OSPF Protocol

Marks:- 10+

Name of student: Subhendu Pal

Faculty Name: Priya Sawant 

Roll No: 1503

Date: 31-01-2018

Session Name: OSPF (Open shortest path first )

OSPF (Open shortest path first ):-

OSPF provides a fast convergence with triggered, incremental updates via link-state advertisements (LSAs). OSPF is a classless protocol and allows a hierarchical design with VLSM and route summarization.

The features of OSPF are as follows:-

1. It was developed by IETF
2. Open standard protocol, not Cisco proprietary
3. It is the successor of RIP
4. Uses SPF algorithm developed by Edgar Dijkstra
5. Link state routing protocol
6. Autonomous systems are divided into multiple areas for easy and efficient routing
7. Classless routing protocol
8. Works with Link state Advertisements
9. Updates are through Multicast (224.0.0.5)
10. Hello packets are sent every 10 secs
11. Hierarchical design with Multiple areas
12. Area 0 is called the backbone area

Functioning of OSPF:-

Unlike RIP, which broadcasts out its entire routing table every 30 seconds regardless of whether other routers want to hear it, OSPF takes a more refined approach.

1. First OSPF sends out hello messages to neighboring routers to announce itself as an OSPF router and discovers who its neighbor routers are. Routers have to agree on certain parameters such as timers and being on a common subnet before they can become neighbors

2. After OSPF neighbor routers are discovered, the router determines whether it should exchange routes with those neighbours. If they decided to do so then they become adjacent.

3. If they are adjacent , they begin to exchange routing information using LSAs. After exchanging all routes, the routers send out updates only when there is a change, and they send information only for that affected link, not the entire routing table.

4. Routers take the LSAs heard from adjacent routers and place those routes in its link-state database. (This is similar to topology database in EIGRP)

• To determine the best path, OSPF uses a metric called cost, which Cisco defines as 108/bandwidth. If you have a 100Mbps link, the cost would be 1. Some other costs are:

10 Mbps - 10

1.544 Mbps - 64

64 kbps - 1562

OSPF Interface Cost Table:-

Questions:-

1. Explain OSPF
2. Functions of OSPF