CCNA Introduction to QoS
Notes
Chapter #2 – What is QoS?
Networks experience congestion — like traffic congestion on the road — when a link has more traffic than it can handle. Once this happens, some packets are dropped.
QoS helps by prioritising important traffic over less important traffic. If traffic must be dropped, it’s the less important traffic. QoS doesn’t magically make links better, it just manages the traffic on them better. If a link is regularly full, it’s time to upgrade that link.
Chapter #3 – Application Traffic
Different traffic needs to be handled in different ways. Some traffic, like voice (phone calls, Teams, Zoom), may not need more bandwidth — it just needs to be delivered reliably and on time. Other traffic can be shaped (or buffered).
Real-time traffic (voice, video calls) needs to be handled differently to streaming traffic (Netflix, YouTube), and to transactional traffic (HTTP, SQL).
There are four important traffic characteristics:
- Bandwidth — the amount of data transferred per second
- Loss — when traffic is lost before it reaches its destination
- Latency — the time it takes traffic to pass from one endpoint to another (RTT is similar, but measures return time too)
- Jitter — the variance in latency
Chapter #4 – Classification and Marking
Every device in the network has its own QoS configuration. Sometimes this means no configuration. This is known as Per Hop Behaviour (PHB), as each device needs to make its own decisions.
Each device needs to classify packets as they arrive at the router. Other actions are applied as the packets are forwarded out.
Each device needs to work toward a common goal, so each device should be configured in a way that helps the entire network. This is called End-to-End QoS.
Marking packets is where the router will add extra information to a packet. This is done in the IP header or the Ethernet header. By marking a packet, other devices can make smarter decisions when deciding what to do with it.
The old way of marking was a 3-bit field in the IP header (ToS) called IP Precedence. This has been replaced with DSCP/DiffServ, which is 6-bits. This is backward-compatible with IP Precedence.
IP Precedence allows for up to 8 classes, called CS0–CS7. DSCP allows for many more combinations, in addition to the CS classes. Some of the new classes (AF, or Assured Forwarding) have a drop-probability, which is similar to a sub-class.
Chapter #5 – Cisco’s MQC
MQC, or Modular QoS CLI, is Cisco’s name for the QoS configuration hierarchy.
Class-maps are used to identify and classify traffic. This can be done solely within the class-map, or with the help of an access-list.
Policy-maps assign actions to the class-maps. This could be marking traffic, or it could be something like reserving bandwidth.
A service-policy assigns a policy-map to an interface. This can apply to the ‘in’ or ‘out’ direction. Some actions, like marking, should be applied as traffic comes in. Other actions, like reserving bandwidth, should be applied as traffic leaves the interface.
We could optionally use NBAR to classify traffic. This is a Cisco feature that identifies traffic based on its application.
Command Summary
| Command | Mode | Description |
|---|---|---|
| ip access-list extended <name> | Global Config | Create an extended ACL |
| class-map [match-all|match-any] <name> | Global Config | Create a new class map |
| match access-group name <name> | Class-map Config | Match criteria within an ACL |
| policy-map <name> | Global Config | Create a policy |
| class <name> | Policy-map Config | Configure class actions within a QoS policy |
| bandwidth percent <value> | Policy-map Class Config | Reserve bandwidth for a class |
| service-policy <name> | Interface Config | Apply a QoS policy to an interface |
| bandwidth <value> | Interface Config | Configure the bandwidth of an interface |
| show policy-map interface <interface> | Privileged Exec | Show QoS settings on an interface |