In the “good old days” we've been teaching our students that although a router can act as a Frame Relay switch, it supports only the rudimentary functionality of switching the packets, but not the policing/marking features available in Frame Relay switches. That hasn't been true for a while - in IOS release 12.1T, Cisco has introduced the congestion management features. You can specify the congestion management per-interface (with the frame-relay congestion-management interface configuration command) and set the DE drop/ECN mark percentages for all PVCs on the interface or you can set the parameters within a map-class.

I don't know how useful this feature is to you; I was fond of finding it because it solves some interesting problems I had a (long) while ago. If you need more in-depth description or actual configurations, post a comment or send me a message.

Using DHCP to assign server IP addresses is usually not a wise decision. To start with, you have to define static DHCP mappings, which rely on client-id attribute in the DHCP request (usually the MAC address of the client). For me, the easiest way to find the correct client ID is as follows:

• Use DHCP to assign the IP address to the server
• Note the newly assigned IP address
• Use the show ip dhcp bindings | include ip-address command to display the client-id to IP address binding.
• Create a static DHCP mapping (for example, by configuring a host DHCP pool on the router) and release/renew IP address on the server

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.

The per-prefix accounting is the easiest one to understand: every time a packet is forwarded through a CEF lookup, the counters attached to the CEF prefix entry are increased. To clear the CEF counters, you can use the clear ip cef address prefix-statistics command. The per-prefix counters are also lost when the IP prefix is removed from the CEF table (for example, because it temporarily disappears from the IP routing table during network convergence process). The CEF per-prefix accounting is thus less reliable than other accounting mechanisms (for example, IP accounting).

Note: The CEF per-prefix counters are always present; if the CEF per-prefix accounting is not configured, they simply remain zero.

Last but not least, you don't need the detail keyword if you want to display the CEF accounting data for a particular prefix. The show ip cef address mask command is enough. And, finally, if you're running IOS release 12.2SB or 12.2XN, you can inspect the CEF counters with SNMP.

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:

router#tclsh
router(tcl)#package require dns
1.3.1
router(tcl)#

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:

If you'd like to implement persistent DHCP bindings on Cisco IOS, but cannot store them on an external server, you could always use the on-board NVRAM. Simply configure ip dhcp database nvram:dhcp.txt. Later on, you can examine the contents of the dhcp.txt file with more nvram:dhcp.txt command.

This post was written in 2007, when a lot of low-end Cisco routers still shipped with flash formatted in the “old” Cisco format and the flash was not really usable to store ever-changing files. For more details on storing DHCP bindings in onboard flash, read the Flash-based DHCP Database blog post.

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
$

• Edit the pkgIndex.tcl file created with the pkg_mkIndex command and set the $dir variable to the IOS directory before the first package command (for example, set dir "flash:tcl/").
• Alternatively, add the Tcl command set dir [file dirname [info script]] in front of the first package command. This command sets the $dir variable to the path of the pkgIndex.tcl file.
• Transfer all the source files into a directory on the router's flash (or any other local storage device).
• Configure the execution of the pkgIndex.tcl file at tclsh startup with the scripting tcl init configuration command (for example, scripting tcl init flash:tcl/pkgIndex.tcl).

When you have completed these steps, the pkgIndex.tcl file will be executed every time the Tcl shell is started in Cisco IOS, defining all the packages you've prepared. Now you can use the package require name Tcl command to load the packages you need in your Tcl script.

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.

After replacing the software (didn't help) and tweaking DHCP timers (no change), it finally dawned on me: the ethernet ports are switched, so the spanning tree was playing tricks with me. Disabling spanning tree with the spanning-tree portfast interface configuration command solved the problem.

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).

As the careful readers of my MPLS books know, it’s dangerous to run LDP with your customers; the moment you run LDP with them (Carrier’s carrier model is an exception), they can insert any labeled packet into your network, bypassing inbound access lists and sending traffic where it’s not supposed to go (even into another VPN). It’s vital that you consider security implications before deploying MPLS LDP autoconfiguration.

Using this feature on P routers is absolutely safe, as they have no customer links. You have to be more careful on the PE routers, more so if you run routing protocols with your customers. The safest configuration method would be to configure LDP autoconfiguration inside a single OSPF area, but even then, a configuration error (placing a PE-CE interface in a wrong area) could open your network to MPLS-based attacks.