4.4QoS, SSH, and file transfer

Because bandwidth is finite, treating delay-sensitive traffic like voice/video at the same priority as backup traffic that tolerates delay causes calls to break up. So we prioritize important traffic with QoS. Also, if the path for remote device management is plaintext, credentials can be stolen, so we encrypt it with SSH and choose a transfer protocol suited to the task for moving IOS and config files. This section treats these as practical judgment: which to choose for a requirement, and how to configure it.

4.4.1The per-hop behavior of QoS

• QoS controls how each hop (device) treats a packet (per-hop behavior). First, classification identifies traffic by type (via ACL, NBAR, etc.), and marking stamps a priority onto the packet. At L3 this uses the IP header's DSCP (6 bits; voice is typically EF = DSCP 46), and at L2 the 802.1Q CoS; downstream devices trust this mark and prioritize accordingly.
• Congestion handling includes queuing (controlling send order, e.g., putting voice in the low-latency priority queue LLQ) and congestion avoidance (e.g., WRED, dropping some packets early before the queue fills). Rate enforcement of excess traffic comes in two forms: policing drops (or re-marks) excess and does not buffer—so it adds no delay but causes drops, while shaping buffers excess and sends it later—smoothing traffic but adding delay (latency).