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 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 No areas. OSPF vs RIP

A comparison of routing philosophies Distance-vector routingLink-state routing Each 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 Eample A) Using the topological below, write the routing table for Router A if it runs: RIP OSPF B) 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 DestcostNext hop CADCAD DestcostNext hop BAEBAE DestcostNext hop EBDEBD DestcostNext hop ABCDABCD DestcostNext hop DACEDACE A's RT C's RT B's RT E's RT D's RT

6 DestcostNext hop ABCDABCD A's RT Destcost BAEBAE From B Destcost CADCAD From C From D Destcost DACEDACE DestcostNext hop BAEBAE BBBBBB 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. DestcostNext hop ABCDEABCDE B----B A's modified table A's final routing table

7 B) 1- RIP scenario If 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 & D A & 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 Destcost ABCDEABCDE C's update Destcost ABCDEABCDE 011∞2011∞2 Destcost ABCDEABCDE ∞2101∞2101

8 C sends its update with cost to both A & D A will add the cost to reach C and update its table as well as D. The final table for A & D X DestcostNext hop ABCDEABCDE CB---CB A's RT DestcostNext hop ABCDEABCDE CE---CE--- 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.

10 PermanentTentative 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 The permanent list becomes the routing table for A

11 Router A's Table: shortest path Now RouterA knows the best path to each network. DestcostNext hop ABCDEABCDE CC---CC 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. DestcostNext hop ABCDEABCDE CC---CC 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.