IP Services · Topic 7 of 9
QoS Per-Hop Behavior
Understand how QoS treats traffic hop by hop, including classification, marking, queuing, congestion avoidance, policing and shaping, in plain English.
- 4 min
- Exam objective 4.7
- QoS decides treatment when the network is congested.
- PHB means each device chooses its own treatment from the marking.
- The order is classification, marking, then queuing, congestion handling and rate limiting.
What it is
QoS lets a network treat some traffic better than other traffic.
Without it, every packet is handled first in, first out. That is fine until the network is busy. Then a video call and a large download compete for the same link, and the call suffers.
QoS protects traffic that is sensitive to four problems:
- Bandwidth: not enough capacity for everything.
- Delay: packets take too long to arrive.
- Jitter: delay that varies from packet to packet, which hurts voice and video.
- Loss: packets dropped, often because a queue is full.
QoS does not create bandwidth. It decides who goes first and who waits.
Per-hop behavior (PHB) means each device decides on its own how to treat a packet, based on its marking. If one device ignores the marking, the special treatment ends there.
How it works
QoS is a chain of steps that each device applies, in this order.
Classification and marking
Classification sorts traffic into groups, using an access list, a protocol or an existing marking. Marking writes a label on the packet so later devices need not classify it again.
Three common markings:
- CoS: a 3-bit field in the 802.1Q tag on Layer 2 frames, values 0 to 7.
- IP Precedence: a 3-bit field in the IP header, values 0 to 7.
- DSCP: a 6-bit field in the same IP header byte, values 0 to 63. It is the more detailed replacement for IP Precedence.
Well-known DSCP values show how PHBs are named:
- Default (0): best effort, no special treatment.
- EF (46): low delay and low loss, used for voice.
- AF: written AFxy, where x is the class and y is the drop likelihood under congestion. A higher y means dropped sooner.
- CS: values that match the older IP Precedence.
Voice is EF, DSCP 46. In AFxy, a higher class x gets better treatment, but a higher drop value y means dropped sooner.
Queuing
Queuing decides the order in which waiting packets leave.
| Method | Idea |
|---|---|
| FIFO | First in, first out, no preference |
| WFQ | Weighted fair queuing, shares the link among flows |
| CBWFQ | Class based, guarantees bandwidth per traffic class |
| LLQ | CBWFQ plus a strict priority queue for voice |
Congestion avoidance
Tail drop discards new packets when a queue is full, hitting all traffic equally. WRED drops a few packets early, favouring less important ones, so TCP senders slow down before the queue overflows.
Policing and shaping
Both limit traffic to a rate, but they act differently.
| Policing | Shaping | |
|---|---|---|
| Excess traffic | Dropped or re-marked | Held and sent later |
| Effect on delay | None added, more loss | Adds delay, smooths flow |
| Direction | Inbound or outbound | Outbound only |
Shaping is typical when a provider enforces a rate lower than your link speed.
The trust boundary
The trust boundary is the point where you decide to accept markings from a device. A phone on a switch may be trusted, while a PC may not be. Traffic is usually re-marked at the edge so devices further along can rely on the labels.
What to remember for the exam
Nine points matter most.
- QoS decides treatment when the network is congested.
- PHB means each device chooses its own treatment from the marking.
- The order is classification, marking, then queuing, congestion handling and rate limiting.
- CoS is Layer 2. IP Precedence and DSCP are Layer 3.
- DSCP has 6 bits. EF, value 46, is the voice marking.
- LLQ is CBWFQ with a strict priority queue.
- WRED drops early to avoid the sudden loss of tail drop.
- Policing drops or re-marks the excess. Shaping delays it.
- Voice needs low delay, jitter and loss. Cisco's design guidance for voice gives targets of about 150 ms one-way delay, 30 ms jitter and 1 percent loss.
Common mistakes
These five errors are frequent.
Believing QoS adds bandwidth. If a link is permanently too small, QoS cannot fix it.
Mixing up policing and shaping. Shaping buffers and delays. Policing drops or re-marks.
Mixing up CoS and DSCP. CoS lives in the VLAN tag and is lost when a frame is routed. DSCP travels end to end in the IP header.
Assuming markings are global. Each hop decides whether to honour them.
Reading higher AF numbers as always better. Class x is better when higher, but drop value y is worse when higher.
Giving every traffic class top priority removes the point of having priorities.
Check yourself
Answer these three prompts.
What do classification and marking each do, and why does marking help later devices?
Classification sorts traffic into groups, and marking writes a label on the packet. Later devices then need not classify it again.
How do policing and shaping differ in what happens to excess traffic?
Policing drops or re-marks the excess traffic. Shaping holds it in a queue and sends it later.
Why does voice go in a priority queue, and which DSCP value commonly identifies it?
Voice needs low delay, jitter and loss, so LLQ gives it a strict priority queue. It is marked EF, DSCP 46.
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