CS440 Computer Networks 1

Link State Routing and OSPFLink State Routing and OSPF

Neil TangNeil Tang10/31/200810/31/2008

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OverviewOverview Basic Idea

Flooding

Route Calculation

Distance Vector VS. Link State

OSPF

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Basic IdeaBasic Idea Each router gets information of its direct neighbors and the cost of the

link to each of them by exchanging Hello messages periodically.

Each router disseminates the neighbor and link cost information to all other routers in the network such that each router will have a complete view of the network topology and the link costs.

Each router uses Dijkstra’s algorithm to calculate the shortest paths to all other routers.

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Link State Packet (LSP)Link State Packet (LSP) LSP is used to record the link state (cost) information. It will be

flooded into the network by each router.

An LSP includes the following fields - The address of the router creating this packet - A list of directly connected neighbors, with the cost of the link to each one. - A sequence number - TTL

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FloodingFlooding A router will generate an LSP and broadcast it to all its neighbors. Tho

se neighbors will continue to broadcast it. Next time when the router generates a new LSP, the Sequence Number (SN) will be increased by one.

For a particular router, every other router only keeps the LSP with largest SN from it. When receiving the old (smaller SN) or duplicate (same SN) LSPs, the routers will simply drop them and will not re-broadcast it.

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FloodingFlooding

(a)

X A

C B D

(b)

X A

C B D

(c)

X A

C B D

(d)

X A

C B D

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FloodingFlooding Efficient: Each router must be aware of the change of network status a

s early as possible. LSPs will be flooded periodically and whenever the network status changes.

Low Overhead: Flooding is only conducted when really necessary. A relatively long timer can be set for periodical flooding.

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Route CalculationRoute CalculationThe Dijkstra’s Shortest Path Algorithm (Refer to pp.280 for formal presentation)

10

5

0

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52

1

936

8

5

14

0

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1

936

7

8

5

13

0

10

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1

936

7

8

5

9

0

10

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1

936

7

8

5

9

0

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1

936

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6

1

0

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93

A

B

C

D

E

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Route CalculationRoute CalculationForward Search Algorithm (An implementation of the Dijkstra’s algorithm)

Step Confirmed Tentative

1 (A,0,-) (B,10,B)(C,5,C)

2 (A,0,-)(C,5,C)

(B,8,C)(D,14,C)(E,7,C)

3 (A,0,-)(C,5,C)(E,7,C)

(B,8,C)(D,13,C)

4 (A,0,-)(C,5,C)(E,7,C)(B,8,C)

(D,9,C)

5 (A,0,-)(C,5,C)(E,7,C)(B,8,C)(D,9,C)

6

1

10

52

93A

B

C

D

E

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Distance Vector (DV) VS. Link State (LS)Distance Vector (DV) VS. Link State (LS) Difference: In DV, the distributed Bellman-Ford algorithm is used. Ever

y node talks to only its neighbors but tells them everything it has learned; In LS, the centralized Dijkstra’s algorithm is used. Every node talks to all other nodes via flooding but tells them only its neighborhood information.

Strength: DV needs to store relatively small amount of routing information in each router. LS stabilizes very quickly.

Weakness: DV stabilizes slowly. LS needs to store a large amount of information in each router.

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Open Shortest Path First (OSPF) RoutingOpen Shortest Path First (OSPF) Routing Authentication of routing messages: 8-byte password is used for

authentication to prevent routing hole.

Load balancing: every router maintains multiple routes for a particular destination.

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Control MessagesControl Messages Hello Message

Link State Advertisement (LSA) Message: It advertises the costs of all links incident to that router.

LS Age Options Type=1

0 Flags 0 Number of links

Link type Num_TOS Metric

Link-state IDAdvertising router

LS sequence number

Link IDLink data

Optional TOS informationMore links

LS checksum Length