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| 6.1 |  | OSPF Overview |
| 6.1.1 |  | Issues addressed by OSPF |

OSPF uses link-state technology. OSPF will choose the path via the T3 links. This is opposed to distance vector technology used by protocols such as RIP. RIP will choose the path via the 64k links. Link-state routers maintain a common picture of the network and exchange link information upon initial discovery or network changes. Link-state routers do not broadcast their routing tables periodically like distance vector routing protocols do. While RIP is appropriate for small networks, OSPF was written to address the needs of large, scalable internetworks. OSPF addresses the following issues:

- Speed of convergence In large networks, RIP convergence can take several minutes, since the entire routing table of each router is copied and shared with directly connected neighboring routers. In addition, a distance vector routing algorithm may experience hold down or route aging periods. With OSPF, convergence is faster because only the routing changes, not the entire routing table, are flooded rapidly to other routers in the OSPF network.
- Support for Variable-Length Subnet Masking (VLSM) RIP v1 is a classful protocol and does not support VLSM. In contrast, OSPF, a classless protocol, supports VLSM.

  Note: RIP v2 supports VLSM.

- Network size In a RIP environment, a network that is more than 15 hops away is considered unreachable. Such limitations restrict the size of a RIP network to small topologies. On the other hand, OSPF has virtually no distance limitations and is appropriate for intermediate to large size networks.
- Use of bandwidth RIP broadcasts full routing tables to all neighbors every 30 seconds. This is especially problematic over slow WAN links because these updates consume bandwidth. Alternately, OSPF multicasts minimize the size of link-state updates and send the updates only when there is a network change.
- Path Selection RIP selects a path by measuring the hop count, or distance, to other routers. It does not take into consideration the available bandwidth on the link or delays in the network. In contrast, OSPF selects optimal routes using cost as a factor. Note: "Cost" is a metric based on bandwidth.
- Grouping of members RIP uses a flat topology and all routers are part of the same network. Therefore, communication between routers at each end of the network must travel through the entire network. Unfortunately, changes in even one router will affect every device in the RIP network. OSPF, on the other hand, uses the concept of areas and can effectively segment a network into smaller clusters of routers. By narrowing the scope of communication within areas, OSPF limits traffic regionally and can prevent changes in one area from affecting performance in other areas. This use of areas allows a network to scale efficiently.

Although OSPF was written for large networks, implementing it requires proper design and planning, which is especially important if the network has more than 50 routers. At this size, it is important to configure the network to let OSPF reduce traffic and combine routing information whenever possible.

| Interactive Media Activity Checkbox: RIP versus OSPF Upon completion of this activity, the student will be able to compare routing issues between RIP and OSPF.

| Web Links Open Shortest Path First http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_doc/ospf.ht