In response to my Never-Ending Story of IP Fragmentation, Stojanco Cavdarov made an interesting observation: routers are not allowed to fragment IPv6 packets, they have to respond back with ICMP unreachable (effectively, routers behave as if IPv6 packets would have an implicit don't fragment bit).

To make life easier for non-TCP IPv6 applications (TCP is supposed to use Path MTU Discovery), the minimum IPv6 packet size that has to be supported on all links was increased to 1280 bytes (which, incidentally, fits very nicely into GRE+IPSec envelope transported across links with 1500-byte MTU).

Whisper asked an interesting question …

“What I would like to know is, on my PPP negotiated ADSL connection, how the ISP assigns me a /32 ip address.”

… which prompted me to test various WAN encapsulations and address assignment rules. Here are the results:

In a previous post, I've described how you can use Tcl shell to upload text content into the router's flash if the router has no connectivity to a suitable file server (or you don't have FTP or TFTP server handy).

The trick works flawlessly, but typing the same obscure Tcl commands gets tedious after a while, so the first time I had to use this solution to develop a Tcl script, I quickly wrote another script that takes a file name as a parameter and hides all the other murky details.

Debugging time-based configurations could be a nightmare, as you have to switch router's time back and forth trying to debug your configuration and wait for the desired event to occur. When I was debugging my EEM-based solution to time-based BGP policy routing, I simply defined two aliases that would set the clock to 30 seconds before the event I wanted to test:

alias exec 859 clock set 08:59:30
alias exec 900 clock set 09:00:30

Obviously, these tests are best done in a lab setup … and you have to turn off NTP or any other form of time synchronization.

When Kate Gerwig, my wonderful editor from SearchTelecom.com, and myself agreed on the contents of the “Building customer-resilient BGP networks” article, we had no idea that it would become extremely relevant just days before it was published. The article describes the tools a Service Provider should use to ensure that its customers cannot harm its BGP routing data (and consequently its other customers and the Internet at large).

On February 24th, someone in Pakistan decided to block local access to YouTube … and someone else decided that the best way to approach the problem was to block the whole world’s access to YouTube.

Rob van Ooijen has sent me a really interesting question:

I've configured a switch port to be unconditionally a trunk with the switchport mode trunk configuration command. However, when the interface was enabled, I've got a dynamic trap indicating the trunking was still dynamic (and the show commands also showed negotiation of trunking is enabled).

In fact, adjacent layer-2 devices can negotiate a number of things these days, among them:

Someone has sent me an interesting question a while ago: he's changed the configuration of a single VTY line and got three blocks of VTY configuration commands, similar to this:

line vty 0 2
 login
line vty 3
 password secret
 login
line vty 4
 login

He wanted to merge the three configuration blocks back into a single one but somehow didn't know how to do it.

One of the most tedious tasks in the initial lab setup (at least for me) is the IP address configuration, which usually includes a number of typos and mixups on the WAN links. You can simplify then WAN address configuration if you configure only one end of the WAN link and let PPP do the rest. For example, you could use the following configuration to configure WAN link on your core router …

Petr Lapukhov describes an interesting scenarion in his post BGP Time-Based Policy Routing: a multi-homed customer that uses one upstream link (for example, more reliable but slower one) during the work hours, switching to the other upstream link (faster, less reliable) after that.

He uses BGP communities to achieve the switch (perfect solution if your ISP supports them) and time-based ACL in a route-map to set the community based on time-of-day. As Cisco changed the way BGP imports local routes in IOS release 12.3T, he then devises an ingenious solution based on reliable static routing to trigger a change in the IP routing table.

The optimum solution is way simpler: you just configure two EEM applets to perform clear ip route network command at appropriate times.

I've finally found the EEM reference documentation that specifies the side effects (changes in environment variables) of all action commands. You can use the changed environment variables in subsequent action commands by prefixing the variable name with the $ sign (similar to the EEM applet where I've included router's name in an outgoing e-mail).