Here ye, here ye, VTP experts. (We are not referring to the Vandenberg Test Program, although they are very likely experts in their field as well.  :)) Can you predict the results of a 3 switch VTP client/server scenario? SW1-3, are connected, as shown in the diagram. Here is the initial output of show VTP status, and show VLAN brief on each. Note that SW1 and SW3 are servers, while SW2 is a client.   We will be adding a failure to the network in just a moment. Read More
In this post we will look at the basic classification and marking features available in the 3550 and 3560 switches. Basic features include packet marking using port-level settings and port-level policy-maps. Discussing Per-VLAN classification is outside the scope of this document. The Catalyst QoS implementation bases on Differentiated Services model. In a few words, the ideas of this model could be outlined as follows: 1) Edge nodes classify ingress packets based on local policy and QoS label... Read More
People are often confused with per-VLAN classification, policing and marking features in the Catalyst 3550 and 3560 models. The biggest problem is lack of comprehensive examples in the Documentation CD. Let's quickly review and compare traffic policing features available on both platforms. The material below is a condensed excerpt of several Catalyst QoS topics covered in the “QoS” section of our IEWB VOL1 V5. You will find more in-depth explanations and large number of simulation-based... Read More
I found it important to make a small post in reply to the following question: Read More
Generally, flow-control is a mechanics allowing the receiving party of a connection to control the rate of the sending party. You may see many different implementations of flow-control technologies at different levels of OSI model (e.g. XON/XOFF for RS232, TCP sliding window, B2B credits for Fibre Channel, FECN/BECN for Frame-Relay, ICMP source-quench message, etc). Flow-Control allows for explicit feedback loop and theoretically implementing loss-less networks that avoid congestion. For the... Read More
UDLD (Unidirectional Link Detection) is Cisco proprietary extension for detecting a mis-configured link. The idea behind it is pretty strighforward - allow two switches to verify if they can both send and receive data on a point-to-point connection. Consider a network with two switches, A and B connected by two links: "A=B". Naturally, if "A" is the root of spanning tree, one of the ports on "B" will be blocking, constantly receiving BPDUs from "A". If this link would turn uni-directional and... Read More
The 3560 QoS processing model is tightly coupled with it’s hardware architecture borrowed from the 3750 series switches. The most notable feature is the internal switch ring, which is used for the switch stacking purpose. Packets entering a 3560/3750 switch are queued and serviced twice: first on the ingress, before they are put on the internal ring, and second on the egress port, where they have been delivered by the internal ring switching. In short, the process looks as follows:... Read More
Catalyst QoS configuration for IP Telephony endpoints is one of the CCIE Voice labs topics. Many people have issues with that one, because of need to memorize a lot of SRND recommendations to do it right. The good news is that during the lab exam you have full access to the QoS SRND documents and UniverCD content. The bad news is that you won’t probably have enough time to navigate the UniverCD with comfort plus the reference configurations often have a lot of typos and mistakes in them. Read More
QoS features available on Catalyst switch platforms have specific limitations, dictated by the hardware design of modern L3 switches, which is heavily optimized to handle packets at very high rates. Catalyst switch QoS is implemented using TCAM (Ternary Content Addressable Tables) - fast hardware lookup tables - to store all QoS configurations and settings. We start out Catalyst QoS overview with the old, long time available in the CCIE lab, the Catalyst 3550 model. Read More
You may want to see the updated version of this post at: Read More

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