📘 CCNA 200-301 v1.1

4.7 Explain the forwarding per-hop behavior (PHB) for QoS such as classification, marking, queuing, congestion, policing, and shaping

PHB (Per-Hop Behavior) is part of Quality of Service (QoS) in networking. QoS helps networks manage traffic so that important data gets priority and network performance stays good, even when traffic is heavy. PHB describes how each network device (like a router or switch) handles packets as they pass through it.

Imagine data moving from one device to another. At every hop (every router or switch along the path), the device decides what to do with that packet. This is the “per-hop behavior.”

There are several key parts of PHB: Classification, Marking, Queuing, Congestion Management, Policing, and Shaping. Let’s go step by step.

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1. Classification

• What it is: Sorting packets into groups based on rules.
• Why it matters: Different types of traffic might need different priorities.
• How it works in IT:
  • Routers or switches examine the packet headers (like source/destination IP, port numbers, or protocol type).
  • Example: Voice packets (VoIP) vs. Email packets. Voice packets can be classified as “high priority,” Email as “normal priority.”
• Key point: Classification does not change the packet, it just labels it for the next step.

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2. Marking

• What it is: Adding a “label” or “tag” to packets to indicate their priority.
• Why it matters: Downstream devices know how to handle traffic based on the markings.
• How it works in IT:
  • Routers can mark packets using DSCP (Differentiated Services Code Point) or IP Precedence in the IP header.
  • Example: Voice traffic can get a DSCP value of 46 (high priority), while email gets 0 (default).
• Key point: Marking does not drop or delay packets; it just tells the network how to treat them.

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3. Queuing

• What it is: Managing packets waiting to be sent out of an interface.
• Why it matters: When traffic exceeds link capacity, we need to decide the order packets leave.
• How it works in IT:
  • Packets are placed in queues based on priority.
  • High-priority traffic (like VoIP) can go into a priority queue and get sent first.
  • Normal traffic waits in lower-priority queues.
• Key point: Queuing ensures important traffic doesn’t get stuck behind less important traffic.

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4. Congestion Management

• What it is: Deciding which traffic gets sent when a device is congested.
• Why it matters: Links can get full. The network must manage traffic to avoid dropping critical packets.
• How it works in IT:
  • Routers use queuing strategies like Weighted Fair Queuing (WFQ) or Low-Latency Queuing (LLQ).
  • LLQ gives guaranteed bandwidth to high-priority traffic.
• Key point: Congestion management controls order and timing, not volume.

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5. Policing

• What it is: Limiting traffic to a certain rate.
• Why it matters: Prevents devices from sending more traffic than allowed, protecting the network.
• How it works in IT:
  • If a user or application sends more data than allowed, extra packets are dropped or remarked.
  • Example: A company allows a video stream to use 5 Mbps. If the stream sends 7 Mbps, 2 Mbps gets dropped or downgraded.
• Key point: Policing is strict—extra traffic is discarded immediately.

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6. Shaping

• What it is: Controlling the traffic rate smoothly, delaying excess packets instead of dropping them.
• Why it matters: Helps avoid sudden bursts that overwhelm the network.
• How it works in IT:
  • Excess packets are buffered and delayed so traffic flows at the allowed rate.
  • Example: A router shapes traffic to 5 Mbps by holding extra packets and sending them gradually.
• Key point: Shaping delays traffic, while policing drops traffic.

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Putting it all together

Here’s a simple flow of PHB in a router:

1. Classification: Identify packet type (voice, video, data).
2. Marking: Label packets with priority values (DSCP).
3. Queuing: Place packets in the correct queue based on priority.
4. Congestion Management: Decide which queue gets bandwidth if congested.
5. Policing/Shaping: Ensure traffic doesn’t exceed allowed limits—drop (policing) or delay (shaping) excess traffic.

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✅ Key CCNA points to remember for the exam:

• PHB is part of DiffServ QoS model.
• Classification → Marking → Queuing → Congestion → Policing/Shaping.
• Policing drops, Shaping delays.
• DSCP values and IP Precedence are used for marking.
• Queues ensure priority traffic gets delivered first during congestion.

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