Flooding Domains versus Areas

At a fundamental level, OSPF and IS-IS are similar in operation. They both build neighbor adjacencies. They both use Dijkstra’s shortest path first (SPF) to find the shortest path to every destination in the network. They both advertise the state of each link connected to a network device. There are some differences, of course, such as the naming (OSI addresses versus IP addresses, intermediate systems versus routers). Many of the similarities and differences don’t play too much in the design of a network, though.

One difference that does play into network design, however, is the way in which the two protocols break up a single failure domain into multiple failure domains. In OSPF we have areas, while in IS-IS we have flooding domains. What’s the difference between these two, and how does it effect network design? Let’s use the illustration below as a helpful reference point for the two different solutions.

In the upper network, we have an illustration of how OSPF areas work. Each router at the border of a flooding domain (an Area Border Router, or ABR), has a certain number of interfaces in each area. Another way of saying this is that an OSPF ABR is never fully within a single area, but rather participates in many areas—thus each area is bounded within or on a router. Thus OSPF areas can be seen as a collection of flooding domains connected via hard boundaries at the ABRs.

In the lower network, we have an illustration of how IS-IS flooding domains work. Each intermediate system is entirely within a single flooding domain; every interface on the intermediate system is part of each flooding domain the IS itself is a part of. Any given IS may be a part of multiple flooding domains, and hence provide connectivity between the flooding domains it’s in. The easiest way to understand IS-IS flooding domains is as a set of overlapping instances of IS-IS; some intermediate systems just happen to be connected to more than one instance, and thus can provide routing between them.

The difference between these two is, by the way, related to the protocol suite in which each was developed. In the ISO suite, each device has a single address; in IP, each interface has an address. Hence, in OSPF, area boundaries are thought of as occurring between interfaces, while in ISO, flooding domain boundaries are thought of as occurring between devices. The old saw is “OSPF areas break on devices, IS-IS flooding domains break on wires.” This isn’t absolutely true, as IS-IS flooding domains still “break” on devices—it’s just that each device is entirely contained in a single flooding domain.

This fundamental difference leads to other, less obvious differences. For instance—

• Links in an IS-IS network can be configured in multiple flooding domains, as can a device. In OSPF, each link is in precisely one area (although there are workarounds to place a single link in multiple areas).
• OSPF assumes reachability information at the link (or interface) level, which means reachability information is automatically carried from one area to another. IS-IS assumes reachability at the device level, which means information about link or interface reachability is not automatically carried between flooding domains. Rather, interface level reachability must be leaked, or more properly redistributed, between flooding domains (remember, each flooding domain can be seen as a separate instance of IS-IS, and devices just happen to be in more than one flooding domain).

Learning to design with IS-IS isn’t just “OSPF on stilts;” it’s a different way of looking at the problem space. If you want to know more, take a look at my recently published LiveLesson on the IS-IS protocol.