Category: You've Asked for It

Shivlu Jain sent me an interesting question:

I'm wondering whether a router performing penultimate hop popping (PHP) imposes an IGP label or not.The value of implicit null is 3; does it mean the router imposes this label (and adds four bytes to the packet)?

The penultimate router does not impose the IGP label (that's why this behavior is called penultimate hop popping). However, the egress router has to signal to its upstream neighbor (the penultimate router) that it should NOT impose a label, so it uses "implicit null" label (= 3) in TDP/LDP updates to signal that the top label should be popped, not rewriten.

Ken McCoy sent me a short question:

At one point someone posted an article about a command you could run on the Catalyst switch that would give you back the distance of the cable between the switch and end device, but now I can't find it.

I remembered reading the same article and after I've figured out the underlying technology is called TDR (Time Domain Reflectometer), uncle Google immediately provided a reader tip from Csaba Farkas.

Cheng sent me an interesting question:

I'm reading your book MPLS and VPN Architectures and I've found the ip vrf forwarding name fallback global command in the “Additional Lookup in the Global Routing Table” section. I can only find this command in Junos, but not in IOS.

… and he was right. When we were writing the book, we described several features that were still in development as it looked like they would be in the production code by the time the book was published. Many of them made it into the public IOS releases (for example, the Carrier's Carrier architecture), but some of them (like this command) simply vanished from the surface.

Marcus Jensen asked me a very interesting question:

I want to send 3 lines of configuration to a remote router, but I know the first line will kill my connection. Can I save these 3 lines of code to a text file, and then issue a Tcl command to add those to the running config?

The solution is much simpler and does not have to involve Tcl at all. The copy something system:running-config command merges the configuration commands in the source file with the current running configuration.

Mohammad Faraz Rehan has encountered an interesting problem when using IP access-list based class/policy maps on Catalyst 3750:

When I try to apply the same service-policy to more than 15 interfaces, the configuration command fails and the switch generates the following syslog message:

%QOSMGR-4-POLICER_PLATFORM_NOT_SUPPORTED: Policer configuration has exceeded hardware limitation for policymap …

I've tried to help him with various TCAM-related information and in the end he found an interesting solution to the problem:

It looks like there is a limit related to using the same access-list/class-map/policy-map on multiple interfaces.

The first time I was applying the same policy-map (19 classes/19 ACLs/46 ACL lines) on all interfaces, but the switch would not accept it on more than 15 interfaces. Another test scenario had 18 classes/18 ACLs/52 ACL entries and the same policy-map would only work on 13 interfaces.

Now we implemented 24 different policy-maps, class-maps and ACLs remained the same, and the switch is happy.

Jeff West has asked me to document the printout produced by the mturoute utility. Here's the first part of the documentation.

mturoute works in two modes:

• Without the -t flag, it sends variable-lenght ICMP echo packets to the specified destination address, trying to figure out the largest packet that is successfully propagated to the destination.
• With the -t flag, it uses traceroute-like algorithm to find the hop-by-hop IP addresses (the source IP addresses of the ICMP TTL exceeded replies) and uses the same packet-size-calculating algorithm to measure the path MTU to each hop.

Today we'll focus on the non-trace mode. It tries to measure the path MTU with a bisection method varying the packet sizes between minimum MTU (92) and maximum MTU (specified with the -m parameter, default is 10000 bytes). The payload size of the first packet (without the -m flag) is thus 5046 bytes ((10000 + 92)/2).

On each iteration, the algorithm prints a “cryptic” sign indicating whether the ping with the current payload size succeeded. The following indications are given:

• '+': ICMP echo reply arrived
• '-': The ping failed (for various reasons, including exceeding the path MTU)
• 'u': ICMP destination unreachable response arrived, indicating blackhole or access-list.

ICMP unreachable is considered a successful response; at least we're measuring the path MTU up to the failure point

Shaun needed an extra TFTP server in CCNP labs and asked whether you could use a router to act as one. The read-only (download only) TFTP functionality has been available in Cisco IOS for a long time, but the common wisdom was that you could only use the TFTP server function to serve current IOS image.

Fortunately, as of IOS 11.0, the function is more generic; you can serve any file residing on the router (you still cannot upload files), but you have to declare each file to be served with the tftp-server path global configuration command. You could even specify an alias to have the file available under a different name and attach an access list to each configured file to restrict its availability.

Note: This article is part of You've asked for it series.

In his comment, Michal has asked about the ability to execute IOS commands with prompts from Tcl shell. I haven't found a generic solution yet, but you can reload a router from a Tcl script. First you have to define an EEM applet that reloads the router and can be triggered from command-line interface:

event manager applet forceReload
 event none
 action 1.0 reload

Now you can use the exec "event manager run forceReload" Tcl command in your Tcl script to run the applet (and reload the router).

read more see 2 comments

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" ]

read more see 10 comments

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.