notes: QoS Overview

Voice, video, and data travel side by side over today’s converged networks. Some of these traffic types (for example, VoIP) need better treatment (that is, higher priority) than other types of traffic (for example, FTP). Fortunately, Cisco offers a suite of QoS tools for providing special treatment for special traffic.
In the absence of QoS, traffic might suffer from one or more of the following symptoms:

1.  Delay (latency): Excessive time required for a packet to traverse the network
2.  Delay variation (jitter): The uneven arrival of packets, which in the case of VoIP can be interpreted by the listener as dropped voice packets
3.  Packet loss: Dropping packets, especially problematic for User Datagram Protocol (UDP) traffic (for example, VoIP), which does not retransmit dropped packets.

- The TX-Ring/Hardware queue is always FIFO. It can be seen with the "sh controllers" command.
- QOS affects how traffic is processed in the output queue/software queue before the hardware queue.
- Queueing is always applied outbound to the interface.
- Shaping is always applied outbound to the interface.
- Policing can be applied inbound or outbound to the interface.
- The default input hold-queue limit is 75 packets. 10 packets for async interfaces.
- The default output hold-queue limit is 40 packets. 10 packets for async interfaces.
- A length of 1000 will normally resolve problems caused by input queue drops of TCP ACKs, but will introduce bigger delay.

commands:

- Shows the TX queue length for an interface

sh controllers Se0/0 | i tx_limit

- Changes the (default=6) telnet marking for telnets from the local router

ip telnet tos {tos-value} 

- Changes the TX queue length for an interface
- Sets the length of time used for load calculations
- This command limits the size of the IP queue on an interface

interface S0/0

tx-ring-limit {number}

load-interval seconds

hold-queue {length} {in|out}