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Flow-Based Weighted Fair Queueing

A Cisco Router Tutorial

By Chris Bryant, CCIE #12933

What's so "fair" about Weighted Fair Queueing (WFQ)? WFQ prevents one particular stream of network traffic, or flow, from using most or all of the available bandwidth while forcing other streams of traffic to sit and wait. These flows are defined by WFQ and require no access list configuration. Flow-based WFQ is the default queueing scheme for Serial interfaces running at E1 speed or below.

Flow-Based WFQ takes these packet flows and classifies them into conversations. WFQ gives priority to the interactive, low-bandwidth conversations, and then splits the remaining bandwidth fairly between the non-interactive, high-bandwidth conversations.

In the following exhibit, a Telnet flow reaches the router at the same time as two FTP flows. Telnet is low-volume, so the Telnet transmission will be forwarded first. The two remaining file transfers will then be assigned a comparable amount of bandwidth. The packets in the two file transfers will be interleaved - that is, some packets for Flow 1 will be sent, then some for Flow 2, and so on. The key here is that one file transfer flow will not have priority over the other.

WFQ Flow-Based

Enabling flow-based WFQ is simple enough. We don't even have to configure it on the following Serial interface, since WFQ is enabled by default on all serial interfaces running at or below E1 speed, but let's walk through the steps:

R1(config)#int serial0
R1(config-if)#fair-queue ?
<1-4096> Congestive Discard Threshold
<cr>

The Congestive Discard Threshold dictates the number of packets that can be held in a single queue. The default is 64. Let's change it to 200.

R1(config)#int serial0
R1(config-if)#fair-queue ?
<1-4096> Congestive Discard Threshold
<cr>

R1(config-if)#fair-queue 200

To verify your queuing configuration, run show queue followed by the interface type and number.

R1#show queue serial0
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: weighted fair
Output queue: 0/1000/200/0 (size/max total/threshold/drops)
Conversations 0/0/256 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 1158 kilobits/sec

IOS Help shows other WFQ Options:

R1(config-if)#fair-queue 200 ?
<16-4096> Number Dynamic Conversation Queues
<cr>

The Dynamic Conversation Queues are used for normal, best-effort conversations. We'll change that to 200 as well.

R1(config-if)#fair-queue 200 200
Number of dynamic queues must be a power of 2 (16, 32, 64, 128, 256, 512, 1024)

Then again, maybe we won't. Let's change it to 256 instead and use IOS Help to show any other options.

R1(config-if)#fair-queue 200 256 ?
<0-1000> Number Reservable Conversation Queues

The final WFQ option is the number of Reservable Conversation Queues. The default here is zero. These queues are used for specialized queueing and Quality of Service features like the Resource Reservation Protocol (RSVP). We'll set this to 100.

R1(config-if)#fair-queue 200 256 100

show queue verifies that all three of these values have been successfully set, as does show queueing fair.

R1#show queue serial 0
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: weighted fair
Output queue: 0/1000/200/0 (size/max total/threshold/drops)
Conversations 0/0/256 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 1158 kilobits/sec

Earlier in this section, I mentioned that serial interfaces running at E1 speed or lower will run WFQ by default. However, if any of the following features are running on the interface, WFQ will not be the default.