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QoSCCNA 200-301

WFQ Not Configured — Interactive Traffic Getting Poor Performance

Presenting Symptom

Interactive traffic (e.g., VoIP, Telnet) experiences high latency and jitter, while bulk data transfers perform well.

Network Context

A small branch office with a single Cisco 4321 router (IOS XE 16.9) connecting to the WAN via a 10 Mbps serial link. The router has multiple traffic types: VoIP (EF), interactive (AF21), and bulk data (default). No QoS policy is applied to the serial interface, and the default FIFO queue is in use.

Diagnostic Steps

1

Check interface queueing and congestion drops

show interfaces serial 0/0/0
Serial0/0/0 is up, line protocol is up 
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 20000 usec, 
  reliability 255/255, txload 1/1, rxload 1/1 
  Encapsulation HDLC, loopback not set 
  Keepalive set (10 sec) 
  Queueing strategy: fifo 
  Output queue: 0/40 (size/max) 
  5 minute input rate 0 bits/sec, 0 packets/sec 
  5 minute output rate 0 bits/sec, 0 packets/sec 
     0 packets input, 0 bytes, 0 no buffer 
     Received 0 broadcasts (0 IP multicasts) 
     0 runts, 0 giants, 0 throttles 
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 
     0 watchdog, 0 multicast, 0 pause input 
     0 packets output, 0 bytes, 0 underruns 
     0 output errors, 0 collisions, 2 interface resets 
     0 unknown protocol drops 
     0 babbles, 0 late collision, 0 deferred 
     0 lost carrier, 0 no carrier, 0 pause output 
     0 output buffer failures, 0 output buffers swapped out

The 'Queueing strategy: fifo' indicates FIFO (default) is used, not WFQ. The output queue size is 40 (default for FIFO). If there were drops, they would show as 'output queue drops'. No drops here, but under load, FIFO can cause latency for interactive traffic.

2

Check if WFQ is enabled on the interface

show queueing interface serial 0/0/0
Interface Serial0/0/0 queueing strategy:  fifo 
  Output queue utilization (queue/count) 
    0/0

Confirms FIFO is active. WFQ would show 'fair' or 'weighted fair' and list queues per flow.

3

Monitor interface congestion and drops during traffic

show interfaces serial 0/0/0 | include output drops|queue
  Queueing strategy: fifo 
  Output queue: 0/40 (size/max) 
  0 output buffer failures, 0 output buffers swapped out 
  0 output errors, 0 collisions, 2 interface resets 
  0 output queue drops

If no drops, congestion may be low. But interactive traffic still suffers due to FIFO's lack of prioritization. Under load, drops would appear here.

4

Check for any existing QoS policy on the interface

show running-config interface serial 0/0/0
interface Serial0/0/0
 ip address 10.1.1.1 255.255.255.252
 encapsulation hdlc
 no fair-queue
 clock rate 2000000

The 'no fair-queue' command explicitly disables WFQ. This confirms WFQ is not configured. The absence of any 'service-policy' indicates no MQC policy either.

Root Cause

The serial interface has FIFO queueing enabled (default) and WFQ is explicitly disabled with 'no fair-queue'. Without WFQ, all traffic is treated equally, causing interactive traffic (e.g., VoIP, Telnet) to experience high latency and jitter when the link is congested, as bulk data fills the queue.

Resolution

Enable WFQ on the serial interface to provide fair queuing per flow, which gives interactive flows better service. Commands: ``` interface serial 0/0/0 fair-queue ``` Alternatively, if using MQC, apply a policy-map with class-based WFQ (CBWFQ) or LLQ. For CCNA, the simple 'fair-queue' command is sufficient.

Verification

Run 'show queueing interface serial 0/0/0' to confirm WFQ is active. Expected output: ``` Interface Serial0/0/0 queueing strategy: weighted fair Output queue utilization (queue/count) 0/0 Conversation 0, link none, flags 0x0 Conversation 1, link none, flags 0x0 ... ``` Also run 'show interfaces serial 0/0/0' to see 'Queueing strategy: fair'.

Prevention

1. Always enable WFQ on low-speed WAN interfaces (default on serial interfaces below 2 Mbps, but verify). 2. Use MQC with LLQ for critical traffic like VoIP to guarantee bandwidth and low latency. 3. Monitor interface queue drops and latency regularly using 'show interfaces' and 'show queue'.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests understanding of QoS queueing mechanisms. Expect multiple-choice questions asking which queueing method provides per-flow fairness (WFQ) or which command enables WFQ on a serial interface. The exam may also present a troubleshooting scenario where FIFO causes poor performance for interactive traffic, and the candidate must identify the fix (enable fair-queue).

Exam Tips

1.

Remember that WFQ is enabled by default on serial interfaces with bandwidth below 2 Mbps, but can be disabled with 'no fair-queue'.

2.

Know that FIFO is the default on interfaces above 2 Mbps and does not provide any prioritization.

3.

The command 'fair-queue' under interface configuration enables WFQ; it is a simple, quick fix for per-flow fairness.

Commands Used in This Scenario

show interfaces

Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.

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