When I was discussing the details of the BGP troubleshooting video with one of my readers, he pointed out that I should mention the need for CEF switching in EBGP multipath scenario. My initial response was “Why would you need CEF? EBGP multipath is older than CEF” and his answer told me I should turn on my gray cells before responding to emails: “Your video as well as Cisco’s web site recommends CEF for EBGP multipath design… but interestingly, it does work without CEF”.

The real reason we need CEF in EBGP load sharing designs is the efficacy of load distribution. Without CEF, the router will send all traffic toward a single BGP prefix over one of the links (fast switching performs per-destination-prefix load sharing). With CEF, the load is distributed based on the source-destination IP address pair combinations. Even if multiple clients send the traffic toward the same server, the load is spread across available links.

A few days ago, a reader sent me an e-mail titled “Telnet Automation from a Cisco Router” and complained that IOS Tcl does not support the expect commands (spawn, send and expect). Since Expect is a Tcl extension, not part of the core Tcl, it’s not included in Cisco IOS, which was the only answer I could give.

Brain dumps are the biggest threat to the certification industry these days, significantly devaluing certifications that rely primarily on multiple-choice answers. Similarly to the threat-prevention measures adopted by airport security (read the insightful analysis of their behavior from Bruce Schneier, a renowned security guru), IT vendors are responding with high-tech measures.

The idea to write an article describing how you can use MPLS VPN-enabled NAT to implement flexible extranets that allow participants to retain their existing (and sometimes overlapping) IP address space has been sitting in my to-do list for over a year.

After I’ve finally written it (without even hinting what I’ve been working on), I got several e-mails from my readers asking the questions this article answers, so it looks like the topic has suddenly become very hot. Do you have any ideas why that would be the case?

You’ll find the original article somewhere in this list.

Numerous companies use certifications to screen job candidates. Even if all the caveats associated with this process are given, you might encounter candidates who have multiple high-level certifications but cannot differentiate a router from a box of cheese. How can you identify (and reject) such people?

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.

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.”

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.

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…

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.

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.

Installation

• 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 
#           CISCO-CLASS-BASED-QOS-MIB.
#

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
printQosClassIndex

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#tclsh
c7200(tcl)#snmp_getone Test cbQosCMPrePolicyBitRate.66.383777
{<obj oid='cbQosCMPrePolicyBitRate.66.383777' val='0'/>}

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).