Deconfusing the Static Route

Configuring a static route is just like installing an entry directly in the routing table (or the RIB).

I have been told this many times in my work as a network engineer by operations people, coders, designers, and many other folks. The problem is that it is, in some routing table implementations, too true. To understand, it is best to take a short tour through how a typical RIB interacts with a routing protocol. Assume BGP, or IS-IS, learns about a new route that needs to be installed in the RIB:

  • The RIB into which the route needs to be installed is somehow determined. This might be through some sort of special tagging, or perhaps each routing process has a separate RIB into which it is installing routes, etc.. In any case, the routing process must determine which RIB the route should be installed in.
  • Look the next hop up in the RIB, to determine if it is reachable. A route cannot be installed if there is no next hop through which to forward the traffic towards the described destination.
  • Call the RIB interface to install the route.

The last step results in one of two possible reactions. The first is that the local RIB code compares any existing route to the new route, using the administrative distance and other factors (internal IS-IS routes should be preferred over external routes, eBGP routes should be preferred over iBGP routes, etc.) to decide which route should “win.” This process can be quite complex, however, as the rules are different for each protocol, and can change between protocols. In order to prevent long switch statements that need to be maintained in parallel with the routing protocol code, many RIB implementations use a set of call back functions to determine whether the existing route, or the new route, should be preferred.

In this diagram—

  1. The IS-IS process receives an LSP, calculates a new route based on the information (using SPF), and installs the route into the routing table.
  2. The RIB calls back to the owner of the current route, BGP, handing the new route to BGP for comparison with the route currently installed in the RIB.
  3. BGP finds the local copy of the route (rather than the version installed in the RIB) based on the supplied information, and determines the IS-IS route should win over the current BGP route. It sends this information to the RIB.

Using a callback system of this kind allows the “losing” routing protocol to determine if the new route should replace the current route. This might seem to be slower, but the reduced complexity in the RIB code is most often worth the tradeoff in time.

The static route is, in some implementations, and exception to this kind of processing. For instance, in old Cisco IOS code, the static route code was part of the RIB code. When you configured a static route, the code just created a new routing table entry and a few global variables to keep track of the manual configuration. FR Routing’s implementation of the static route is like this today; you can take a look at the zebra_static.c file in the FR Routing code base to see how static routes are implemented

However, there is current work being done to separate the static route from the RIB code; to create a completely different static process, so static routes are processed in the same way as a route learned from any other process.

Even though many implementations manage static routes as part of the RIB, however, you should still think of the static route as being like any other route, installed by any other process. Thinking about the static route as a “special case” causes many people to become confused about how routing really works. For instance—

Routing is really just another kind of configuration. After all, I can configure a route directly in the routing table with a static route.

I’ve also heard some folks say something like—

Software defined networks are just like DevOps. Configuring static routes through a script is just like using a southbound interface like I2RS or OpenFlow to install routing information.

The confusion here stems from the idea that static routes directly manipulate the RIB. This is an artifact of the way the code is structured, however, rather than a fact. The processing for static routes are contained in the RIB code, but that just means the code for determining which route out of a pair wins, etc., is all part of the RIB code itself, rather than residing in a separate process. The source of the routing information is different—a human configuring the device—but the actuall processing is no different, even if that processing is mixed into the RIB code.

What about a controller that screen scrapes the CLI for link status to discover reachable destinations, calculates a set of best paths through the network based on this information, and then uses static routes to build a routing table in each device? This can be argued to be a “form” of SDN, but the key element is the centralized calculation of loop free paths, rather than the static routes.

The humble static route has caused a lot of confusion for network engineers, but clearly separating the function from the implementation can not only help understand how and why static routes work, but the different components of the routing system, and what differentiates SDNs from distributed control planes.