The Perils of Default

Thomas Chipman Over the years, you’ve set up many Cisco routers and switches. Abbreviated commands, memorized defaults and simplified navigation make it easy to navigate through configuration mode. The once-strategic, meticulous, and tedious configuration method you learned back in your CCNA days is slowly being replaced by something simpler and more efficient – something shorter and faster. But is it really better? Is it really “better?”
Despite the uniformity Cisco presents across its product lines in terms of appearance, you will find a frustrating number inconsistent settings between different hardware platforms and software revisions. We can easily get burned by inconsistencies, and eventually learn that pain is a better teacher than any other.
Consider, for example, the default summarization setting for an Enhanced Interior Gateway Routing Protocol Autonomous System (AS), configured on a 2800 series Integrated Services Router with IOS revision 15.0. Will the routing process automatically summarise routes based on their classful bits boundaries? If you know the correct default behavior, it’s one less command that you’ll need when configuring EIGRP for that router. However, if you don’t, you could just break your network.
Although Cisco documentation should give us an answer, it is not always as simple as one would think. For our automatic summarization example: Cisco refers to the default behavior in Cisco IOS IP Routing EIGRP Command Reference as the following:
Command Default This command’s behavior is disabled by default. The software does not send subprefix routing data across classful network borders. Cisco IOS Release 15.0(1)M. 12.2(33.SRE, 12.2 (33.XNE), Cisco IOS XE Version 2.5, Cisco IOS 12.2(33.SXI4 & Later Releases. The behavior of this command will be disabled by default. The software does not send subprefix routing information over classful network borders. Is automatic summarization enabled by default or disabled? Perhaps we just need another reference. Let’s have a look at Cisco’s statement on automatic summarization in IP routing: Cisco IOS Release 15M&T, EIGRP Configuration Guide
Route Summarization EIGRP can be configured to automatically summarize subnet routes into network level routes. You can, for example, make subnet 172.16.1.0 appear as 172.16.0.0 on interfaces that were configured with subnets of192.168.7.0. Automatic summarization occurs when multiple networkrouter configurations or address family configuration commands have been configured for an EIGRP process. This feature is automatically enabled. Is it clear enough that you would rather try the default setting than simply entering the no auto summary command?
It is possible to avoid many of these inconsistencies by not assuming any relevant default behaviors during configuration. It may be more time-consuming and even less efficient, but it is better to issue an extra command mimicking the default behavior of a platform than to discover that it wasn’t what you expected when the device breaks your production environment.
“Oh, but I am safe. You say that all of my hardware is the same platform and runs the same software version. Let’s take a look at another scenario. Let’s suppose you work for an organization with a remote site that uses a Cisco Catalyst2960 series switch. This remote site has requested a second Catalyst2960 switch. You have been assigned to configure the switch in your staging environment and box it up before sending it out for installation. Your organization uses only Catalyst 2960 switches.