Junos Packet Flow: A Deep Dive

The Juniper Networks Certified Specialist Enterprise Routing and Switching (JNCIS-ENT) certification is a credential that validates your understanding of networking technologies and related platform configuration and troubleshooting skills. One of the key topics covered in this certification is the Junos packet flow. This blog post will provide a deep dive into this topic.

Understanding Packet Flow

Packet flow in Junos is the process by which packets are processed by a Juniper router or switch. This process involves several stages, including ingress, processing, and egress.


The ingress stage is where the packet enters the device. The packet is first received by the Physical Media Dependent (PMD) layer, which is responsible for the physical connection to the network. The packet is then passed to the Media Access Control (MAC) layer, which adds the MAC address of the device to the packet.


The processing stage is where the packet is examined and decisions are made about where it should be sent. This stage involves several steps:

  1. Input Check: The packet is checked for errors. If any are found, the packet is discarded.

  2. Filtering: The packet is checked against any filters that have been set up. If the packet matches a filter, the appropriate action is taken.

  3. Routing: The packet is sent to the appropriate routing table, based on its destination IP address.

  4. Forwarding: The packet is sent to the appropriate interface for transmission to its destination.


The egress stage is where the packet leaves the device. The packet is sent to the MAC layer, which adds the MAC address of the next hop to the packet. The packet is then sent to the PMD layer, which transmits the packet over the physical network connection.

Packet Flow in Junos

In Junos, packet flow is handled by two main components: the Packet Forwarding Engine (PFE) and the Routing Engine (RE).

The PFE is responsible for the ingress and egress stages of packet flow. It receives packets, processes them, and sends them out. The PFE also handles tasks such as filtering and classifying packets.

The RE is responsible for the processing stage of packet flow. It makes decisions about where packets should be sent, based on information in the routing table.

Understanding packet flow in Junos is crucial for troubleshooting network issues and for optimizing network performance. By understanding how packets are processed by a Juniper device, you can better understand how to configure and manage your network.

In conclusion, Junos packet flow is a complex process that involves several stages and components. However, with a solid understanding of this process, you can effectively manage and troubleshoot your Juniper network.

© Ben Jacobson.RSS