Interior Gateway Protocols (RIP, OSPF)continued…

2

1. It is link-state

2. It has faster convergence

3. No hop limitation

4. Update efficiency: OSPF only sends out changes when they occur.

5. OSPF uses metric to calculate the cost between nodes

6. OSPF uses the concept of area to implement hierarchical routing

1. Distance-vector

2. Can be quite slow to converge

Because of hold-down timer.

3. RIP to limited to 15 hops.

4. RIP sends entire routing table every 30 seconds.

5. Only uses hop count of 1.

6. No areas.

1. Distance-vector

2. Can be quite slow to converge

Because of hold-down timer.

3. RIP to limited to 15 hops.

4. RIP sends entire routing table every 30 seconds.

5. Only uses hop count of 1.

6. No areas.

OSPF vs RIP

A comparison of routing philosophies

Distance-vector routing Link-state routingEach router sends routing information (Table) to its neighbours

The information sent is an estimate of its path cost to all networks

Information is sent on a regular periodic basis.

A router determines next-hop information by using the distributed Bellman-Ford algorithm on the received estimated path costs

Each router sends routing information (Link state) to all other routers

The information sent is the exact value of its link cost to adjacent networks.

Information is sent when changes occur.

A router first builds up a description of the topology of the internet and then may use any routing algorithm to determine next-hop information (usually Dijkstra’s shortest path)

4

A C

D

2

5

B

E

15 2

102

EampleA) Using the topological below, write the routing table for Router A

if it runs: RIP OSPFB) What happens if the link between A & D goes down.

5

A) 1- RIP scenario RIP only uses hop count of 1. Each router will flood its routing table to adjacent (neighbours). Bellman algorithm is used to determine the shortest path. After a slow converge, the entire routing table for every node will

be shared again if there is change in the topology or after a fix time

Dest cost Next hop

CAD

011

---

Dest cost Next hop

BAE

011

---

Dest cost Next hop

EBD

011

---

Dest cost Next hop

ABCD

0111

----

Dest cost Next hop

DACE

0111

----

A's RT

C's RT

B's RT

E's RT

D's RT

6

Dest cost Next hop

ABCD

0111

----

A's RTDest cost

BAE

011

From B

Dest cost

CAD

011

From C From D

Dest cost

DACE

0111

Dest cost Next hop

BAE

122

BBB

Assuming that B's news arrive first

The received table is updated with the cost to reach B then it is compared with A's orignal table

(E,2,B) will be added to A's RT since it does not have this entry

The same process will be repeated for the flooded table from D but this time the entries in A will not change because RIP keeps the older value if no better path (less hops) is found.

Dest cost Next hop

ABCDE

01112

----B

A's modified table

A's final routing table

7

B) 1- RIP scenarioIf the link A-D fails, both routers will update their routing table with

cost ∞(unreachable).Here we don't have instability problem since C has a direct

connection to A & DA & D will send triggered updates (their entire table to their

neighbors. This causes the network to recalculate Bellman algorithm. Finally the network will converge after a while.

X

A's outgoing Update

D's outgoing Update

Dest cost

A

B

C

D

E

1

2

1

1

2

C's update

Dest cost

ABC

DE

011

∞2

Dest cost

ABCDE

∞2101

8

C sends its update with cost to both A & DA will add the cost to reach C and update its table as well as D.

The final table for A & D

X

Dest cost Next hop

A

B

C

D

E

0

1

1

2

2

-

-

-

C

B

A's RT

Dest cost Next hop

A

B

C

D

E

2

2

1

0

1

C

E

-

-

-

D's RT

9

A) 2- OSPF Each router will send LSA to announce its links. The Dijkstra algorithm is calculated to find the best path to each node

and leaf network.

A C

D

2

B

E

2

10

Permanent Tentative

A is the root

C is added to P list and its table is updated with the cost and added to the T

list

D is added to P list

E is added to P list

B is added

A,0,-

C,2,-

D,4,C

E,14,D

B, 15, -

A, 0, -B, 15 ,-C, 2, -

D,5,- updated

D, 4, C

E, 14,D

A C

D

2

B

E

2The permanent list becomes the routing table for A

11

Router A's Table: shortest path Now RouterA knows the best path to each network.

A C

D

2

5

B

E

15 2

102

Dest cost Next hop

A

B

C

D

E

0

15

2

4

14

-

-

-

C

C

A's RT

12

B) when A-D goes down A and D will send out LSA with only one entry to inform the

adjacent nodes. Since this link is not used in the shortest path tree for any node,

OSPF routing table will not change.

A C

D

2

5

B

E

15 2

102

Dest cost Next hop

A

B

C

D

E

0

15

2

4

14

-

-

-

C

C

A's RT: did not change

X

OSPF areas• Partitioning the networks and routers in an AS into subsets called areas.

• Knowledge of an area’s topology remains hidden from other areas. The topological databases have to be maintained only inside the area, so the link state advertisements LSA are flooded only inside that area.

• Less network traffic.

• Less CPU power required in each router.

• Thus, permits growth and makes the networks in an AS easier to manage.

• The areas are connected by a backbone over which the area border routers (ABR) communicate.

• 2 types of routers

– Internal routers having interfaces only in one area or only in backbone.

– Area border routers (ABR) which attach to multiple areas, including the backbone.

What does a OSPF router do? (contd..)

• If it is an AS boundary router :

– Originates AS external link advertisements for each known AS external destination.

• If it is an Area Border Router ABR

– Originates a summary links advertisement describing each known inter-area destination for routers inside the area.

– Originates a summary links advertisement describing each of the networks inside the area, for use of other ABRs

• If it is a Designated router for multicast:

Originates a network link advertisement giving the address of the multicast group, and the router IDs of the routers attached to it.