Weighted Fair Queuings

Weighted fair queuing (WFQ)
provides equal amounts of bandwidth to each conversation that
traverses the interface. WFQ uses a process that refers to the timestamp found on the last bit of
a packet as it enters the queue.
Assigning Priorities
WFQ assigns a high priority to all low-volume traffic. Figure 30.3 demonstrates how the timing
mechanism for priority assignment occurs. The algorithm determines which frames belong to either
a high-volume or low-volume conversation and forwards the low-volume packets from the queue
first. Through this timing convention, remaining packets can be assigned an exiting priority.
In Figure 30.3, packets are labeled A through F. As depicted in this figure, Packet A will
be forwarded first because it’s part of a low-volume conversation, even though the last bit
of session B will arrive before the last bit of the packets associated with Packet A did. The
remaining packets are divided between the two high-traffic conversations, with their timestamps
determining the order in which they will exit the queue.
FIGURE 3 0 . 3
Priority assignment using WFQ
Assigning Conversations
We’ve discussed how priority is assigned to a packet or conversation, but it’s also important to
understand the type of information that the processor needs to associate a group of packets with
an established conversation.
The most common elements used to establish a conversation are as follows:

Source and destination IP addresses

MAC addresses

Port numbers

Type of service

DLCI number assigned to an interface
Say a router has two active conversations, one a large FTP transfer and the other an HTTP
session. The router, using some or all of the factors just listed to determine which conversation
a packet belongs to, allocates equal amounts of bandwidth to each conversation. Each of the
two conversations receives half of the available bandwidth.

IOS Queuing Options

As we’ve said, if your serial links are not congested, you do not need to implement queuing.
However, if the load exceeds the transmission rate for small periods of time, you can use a Cisco
IOS queuing option to help the congestion on a serial link.
To effectively configure queuing on a serial link, you must understand the types of queuing
available. If you choose the wrong type of queuing, you can do more harm on the link than help.
Also, this is not a one-time analysis of traffic patterns. You must constantly repeat your analysis
of your serial link congestion to make sure you have implemented the queuing strategy correctly.
Figure 30.2 shows the queuing options available from Cisco.
FIGURE 3 0 . 2
Queuing options
The following steps and Figure 30.2 describe the analysis you should make when deciding on
a queuing policy:
1.
Determine whether the WAN is congested.
2.
Decide whether strict control over traffic prioritization is necessary and whether automatic
configuration is acceptable.
3.
Establish a queuing policy.
4.
Determine whether any of the traffic types you identified in your traffic pattern analysis can
tolerate a delay.