Blog Posts in December 2008

Is it wise to use certifications in the candidate selection process?

My previous certification-related post described how some companies use certifications to filter job applicants for networking-related positions. Should you follow that example? If you’re in a country with a saturated job market, where the number of applicants far exceeds the number of job postings (consider yourself very lucky if you’re an employer), you should certainly use whatever filters you can to screen the hundreds of applications you receive … but be aware that you have potentially lost a few gems hidden in the flood.

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MPLS support on 1800-series routers

Christoph sent me an interesting question a few days ago:

I played a bit arround with 2 Cisco 1803 and I found MPLS related configurations commands in IOS 12.4(15)T (Advanced Enterprise) on this box. MPLS was not listed as a included fearture in the Cisco Feature Navigator for this image and some searching at took me to a 2 year old document telling me that MPLS isn't supported on this series. Some more searching took me back to the Cisco Feature Navigator which lists MPLS as feature for the Cisco 1805 router (which uses the same IOS image, afaik).
So, I'm a bit confused now if MPLS is really working / supported on the low-end Cisco ISR 1800 fixed series?

MPLS was mostly available but never supported on low-end platforms (including Cisco 2600). In those days I've taken some heat for reusing existing 2600-based labs to teach Cisco-internal MPLS courses (since we were teaching the students to configure unsupported devices :).

Anyhow, the "not supported" means exactly that: it may be available (well, it is), it may work (it actually does), but if it's broken (and I've seen at least one low-end-platform-specific bug in the early days) you can't complain.

Is anyone aware whether the official support for the MPLS on 1800 series has changed? If so, please share your information with us.

If you need to offer a production-grade service to your customers, don't use unsupported equipment; if you need a solution for your personal needs or you're building a lab, go ahead.

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Why would I need a Web Application Firewall?

If you have been visited by a friendly Cisco sales engineer recently, you might have already heard about the ACE Web Application Firewall (WAF). If you’re curious enough to start investigating on your own, you might have stumbled across the WAF product description on Cisco’s Web site, which tells you … nothing.

Let’s start with an easy question: if I already have a firewall, why would I need another box with “fire” and “wall” in its name? The short answer is “Because Web programmers rarely know how to write secure Web applications.”

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Book review: Voice over IP Security

Based on the title, I would assume that the Cisco Press book Voice over IP Security: Security best practices derived from deep analysis of the latest VoIP network threats attracts primarily senior voice engineers who know that they have to secure their production networks. The author of the book strongly disagrees with my opinion, however, spending more than a third of the book on baseline explanations of VoIP, SIP, H.323, firewalls, NAT, DES, IPSec…. I enjoyed the overview chapters, as I last configured VoIP before SIP was invented, but an experienced VoIP engineer would be disappointed.

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This is QoS; Who Cares about Real-Time Response?

It all started with a innocuous question: can you detect voice traffic with EEM? Looks simple enough: create a QoS class-map that matches voice calls and read the cbQosClassMapStats table in the CISCO-CLASS-BASED-QOS-MIB. The first obstacle was finding the correct indexes, but a Tcl script quickly solved that; I was ready to create the EEM applet. The applet failed to work correctly and after lots of debugging I figured out the counters in the cbQosClassMapStats table change only every 10 seconds.

I couldn’t believe my eyes and simply had to test other MIB variables as well. As expected, the IF-MIB (standard interface MIB) counters increase in real-time, but obviously someone had the bright idea that we need to detect changes in traffic profile only every now and then. Although I've received numerous suggestions from my readers, none of them works on a Cisco 1800 or a Cisco 7200. Oh, well, Cisco developers from the days when I started working with routers would have known better…

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The most convoluted MIB I’ve seen

Jared Valentine sent me a really interesting problem: he would like to detect voice traffic and start shaping TCP traffic for the duration of the voice call. The ideal solution would be an EEM applet reacting to the changes in the CISCO-CLASS-BASED-QOS-MIB; one of its tables contains the amount of traffic for each class configured in a service policy.

The MIB navigation looks simple: you just read the values from the cbQosClassMapStats table, indexed by policy ID and class ID. The real problem is finding the correct index values. I could walk the MIB manually with a MIB browser or snmp_getnext TCL calls, but this approach is obviously not scalable, so I wrote a script that walks through the cbQosServicePolicy, cbQosObjects, cbQosPolicyMapCfg and cbQosClassMapCfg tables and prints the index values you need.

The following text written by Ivan Pepelnjak in 2008 was originally published on CT3 wiki. That web site became unreachable in early 2019. We retrieved the original text from the Internet Archive, cleaned it up, updated it with recent information if necessary, and republished it on blog on November 17, 2020

This script traverses the Class-based QoS MIB and displays service policies and classes attached to individual interfaces. The policy index and class index values are printed next to the policy/class name to help the operator fetch the desired SNMP variable from the statistics tables of the CISCO-CLASS-BASED-QOS-MIB.


  • Download the source file into flash:cbindex.tcl
  • Configure alias exec cbindex tclsh flash:cbindex.tcl
  • Configure persistent CBQoS indexes with the snmp mib persist cbqos (otherwise the indexes will change after the router reload).

Usage guidelines

Usage: cbindex community

Command line parameters:

  • Community: SNMP community with R/O access to the CISCO-CLASS-BASED-QOS-MIB

Source code

# title:    Displays MQC class map indexes
# name:     cbindex.tcl
# desc:     The script traverses the Class-based QoS MIB and
#           displays service policies and classes attached to 
#           individual interfaces. The policy index and class
#           index values are printed next to the policy/class
#           name to help the operator fetch the desired SNMP 
#           variable from the statistics tables of the 

proc snmpInit { oid } {
  global snmpCommunity
  set getResult [ snmp_getnext $snmpCommunity $oid ]
  if { [ regexp {snmp error} $getResult ] } { 
    puts "SNMP calls with community $snmpCommunity fail"; return 0 
  if { [ regexp {oid='(.*)'} $getResult ignore nxtoid ] } {
    if { [string first $oid $nxtoid] == 0 } { return 1 }
  puts "MIB $oid not implemented in this IOS release"; return 0;
proc snmpGet { oid result } {
  global snmpCommunity
  upvar $result r
  if { [info exists r] } { unset r }

  set getResult [ snmp_getone $snmpCommunity $oid ]
  if { [ regexp {snmp error.*text='(.*)'} $getResult ignore errtxt ] } { 
    error "snmpGet - $errtxt"; return 0 
  if { [ regexp {oid='(.*)'.*val='(.*)'} $getResult ignore oid result ] } {
    if { ! [ string equal $result "NO_SUCH_INSTANCE_EXCEPTION" ] } {
      set r(OID) $oid ;
      set r(VALUE) $result ; 
      return 1;
  return 0;

proc snmpGetNext { oid result } {
  global snmpCommunity
  upvar $result r
  if { [info exists r] } { unset r }

  set getResult [ snmp_getnext $snmpCommunity $oid ]
  if { [ regexp {snmp error.*text='(.*)'} $getResult ignore errtxt ] } { 
    error "snmpGet - $errtxt"; return 0 
  if { [ regexp {oid='(.*)'.*val='(.*)'} $getResult ignore oid result ] } {
    if { ! [ string equal $result "NO_SUCH_INSTANCE_EXCEPTION" ] } {
      set r(OID) $oid ;
      set r(VALUE) $result ;
      set oidSplit [ split $oid "." ]
      set r(NAME)  [ lindex $oidSplit 0 ]
      set r(INDEX) [ lreplace $oidSplit 0 0 ]
      set r(IDXLIST) [ join $r(INDEX) "." ]
      return 1;
  return 0;

proc snmpGetInTable { oid result { parentoid "" }} {
  global snmpCommunity
  upvar $result r

  snmpGetNext $oid r
  if { ! [info exists r(OID)] } { return 0 }
  if { [string equal $parentoid ""] } {
    set oidSplit [ split $oid "." ]
    set parentoid [lindex $oidSplit 0]
  if { [string first $parentoid $r(OID)] != 0 } { return 0 }
  return 1;

proc printQosClassIndex {} {
  global snmpCommunity
  set oid "cbQosIfIndex"
  array set dirLookup { 1 in 2 out }
  set cnt 0
  while { [ snmpGetInTable $oid svcPolicy ] } {
    if { [snmpGet "ifDescr.$svcPolicy(VALUE)" ifDescr] } {
      snmpGet "cbQosPolicyDirection.$svcPolicy(INDEX)" svcDirection
      snmpGetNext "cbQosConfigIndex.$svcPolicy(INDEX)" policyObject
      snmpGet "cbQosPolicyMapName.$policyObject(VALUE)" policyName
      puts "\n$ifDescr(VALUE) ($dirLookup($svcDirection(VALUE))): $policyName(VALUE) ($svcPolicy(INDEX))"
      set coid "cbQosObjectsType.$svcPolicy(INDEX)"
      set parentoid $coid
      while { [ snmpGetInTable $coid svcClass $parentoid ] } {
        if { $svcClass(VALUE) == 2 } {
          snmpGet "cbQosConfigIndex.$svcClass(IDXLIST)" svcClassConfig
          snmpGet "cbQosCMName.$svcClassConfig(VALUE)" svcClassName
          puts "  $svcClassName(VALUE) $svcClass(IDXLIST)"
        set coid $svcClass(OID)
    } else { error "Cannot get interface name for service policy $svcPolicy(VALUE)" }
    set oid $svcPolicy(OID)

set snmpCommunity [lindex $argv 0]
if { [string equal $snmpCommunity ""] } { set snmpCommunity "public" }
if { ! [ snmpInit "cbQosObjectsType" ] } return

Sample usage scenario

The following QoS classes and policies have been configured on the router:

class-map match-all Mail
 match protocol smtp
class-map match-all Web
 match protocol http
class-map match-all SecureWeb
 match protocol secure-http
class-map match-any Surfing
 match class-map Web
 match class-map SecureWeb
class-map match-all Files
 match protocol ftp
policy-map Internet
 class Web
    bandwidth 128
 class SecureWeb
    priority 64
 class Mail
    bandwidth 32
policy-map MailOrFtp
 class Mail
  set ip precedence 0
 class Files
  set ip precedence 0
 class Surfing
    police 16000
 class class-default
   police cir 8000
     exceed-action drop 
interface Serial1/0
 service-policy input MailOrFtp
 service-policy output Internet
interface Serial1/1
 service-policy output MailOrFtp

The cbindex script reported the following SNMP indexes:

c7200#cbindex Test

Serial1/0 (in): MailOrFtp (48)
  Web 48.383777
  Surfing 48.1970017
  Mail 48.4297921
  Files 48.13110129
  class-default 48.14779377
  SecureWeb 48.15077857

Serial1/0 (out): Internet (50)
  Mail 50.10516033
  Web 50.14007809
  SecureWeb 50.14520625
  class-default 50.15008753

Serial1/1 (out): MailOrFtp (66)
  Web 66.383777
  Surfing 66.1584993
  Files 66.4236097
  Mail 66.11615889
  SecureWeb 66.15077857
  class-default 66.15082481

Based on these indexes, you could monitor the bit rate of the Web class in outbound policy configured on Serial 1/1 with SNMP variable cbQosCMPrePolicyBitRate.66.383777.

c7200(tcl)#snmp_getone Test cbQosCMPrePolicyBitRate.66.383777
{<obj oid='cbQosCMPrePolicyBitRate.66.383777' val='0'/>}
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Certifications: a new barrier to entry

Recent blog posts indicate that, in at least some market segments, IT certifications are becoming a new barrier to entry: companies require a specific set of certifications in their job offerings and use those requirements to filter the candidates who are invited to the initial interview. Obviously, IT vendors pushing the certifications are getting some real traction. On the other hand, anecdotal evidence indicates that certification holders are sometimes able to memorize vast amounts of information without being able to put it to use (I don’t want to imply that they used other, less honest methods).

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NAT translation logging

The ip nat log translations syslog command starts NAT logging: every NAT translation created on the router is logged in syslog (which can be sent to console, syslog host or internal buffer). You could use this command as a poor man’s reporting tool if you have to monitor the address translations on your edge routers (for example, due to security policy or auditing requirements). Obviously you should configure the no logging console first in a production environment; otherwise your router will hang a few moments after you’ve enabled NAT logging.

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Online session poll results

I would like to thank everyone that took time and voted on the subject of my next on-line session. We have a clear winner: Dynamic routing across a firewall … although the closeness of the other topics might indicate a helping hand of our polish friend. Unfortunately, I have some bad news to go with the good ones – the “December” session will most likely happen in January or February (I'm simply running out of time).
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Update interval for IOS MIB counters?

Dear readers! This time I really need your help (uncle Google and his relatives gave me only one relevant hit and even that doesn't work on 7200 or 1800).

I'm trying to implement an EEM applet that would detect traffic rate change using CISCO-CLASS-BASED-QOS-MIB. Everything would work perfectly ... if only IOS wouldn't update the MIB counters approximately every 10 seconds, not in real-time. Is anyone aware of a configuration command that would force the router to update these counters any faster?
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Random “Scenic Route Certification” thoughts

The “Sometimes the path is more important than the destination” post has generated numerous highly interesting comments. I already planned to write about some of the issues raised by the readers (certification grind mill) or wrote about others (knowledge or recipes), so I’ll skip those and focus on the other interesting bits-and-pieces (but please make sure you read the original post first).

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Should VTP be disabled by default?

One of my readers sent me a question that triggered one of my old grudges:

In my experience, when you first add a new switch (having a NULL domain) on an existing VTP Domain, it inherits the domain name, regardless of it being a VTP Server. I was wondering if this is a feature (i.e. has proved to be a solution in most cases) or a bug (i.e. has proved to cause problems in most cases). I know it's proved to be the latter for us!

In my personal opinion Cisco at one point in time wanted too much plug-and-play and someone had a great idea that you can just plug another switch into your network and it would autoconfigure itself. We've been suffering because of that "insight" ever since (and the CCIE written test has material for a few more interesting questions :).

I strongly believe that VTP should be turned off by default and should generate a warning before being enabled, but it will probably not happen. What do you think?

Disclaimer: I am not a switching person and have no idea about anything below or above layer 3.

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I’m Too Old … I Prefer CLI over GUI

I was delighted when I got access to Cisco’s Application Control Engine (ACE) XML Gateway/Web Application Firewall (WAF) box. This box is the perfect intersection of three fields that really interest me: networking, security and Web programming. To my huge disappointment, though, all the real configuration can only be done through the Web interface. I understand that casual users of a device prefer a graphical user interface (GUI) over text commands (and Generation Z has never seen a terminal window, DOS prompt or, God forbid, an actual terminal), but you can achieve so much more with a simple text-based configuration approach:

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When Would an MPLS LSR Have Untagged Output Label?

This is a nice MPLS question I’ve received from one of the readers:

I have understood the Penultimate Hop Popping (PHP) process, but I don’t understand when a router would use UNTAGGED instead of POP TAG?

Instead of answering the question directly, let's walk through a series of simple Q&A pairs that will help you understand the whole process (remember: knowledge, not recipes!).

It's highly recommended you read the first few chapters of the MPLS and VPN Architectures book before the rest of this post.
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