The "How could we figure out if any traffic uses the default route" challenge was obviously too easy; a number of readers quickly realized that the CEF accounting can do what we need (and I have to admit I've completely missed it).

However, when I started to explore the various CEF accounting features, it turned out the whole thing is not as simple as it looks. To start with, the ip cef accounting global configuration command configures three completely unrelated accounting features: per-prefix accounting (that we need), traffic matrix accounting (configured with the non-recursive keyword) and prefix-length accounting.

When we were writing the MPLS and VPN architectures books, there was a limit on the number of OSPF processes you could configure per PE-router. The limit was based on the fact that IOS supports up to 32 routing information sources. Two of them are static and connected; you also need an IGP and BGP in the MPLS VPN backbone, resulting in 28 OSPF processes that could be configured on a single PE router. This “feature” severely limited OSPF-based MPLS VPN deployments until IOS release 12.3(4)T when the limitation was removed, resulting in the availability of up to 30 routing processes per VRF.

RIP, BGP, and EIGRP never experienced the same limitations as you configure VRF-specific routing instances within address families of a single routing protocol

Cisco IOS release 12.4(15)T brought (among a plethora of voice features) the logging to non-volatile storage, a nice-sounding name for the ability to write syslog messages into files on your flash memory (or an embedded disk, if you have one). To configure it, use the logging persistent [url directory] [size filesystem-size] [filesize logging-file-size] global configuration command:

• The directory argument specifies where you want the files to be stored (for example, flash:/logging).
• The filesystem-size specifies the maximum disk space the logging files can consume (once you exceed the limit, the oldest file is deleted)
• The logging-file-size parameter specifies the maximum size of each file (once the file grows too large, a new file is created).

Note: You can store the log files on the router's flash memory if it appears as a disk file system (check with the show file systems command). Wouldn't it be great if this feature would also work on USB drives ...

I've ported the dns package of the Tcl standard library to Cisco IOS. You can download it from my web site and install it on your router in just a few steps:

1. Extract all the files from the ZIP archive and copy the Tcl files into a subdirectory on your router's flash (I would recommend you use flash:tcllib/dns).
2. Configure the package initialization script with the scripting tcl init flash:tcllib/dns/pkgIndex.tcl global configuration command

To test the successful installation, start the Tcl shell from the command prompt and try to load the DNS package:

The Static routing with Catalyst 3750 post has generated a lot of good, creative ideas. Some of the proposed solutions were better than the others and some were simply not implementable (but nonetheless, had great creative potential :). Here is my list of the favorites:

A routing protocol: as a few of you have rightly pointed out, this is the best choice.

Aggressive Unidirectional Link Detection (UDLD): this is my second favorite, as it's a reliable link-level mechanism that will detect a break in the fiber cable … exactly the right tool for the job.

In a previous post, I've described how the track interface ip routing command reports incorrect interface state if you use IP Event Dampening feature. To track the actual IP routing readiness of an interface, you could use the following workaround:

• Create a static IP route pointing to the interface you want to test. Make sure this route is not redistributed into any routing protocols.
• Track the reachability of the static route

Here's an interesting scenario:

We have two sites, each using a Catalyst 3750 switch, and routing between them using static routes. There's a primary fiber link between them and we're using twisted-pair-to-fiber converters due to port limitations on Cat3750. These converters do not report fiber link down status correctly (the carrier is still present on twisted pair even if fiber is down), so the primary Ethernet interfaces do not go down if the fiber link breaks and the primary static route is not removed, requiring manual action to switch over to the backup link.

The setup is summarized in this diagram:

Although it's not exactly trivial, you can use standard Tcl packages with Tcl
shell on Cisco IOS by following this procedure:

• Install a Tcl interpreter on your workstation (use ActiveState's ActiveTcl in Windows environment).
• Collect all the source files needed for your set of packages into one directory on your workstation.
• Execute Tcl pkg_mkIndex command in that directory.

$ tclsh
% pkg_mkIndex . *.tcl
% ^Z
$

I've recently replaced my old home router (well, actually a combination of two low-end models, one could handle ISDN and the other one 3DES) with a 1812. After I've struggled past the “interesting” interface names (it has 8 switched ports, named FastEthernet2 to FastEthernet9) and brushed up my BVI/VLAN skills, configuring it was a breeze … only the DHCP server was causing me problems; every time my laptop would wake from the standby mode, it would take almost half a minute before it got the LAN IP address. The obvious suspect (as I've installed the 12.4(15)T on it) was the software, the next one DHCP ping timers.

The on-line configuration help for the ip dhcp conflict logging configuration command (logging: Record address conflicts in a log file) is one of the more misleading texts I've found in Cisco IOS (and the CCO documentation is not much better). Here's how it actually works ...