Were you ever in a situation where a file that would have to be on the router was sitting on your laptop, but you couldn't store it into the router's flash across the Telnet session or through the console port?

If the file in question is a text file, and the router supports Tcl shell, _danshtr_ documented an interesting trick: you create the file in Tclsh interpreter, cut-and-paste the text through the telnet session into a Tcl string and write the string to the file. If you want to have a more cryptic solution here it is:

• Start tclsh;
• Enter puts [open "flash:filename" w+] {. Do not hit the ENTER key at the end of the line
• Copy-paste the file contents. The contents should not include unmatched curly right brackets (every curly right bracket has to be preceded by a matching curly left bracket).
• After the file contents have been pasted, enter } and press ENTER.
• End the tclsh session with tclquit.

In one of his posts, Joe Harris describes a great Linux-based WAN emulation toolkit that you can use to introduce latency, bandwidth constraints or packet loss in your lab environment.

In the last few months I ran across a number of IP fragmentation issues. Unfortunately I also encountered a lot of misconceptions about IP fragmentation, its impact on GRE and IPSec, as well as the fragmentation-related mechanisms like MTU Path Discovery. I documented most of what I found in the The Never-Ending Story of IP Fragmentation.

In a response to my post Redundant DHCP Server I've speculated that a Cisco router should coexist with a Windows-based DHCP server if you configure them with non-overlapping address ranges. I was wrong, Edgar Cahuana discovered that Microsoft's DHCP server wants to have complete control over the LAN it's serving and shuts down if it detects another DHCP server on the same LAN.

Sometimes you end up having wrong BGP AS number throughout your network. It could be a result of an unexpected merger or split or you could have started using a private BGP AS number and realized you have to connect to the Internet using a real AS number. The proper solution would be a total reconfiguration of the whole network, but of course not many engineers have the time and courage to do it ;), so it's time to introduce another kludge: the neighbor local-as configuration command.

If you do a lot of tests in a router lab, you're probably getting upset when you have to retype the login and enable password whenever you log into a router. What I do in my labs is to disable VTY login, set the default privilege level to 15 and disable exec timeout (to stop the router from terminating my session).

You've probably already heard the phrase "When the only tool you have is a hammer, everything looks like a nail" (and seen people acting according to it). Likewise, if you have an IOS release with EEM support, a lot of things that would require smart design could be solved in a brute-force way with a few EEM applets. For example, the problem of the BGP default route could be solved “easily” with a few applets that track syslog messages reporting when the BGP neighbors go up/down.

Cisco IOS allows you to define aliases for the commands you commonly use with the alias global configuration command. The alias command accepts the CLI mode (exec, configuration ...) for the new command and the string that replaces the command name. If you specify additional parameters in the new command, they are appended to the alias string.

For example, if want to have the ipconfig command that displays interface IP configuration, you can configure alias exec ipconfig show ip interface. When you execute ipconfig ifname the alias is expanded into show ip interface ifname and displays the IP configuration of a single interface.

What do geeks do when they wait for Santa? At least some of them tweak their Blogger template ... and so my blog got a completely new look-and-feel (after I've spent three months managing to postpone this task). Any opinions and suggestions are most welcome; after all, I'm not even close to a web designer.

With the introduction of Control Plane Policing features (available from 12.3(4)T), you can easily inspect all the open ports (servers and clients) on a router with the show control-plane host open-ports command, resulting in a printout very similar to the netstat -a printout on a Unix/Windows workstation.