# Exploring Junos CoS Components: Queues, Forwarding Classes, and Policers When studying for the JNCIS-ENT certification, understanding the Junos Class of Service (CoS) components is crucial. In this blog post, we will delve into three key components: Queues, Forwarding Classes, and Policers. ## Queues In Junos, queues are used to manage the traffic flow. Each interface has multiple queues, and each queue has a specific priority level. The traffic is placed into these queues based on its forwarding class. There are four types of queues in Junos: 1. **Best Effort (BE)**: This is the default queue where traffic is placed if no specific forwarding class is assigned. 2. **Assured Forwarding (AF)**: This queue is for traffic that requires a minimum guarantee of bandwidth. 3. **Expedited Forwarding (EF)**: This queue is for traffic that requires low delay, jitter, and loss. 4. **Network Control (NC)**: This queue is for network control traffic, such as routing updates and network management traffic. ## Forwarding Classes Forwarding classes in Junos are used to classify traffic into different categories. Each forwarding class is associated with a queue. When a packet arrives at a router, it is classified into a forwarding class based on its CoS values (such as DSCP or IP precedence). The packet is then placed into the corresponding queue. There are four default forwarding classes in Junos: 1. **Best Effort (BE)** 2. **Assured Forwarding (AF)** 3. **Expedited Forwarding (EF)** 4. **Network Control (NC)** You can also create custom forwarding classes to meet specific network requirements. ## Policers Policers in Junos are used to control the rate of traffic flow. They work by comparing the traffic rate with a predefined limit. If the traffic rate exceeds the limit, the policer takes action, such as dropping the excess traffic or marking it for a lower priority queue. There are two types of policers in Junos: 1. **Single-rate two-color policer**: This policer allows you to define a single rate limit and two actions (one for conforming traffic and one for exceeding traffic). 2. **Two-rate three-color policer**: This policer allows you to define two rate limits and three actions (one for conforming traffic, one for exceeding traffic, and one for violating traffic). In conclusion, understanding how queues, forwarding classes, and policers work in Junos is essential for managing traffic flow and ensuring quality of service in your network. As you prepare for the JNCIS-ENT certification, make sure to get hands-on experience with these CoS components. © Ben Jacobson.RSS