CCIE Candidate

Moving onto frame-relay traffic shaping (FRTS), we have a whole new set of tools that can be used to shape in a frame environment. Here’s a list of the notable differences between class-based and frame-relay traffic shaping.

When configuring a Cisco router to perform traffic-shaping, you have flexibility. You can classify packets and apply various traffic-shaping policies to the different classes. (I’ve used this to make sure that people listening to on-demand audio don’t overwhelm my link and to prevent a fat e-mail upload or download from clogging the pipe.)
The [...]

Traffic-shaping is the idea of delaying a flow of traffic such that it does not exceed a specified bandwidth. In observing an graph of bandwidth utilization of unshaped flow of traffic versus shaped traffic, you might see an irregular utilization of spikes up and down for the unshaped traffic, and an even plateau for [...]

actual queue depth - the number of packets in the queue in a given moment
average queue depth - calculated based on the actual queue depth and the previous average. WRED uses this value to slowly adjust to what may be quick queue depth changes
class-based weighted fair queuing - reserves a minimum amount of pipe [...]

This section of the chapter discusses layer 2 queuing and drop methods on the 3550 platform, with a bit of attention paid to the 2950 by way of comparison. I’ll try to be brief, as I suspect most of us aren’t often worried about managing our “cheap” bandwidth. Many of us are probably [...]

Weighted Random Early Detection is a Cisco-created algorithm used to monitor queue lengths, and preemptively discard packets in an effort to prevent the queue from getting full. WRED monitors a queue, and will discard more and more packets as the queue gets longer and longer. This tends to improve performance of TCP conversations [...]

Low latency queuing is the tool to use when you are providing QoS for delay sensitive traffic. LLQ is the same as CBWFQ, but for the addition of some queues to be tweaked to be low-latency. These special low-latency queues are serviced much like PQ’s “high” queue - LLQ will service packets in [...]

Class-based WFQ and low-latency queuing (which I’ll blog about in the next post) are best of breed queuing methods that take from PQ, CQ and WFQ, plus add new features of their own.
CBWFQ Key Points

Classification - can classify any traffic that MQC commands match

Weighted fair queuing, or WFQ, is a different beast from PQ or CQ. You can’t define your own classes with WFQ. WFQ does the classification for you, by examining flows:

Source IP
Destination IP
Transport layer protocol (TCP or UDP)
TCP or UDP source port
TCP or UDP destination port
IP Precedence

Custom queuing, or CQ, responds to the shortcomings of PQ by guaranteeing that every queue gets some kind of minimum amount of bandwidth. The idea is to avoid queue starvation; queues need to eat, and bandwidth is their food. CQ provides 16 queues you can tweak, plus a system queue that’s always there [...]

Priority queuing is interesting in how it schedules packets, but isn’t very practical for real-world networks. The PQ scheduler has 4 queues: high, medium, normal and low. The PQ scheduler services those queues in that order. If there is a packet in a given queue, that packet is forwarded, and then [...]

Why do routers queue? Because an interface can only send one packet at a time. If that interface is busy, and another packet is ready to go, it must wait in line.
FIFO = first in, first out. A FIFO queue is a software queue to hold packets while they packets wait, in [...]

OECG chapter 15 is a breakdown of the major queuing methods employed in Cisco routers and switches. The mechanics of the queuing methods are described. I’ll break up each queuing method into their own blog post as I go through them. It will make for shorter blog posts, but it should keep [...]

Assured forwarding - a DiffServ PHB defining 4 queueing classes and 3 probabilities that a packet with be dropped within a queue class.
Cell Loss Priority - when this bit is set to “1″ in an ATM cell, it tells an ATM device that this cell should be dropped before others in times of congestion
class-based marking [...]

04/02/2007 - 1.5 hours. Ch. 13 study.
04/05/2007 - 1.5 hours. Ch. 13 study.
04/06/2007 - 1.5 hours. Ch. 13 study.
04/09/2007 - 2.0 hours. Ch. 13 study. (Will this chapter ever end?!?) I just peeked ahead - I’ll finish Ch. 13 tomorrow or the next day. Ch. 14 is pretty [...]