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 ...

The track number interface name ip routing command is supposed to track an interface readiness to forward IP packets. In reality, it only tracks the interface line protocol status plus the IPCP status in case of PPP interfaces (as well as the actual presence of an IP address on the interface). If you configure IP Event Dampening (with the dampening) command, the interface might be suppressed (unavailable for IP routing), but the track object will report it as available (tested on IOS release 12.4(6)T). This could result in suboptimal HSRP/GLBP decisions if you use track objects to influence HSRP/GLBP priority or actual loss of data if you use such a track object to control policy-based routing.

Most MPLS books (mine included) and courses tell you that you have to manually enable MPLS on each interface where you want to run it with the mpls ip interface configuration command. However, this task was significantly simplified in IOS release 12.3(14)T with the introduction of MPLS LDP autoconfiguration. If you use OSPF as the routing protocol in your network, you can use the mpls autoconfig ldp [area number] router configuration command to enable LDP on all interfaces running OSPF (optionally limited to an OSPF area).

I have been aware of the typeahead Tcl command for months, but somehow I never got it to work.

It works perfectly in IOS release 12.4(15)T; this is what you have to do to clear interface counters:

typeahead "y"
exec "clear counter dialer 0";

Warning: if the input is not consumed by the executed commands, it stays in the typeahead buffer; quite dangerous if you have a sequence of commands, as the wrong command could be acknowledged.

Are you as upset as I am with the constant display of the legend in front of the routes displayed with the show ip route command? Two output filters can help you.

One of my readers recently asked an interesting question: “How do you install a static route when an IP address is not reachable?”

Without going into the design reasons that prompted the question, you can actually track when IP SLA measurement fails with an obscure configuration syntax of the track objects that tracks when another track object fails.

Contrary to what the regular reload does, the warm reload does not change the configuration register value (obviously that's done by ROMMON, which is not involved in the warm reload process). If you just did a password recovery and changed the configuration register back to a normal value, you'd thus be unpleasantly surprised when the NVRAM would be ignored (yet again) after a warm reload (I stumbled across this as I was trying a new IOS release with the reload warm file URL command).