Category: QoS

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.

My MPLS QoS: Implementing the best model for guaranteed service article published by SearchTelecom gives you a high-level overview of the pipe and hose QoS models in the MPLS VPN environment. I’m also describing basic DiffServ QoS mechanisms available in an MPLS backbone.

If you’re new to IP QoS, you should start with the IP QoS: Two generations of class-of-service tools article.

My IP QoS: Two generations of class-of-service tools article published by SearchTelecom gives you a very high-level overview of IntServ and DiffServ approaches to IP QoS as well as brief description of various DiffServ tools.

More to explore:

• More QoS articles
• QoS Fundamentals webinar

Policing implementations in Cisco IOS are a bit confusing: IOS supports three different algorithms that are configured with very similar parameters of the police command in modular QoS CLI. There's also the older rate-limit command that uses a limited implementation of one of the three algorithms. You'll find all policing details, including the graphic representations of all three algorithms in the QoS Policing in Cisco IOS article.

Setting the size of the hardware output queue in Cisco IOS with the (then undocumented) tx-ring-limit (formerly known as tx-limit) has been a big deal when I was developing the first version of the QoS course that eventually became the initial release of the Implementing Cisco Quality of Service training.

However, while it's intuitively clear that the longer hardware queue affects the QoS, years passed before I finally took the time to measure the actual impact.