Oct
17
Computing voice bandwidth is usually required for scenarios where you provision LLQ queue based on the number of calls and VoIP codec used. You need to account for codec rate, Layer 3 overhead (IP, RTP and UDP headers) and Layer 2 overhead (Frame-Relay, Ethernet, HDLC etc. headers). Accounting for Layer 2 overhead is important, since the LLQ policer takes this overhead in account when enforcing maximum rate. Read More
Sep
11
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
Aug
26
Note: The following post is an excerpt from the full QoS section of IEWB-RS VOL1 version 5. Peak shaping may look confusing at first sight; however, its function becomes clear once you think of oversubscription. As we discussed before, oversubscription means selling customers more bandwidth than a network can supply, hoping that not all connections would use their maximum sending rate at the same time. With oversubscription, traffic contract usually specifies three parameters: PIR, CIR and Tc –... Read More
Aug
17
Try assessing your understanding of Cisco's CBWFQ by looking at the following example: class-map match-all HTTP_R6 match access-group name HTTP_R6 ! policy-map CBWFQ class HTTP_R6 bandwidth remaining percent 5 ! interface Serial 0/1 bandwidth 128 clock rate 128000 service-policy output CBWFQ and answering a question on the imaginable scenario: Two TCP flows (think of them as HTTP file transfers) are going across Serial 0/1 interface. One of the flows matches the class HTTP_R6, and another flow,... Read More
Aug
12
This post is a partial excerpt from the QoS section of our IEWB-RS VOL1 V5. We'll skip the discussion of how much of a legacy the Custom Queue is, and get stright to the working details. Custom queue feature is similar to WFQ in that it tries to share the bandwidth between packet flows using the max min approach: each flow class gets the guaranteed share proportional to its weight plus any class may claim the “unused” interface bandwidth. However, unlike WFQ, there are no dynamic conversations,... Read More
Jul
09
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
Jul
03
This may seem to be a basic topic, but it looks like many people are still confused by the difference between those two concepts. Let us clear this confusion at once! Read More
Jun
26
The goal of this article is to discuss how would the following configuration work in the 3560 series switches: interface FastEthernet0/13 switchport mode access load-interval 30 speed 10 srr-queue bandwidth shape 50 0 0 0 srr-queue bandwidth share 33 33 33 1 srr-queue bandwidth limit 20 Before we begin, let’s recap what we know so far about the 3560 egress queuing: 1) When SRR scheduler is configured in shared mode, bandwidth allocated to each queue is based on relative weight. E.g. when... Read More
May
08
Hi Brian, Read More

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