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| 7.1 |  | IS-IS Fundamentals |
| 7.1.5 |  | OSPF versus IS-IS |

The configuration of OSPF is based on a central backbone, Area 0, with all other areas being physically attached to Area 0. Because of this, certain design constraints will inevitably exist. A good, consistent IP addressing structure is necessary when this type of hierarchical model is used. It is used to summarize addresses into the backbone, and reduce the amount of information that is carried in the backbone and advertised across the network. In comparison, IS-IS also has a hierarchy with Level 1 and Level 2 routers. With IS-IS, the area borders lie on the links. However, significantly fewer link-state PDUs (LSPs) are used. Therefore, many more routers, up to 1000, can reside in a single area. This capability makes IS-IS more scalable than OSPF. IS-IS allows a more flexible approach to extending the backbone. Adding Level 2 routers can extend the backbone. This process is less complex than with OSPF.

With regard to CPU use and the processing of routing updates, IS-IS is more efficient. Not only are there fewer LSPs to process, as compared to OSPF LSAs, but also the mechanism by which IS-IS installs and withdraws prefixes is less intensive.

Both OSPF and IS-IS are link-state protocols and therefore provide fast convergence. The convergence time depends on a number of factors, such as timers, number of nodes, and types of routers. Based on the default timers, IS-IS will detect a failure quicker than OSPF and therefore should converge more rapidly. If there are many neighbors and adjacencies to consider, the convergence time depends on the processing power of the router. IS-IS is typically less CPU intensive than OSPF. -

The timers in IS-IS allow more tuning than OSPF. There are more timers to adjust, and therefore finer granularity can be achieved. By tuning the timers, convergence time can be significantly decreased. However, this speed may be at the expense of stability, so a compromise may have to be made. A network engineer should understand the implications of adjusting these timers.

| Interactive Media Activity Checkbox: OSPF versus IS-IS Upon completion of this activity, the student will be able to identify the differences between OSPF and IS-IS.