Cis185 bsci-lecture4-single area-ospf-review

Single Area OSPF - Review

CIS 185 Advanced Routing

Rick Graziani

Cabrillo College

[email protected]

Last Updated: Fall 2009

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Note My web site is www.cabrillo.edu/~rgraziani. For access to these PowerPoint presentations and other

materials, please email me at [email protected].

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For further information This presentation is an

overview of what is covered in the curriculum/book.

For further explanation and details, please read the chapter/curriculum.

Book: Routing Protocols

and Concepts By Rick Graziani and

Allan Johnson ISBN: 1-58713-206-0 ISBN-13: 978-58713-

206-3

Introduction to OSPF

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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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The network Command

The area area-id refers 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)# network network-address wildcard-mask area area-id

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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 interval Dead interval Network type 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 address ALLSPFRouters 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 IntervalsRouter(config-if)# ip ospf hello-interval seconds

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

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-id command.

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 1”

Outgoing 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

<output omitted>

R2# show ip protocols




Router ID 10.2.2.2

<output omitted>





Router ID 10.3.3.3

<output omitted>

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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 neighbor’s 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

<output omitted>

Verifying OSPF

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Verifying OSPF





Router ID 10.1.1.1

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

Maximum path: 4

Routing 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 originating from this router

OSPF Neighbors

Administrative Distance

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Verifying OSPFR1# show ip ospf

<some output omitted>

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

Minimum hold 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 this area is 3

Area has no authentication

SPF algorithm last 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 calculation depends 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 recalculate the SPF algorithm, preventing proper convergence.

SPF schedule delay. To minimize this problem, the router waits 5 seconds (5000 msec) after

receiving an LSU before running the SPF algorithm. Minimum hold time:

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

The router waits 10 seconds after running the SPF algorithm before rerunning the algorithm.

R1# show ip ospf




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Verifying OSPFR1# show ip ospf interface serial 0/0/0






<output omitted>

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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: <some code output omitted>

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


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

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 by the system administrator. The lower the cost, the more likely the interface is to be 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 used on all routers so the OSPF routing metric remains consistent.

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

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 interface serial 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)# bandwidth bandwidth-kbps

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

R1(config-if)# bandwidth 64

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


R1(config-if)# end


Serial0/0 is up, line protocol is up




<output omitted>

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 use the ip ospf cost command, which allows you to directly specify the cost of an interface.

This will not change the output of the show ip ospf interface command,

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

R1(config-if)# ip ospf cost 1562

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


R1(config-if)# end


Serial0/0 is up, line protocol is up



<output omitted>

100,000,000/64,000 = 1562

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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DROthers only form full adjacencies with the DR and BDR in the network. send their LSAs to the DR and BDR using the multicast address 224.0.0.6 (ALLDRouters, all DR routers).

R1 sends LSAs to the DR. The BDR listens, too.

The DR is responsible for forwarding the LSAs from R1 to all other routers. DR uses the multicast address 224.0.0.5 (AllSPFRouters, all OSPF routers). Only one router doing all the flooding.

DROther

DROther DROther DROther

DROther

DROther

224.0.0.6

224.0.0.5

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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

Designated Router (ID) 192.168.31.33, Interface address 192.168.1.3

Backup Designated router (ID) 192.168.31.22, Interface address 192.168.1.2


<output omitted>

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 will not become the DR or the BDR even if it has a higher OSPF interface priority or router ID than the current DR or BDR.

DR

BDR


DROther

I am a new router with the highest Router ID. I cannot force a new DR or BDR election, so I am a DROther.

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

DR

BDR


DROther

I’m back but I don’t 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


BDR

Amongst the DROthers I have the highest Router ID, so I 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


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/BDR elections.

How can we make sure RouterB is the DR and RouterA is the BDR, regarless of RouterID values?

Want to be DR

Want to be BDR

Highest Router ID

To simplify our 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 interface command.

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 in

another.

Router(config-if)# ip ospf priority {0 - 255}

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OSPF Interface Priority

The OSPF interface priority can be viewed using the show ip ospf interface command.

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

Designated Router (ID) 192.168.31.33, Interface address 192.168.1.3

Backup Designated router (ID) 192.168.31.22, Interface address 192.168.1.2


<output omitted>

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After doing a shutdown and a no shutdown on the Fast Ethernet 0/0 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 ospf priority 100

Pri = 200

Pri = 100

Highest priority wins

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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 of IP packets still takes the lowest cost route, which might be between two DROthers.

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, the default “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 using the loopback as an exit interface because the ISP router in this topology does not physically exist.

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R3’s Routing Table

R3# show ip route

Gateway of last resort is 192.168.10.5 to network 0.0.0.0

192.168.10.0/30 is subnetted, 3 subnets

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




C 172.16.1.32/29 is directly connected, FastEthernet0/0

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, Loopback0

O 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 that route. 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

Steps to OSPF Operation with States

1. Establishing router adjacencies (Routers are adjacent)Down State – No Hello receivedInit State – Hello received, but not with this router’s Router ID

“Hi, my name is Carlos.” “Hi, my name is Maria.”Two-way State – Hello received, and with this router’s Router ID

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

2. Electing DR and BDR – Multi-access (broadcast) segments onlyExStart State with DR and BDRTwo-way State with all other routers

3. Discovering RoutesExStart StateExchange StateLoading StateFull 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 10.6.0.1 10.5.0.1

Hello 10.5.0.1 10.6.0.1

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. When the router sends a Hello packet to the neighbor with its RouterID and the neighbor

sends a Hello packet packet back with that Router ID, the router’s 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

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 change

in topology (this includes when a routers are first booted). OSPF uses hop-by-hop flooding of LSAs; an LSA received on one

interface are flooded out other OSPF enabled interfaces. If a link state entry in the LSDB (Link State DataBase) reaches an age of 60

minutes (MaxAge) without being updated, it is removed and SPF is recalculated.

Every 30 minutes (LSRefreshTime), OSPF routers flood only their link states 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 a value of 60 minutes (MAXAGE).

Single Area OSPF - Review

CIS 185 Advanced Routing

Rick Graziani

Cabrillo College

[email protected]

Last Updated: Fall 2009

Date post:	09-Jun-2015
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