [110/66] via 99.0.0.5, 00:25:52, FastEthernet0/0

LSA 1s (Within area)

LSA 3s (Other areas)

No LSA 4s (ASBR)

No LSA 5s (External routes)

Default Route (Injected by ABR)

NOTE on default route:

ABR will advertise a default route with a cost of 1 cost of65 = 1 (Default) +1 (Fa) +64 (serial link) The default cost can be modified with the ospf command:

ABR(config-router)# area area-iddefault-

cost cost

Stub Areas


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R3# show ip route




C 9.0.0.0 is directly connected, Serial0/2


O 10.0.0.0 [110/1626] via 11.0.0.1, 00:00:41, Serial0/3




C 99.0.0.0/30 is directly connected, Serial0/0

O 99.0.0.4/30 [110/65] via 99.0.0.2, 00:00:46, Serial0/0

O 99.1.0.0/16 [110/65] via 99.0.0.2, 00:00:46, Serial0/0


O IA 172.16.1.0 [110/1627] via 11.0.0.1, 00:00:31, Serial0/3

O IA 172.16.2.0 [110/1627] via 11.0.0.1, 00:00:31, Serial0/3

C 172.16.10.0 is directly connected, FastEthernet0/0

C 172.16.11.0 is directly connected, FastEthernet0/1172.30.0.0/24 is subnetted, 1 subnets

O IA 172.30.1.0 [110/1628] via 11.0.0.1, 00:00:01, Serial0/3

O 192.168.2.0/24 [110/1563] via 11.0.0.1, 00:00:41, Serial0/3

O*E2 0.0.0.0/0 [110/1] via 11.0.0.1, 00:00:41, Serial0/3

Stub Areas

Notice, there is no automatic default route on ABR, as there are with the

internal stub routers. This default route came from the ASBR. In other words the ABR will inject the default route into the stub

area whether or not it has a default route in its routing table.


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125

Totally Stubby

Area

Totally Stubby


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126

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

Areas

Totally Stubby


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127

R1: (ABR)

router ospf 1

area 1 stub no-summary

^^ Command: area area stubno-summary

R22 andR33: (INTERNAL ROUTERS)

router ospf 1

area 1 stub

^^ Command: area area stub

Areas

Totally

LSA 1s still

sent within

each area.


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128

Stub Area

LSA 4 LSA 4 Blocked

LSA 5 BlockedLSA 5

LSA 3LSA 3

Defaultroute to

ABR

injected

Blocked:

LSA 3s (Inter-Area routes) LSA 4s (reachability to ASBR) LSA 5s (External routes)


Changes in other areas and external routes no longer affect Stub Area routing tables.




No external routes.

Stubby Area

Blocked

BlockedBlocked

Default

route to

ABR

injectedWe only see routes in our area


No inter-area or external routes.

Totally Stubby


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129

R33# show ip route


C 33.33.33.33 is directly connected, Loopback0172.16.0.0/24 issubnetted, 2 subnets






O*IA 0.0.0.0/0 [110/2] via 172.16.1.1, 00:02:13, FastEthernet0/0

Areas

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)

Note: Packet Tracer does not support

Totally Stubby Networks (yet)



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R1# show ip route








O 11.0.0.0 [110/1626] via 10.0.0.2, 00:05:26, Serial0/0


O IA 99.0.0.0/30 [110/1690] via 10.0.0.2, 00:05:26, Serial0/0

O IA 99.0.0.4/30 [110/1691] via 10.0.0.2, 00:05:26, Serial0/0

O IA 99.1.0.0/16 [110/1691] via 10.0.0.2, 00:05:26, Serial0/0



C 172.16.2.0 is directly connected, FastEthernet0/1O IA 172.16.10.0 [110/1627] via 10.0.0.2, 00:05:26, Serial0/0

O IA 172.16.11.0 [110/1627] via 10.0.0.2, 00:05:26, Serial0/0




O 192.168.2.0/24 [110/65] via 10.0.0.2, 00:05:26, Serial0/0

O*E2 0.0.0.0/0 [110/1] via 10.0.0.2, 00:05:26, Serial0/0

y y

Notice, there is no automatic default route on ABR, as thereare with the internal stub routers.

This default route came from the ASBR. In other words the ABR will inject the default route into

the stub area whether or not it has a default route in its

routing table.

Quick Review


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LSA 1s Router LSAs


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132

show ip ospf database Router Link States (LSA 1s)Should display all the RouterIDs of routers in that area,

including its own.

show ip route O routes

Routes within that area

LSA 1sLSA 1s

LSA 1s

LSA 2s Network LSAs


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show ip ospf database Net Link States (LSA 2s)Net Link States (LSA2s) should display the RouterIDs of theDRs on all multi-access networks in the area and their IPaddresses.

show ip route O routes

Routes within that area

LSA 2s

LSA 2s

LSA 3 Summary LSAs


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LSA 3sLSA 3s

LSA 1s

show ip ospf database Summary Net Link States (LSA 3s)Link ID = IP network addresses of networks in other areas

ADV Router = ABR Router ID sending the LSA-3

show ip route IA (Inter-Area Routes)

Routes in other areas



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show ip ospf database Summary Net Link States (LSA 3s)Link ID = IP network addresses of networks in other areas

ADV Router = ABR Router ID sending the LSA-3

show ip route IA (Inter-Area Routes)

Routes in other areas

LSA 1s

ebitLSA 4

LSA 4

R2 (ASBR)

LSA 5 External Link States


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136

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

R2 (ASBR)

router ospf 1

redistribute static

ip route 57.0.0.0 255.0.0.0 ser 0/3

LSA 5sLSA 5

LSA 5

Stub Area LSA 1s stillsent within

each area.


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137

Stub Area

LSA 4 LSA 4 Blocked

LSA 5 BlockedLSA 5

LSA 3LSA 3

Defaultroute to

ABR

injected

Sent by ABR: LSA 3s (Inter-Area routes)

Blocked: LSA 4s (reachability to ASBR) LSA 5s (External routes)


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




No external routes.

Totally

St bb A

Totally Stubby Area LSA 1s stillsent within

each area.


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Stub Area

LSA 4 LSA 4 Blocked

LSA 5 BlockedLSA 5

LSA 3LSA 3

Defaultroute to

ABR

injected

Blocked:

LSA 3s (Inter-Area routes) LSA 4s (reachability to ASBR) LSA 5s (External routes)


Changes in other areas and external routes no longer affect Stub Area routing tables.




No external routes.

Stubby Area

Blocked

BlockedBlocked

Default

route to

ABR

injectedWe only see routes in our area


No inter-area or external routes.


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Multi Area OSPF Part 1 of 2

CIS 185 Advanced Routing

Rick Graziani

Cabrillo College

[email protected]

cis185 route lecture3 ospf part1

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