OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

2.2. Fundamental Link State Concepts

To recap, the two most serious problems with most vector protocols are:

• They sometimes converge slowly.

• They are vulnerable to loops.

This section looks at the basics of link state routing, with emphasis on how it avoids these two problems.

Slow convergence in vector protocols is due to the hop-by-hop, distributed calculation of routes. The more routers on a route—the larger the network is—the longer the convergence time. Suppose that instead of having to perform a route calculation before passing along the results of the calculation, a router could pass an update along to its downstream neighbors as soon as it is received, and then perform its route calculation on a copy of the update afterward? The improvement ...