In a comment to the “Execute multiple commands at once” post, Michal has asked for a complete Tcl-shell-with-parameter example. Here's a short script that shuts down the interface and displays its status:

• Variable ifname is set to the value of the first command-line parameter (in many other programming languages, this would be written as argv[0]);
• If the ifname is empty, the script aborts and prints the usage guidelines (again, in a more human-oriented programming language, this would be if (ifname == “”) ...);
• The show ip interface ifname command is executed. If it fails, the interface name is not correct and the script aborts.
• IOS configuration commands interface ifname and shutdown are executed.
• The show ip interface brief configuration command is executed and filtered with the interface name.

#
# ifname is set to first CLI parameter (interface name)
#
set ifname [lindex $argv 0]
if {[string equal $ifname ""]} { puts "Usage: shutdown ifname"; return; }
if { [ catch { exec "show ip interface $ifname" } errmsg ] } {
  puts "Invalid interface $ifname, show ip interface failed"; return}

ios_config "interface $ifname" "shutdown"
puts [ exec "show ip interface brief ¦ include $ifname" ]

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In his comment to my post about re-enabling debugging after router reload, Mike pointed out an interesting IOS feature: you can execute the do command from a configuration file, not just from the user interface. To make his tip even more useful, you can store the do command(s) in an external file on a TFTP server, not in the startup configuration (which would have to be edited manually). With the boot host URL configuration command you'd then ensure that these commands are executed after the router reload.

Notes:

• The router expects a newline character at the end of the configuration file. The best way to ensure it's always there is to add a comment line at the end of the file
• The configuration file load usually fails immediately after the reboot, as the interfaces and IP routing processes are not yet fully operational. You might thus miss the first few seconds of the router's operations (unless you store the extra configuration file Flash or NVRAM).

Pete Vickers sent me a very interesting configuration sample:

To get an IOS device to upload it’s configuration periodically to an external FTP server:

ip ftp source-interface loopback 0
ip ftp username ftp_username
ip ftp password ftp_password
file prompt quiet
!
kron policy-list backup
 cli copy running-config ftp://10.20.30.40
!
kron occurrence daily-backup at 0:30 recurring
 policy-list backup

The beauty of this example is that you can use it on platforms that don't support Embedded Event Manager (which has a very similar cron functionality) as the kron commands were introduced in 12.2T and 12.3 IOS releases.

One of my readers asked for a working configuration of the conditional OSPF default route advertisement feature. In my scenario, the OSPF default route would be announced whenever an Internet prefix (172.18.0.0/16) would be present in the IP routing table.

After I've published the Tcl script that displays the interface IP parameters in a formatted table, cos quickly pointed out a bug: I've expected the IP addresses in the address mask format. In the meantime, I've figured out the root cause of the problem (our remote labs are set to display IP masks in decimal format for compatibility reasons) and fixed the Tcl script. It temporarily sets the terminal ip netmask-format to bit-count before executing the show command. The new script recognizes three parameters:

The comment to one of my previous posts reminded me of a cool feature that's been available in Cisco IOS for a number of years - you can change how the IP addresses and routes are displayed in various show printouts (but not in the router configuration) with the terminal ip netmask-format bit-count|decimal exec-level command. You can even make the change permanent by configuring ip netmask-format format on console and VTY lines.

And the funniest part of the whole story is that I was utterly impressed with the feature when it was introduced ... and now almost started to reinvent the wheel and implement the same functionality in Tcl

Following my post about the relationship between the MPLS and VPN architectures books and CCIP MPLS exam, Peter Dob had an excellent idea: combine the MPLS and VPN architectures (Volume I, CCIP edition would be even better) with the MPLS fundamentals from Luc de Ghein. By reading Luc's book, you'll also get exposure to other MPLS-related topics (for example, AToM) on top of MPLS TE overview that you need for the exam.

After I've published a post on how you can use the new router-traffic keyword to minimize the Internet-facing access list you use with CBAC, Euphrates Greene pointed out to me that this feature does not work for client DHCP traffic (if the router is acting as a DHCP client, for example, when connected to a MAN Ethernet environment).

Once you start thinking about what's really going on, it all becomes obvious: as the router has no IP address when it sends the DHCP request, and it sends the DHCP request to a broadcast address (as it doesn't know the IP address of the upstream DHCP server), there is no session that could be entered into the CBAC session table. So you still have to allow all DHCP traffic to your router with an access-list similar to this one:

ip access-list extended Internet
 permit udp any eq bootps any eq bootpc
 deny ip any any

Note: Replace the highlighted any keyword with the actual DHCP server's IP adress if you have it available and you want to have an even more secure IP access-list.

If you have an existing BOOTP environment (for example, a set of old Unix workstations and X-terminals) and want to deploy DHCP on the same LAN segment, you could run into interesting compatibility issues, as the DHCP servers by default responds to BOOTP requests.

However, IOS has an interesting feature when you use a router as a DHCP server: you can tell it to ignore the BOOTP requests with the ip dhcp bootp ignore global configuration command (introduced in 12.2T and 12.3). Even more, the router can respond to DHCP requests and forward BOOTP requests to a non-local BOOTP server configured with the ip helper-address interface configuration command.

In response to the post detailing router response to port scans, one of my readers asked an interesting question:

“I was wondering if there was a way to prevent the router from sending those TCP RST packets administratively prohibited ICMP messages back to scanners for TCP and UDP respectively. I basically want my router to drop all packets period without replying back in any way, shape, form, or fashion.”

Here's how you do it: