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Router Data Flow Overview

Flow of Routing Information

Routing information is the information about routes learned by the routing protocols from a router’s neighbors. This information is stored in routing tables. The routing protocols advertise active routes only from the routing tables. An active route is a route that is chosen from all routes in the routing table to reach a destination.

To control which routes the routing protocols place in the routing tables and which routes the routing protocols advertise from the routing tables, you can configure routing policies, which are sets of rules that the policy framework uses to preempt default routing policies.

The Routing Engine, which is the router’s control plane, handles the flow of routing information between the routing protocols and the routing tables and between the routing tables and the forwarding table. The Routing Engine runs the Junos OS and routing policies and stores the active router configuration, the master routing table, and the master forwarding table,

Flow of Data Packets

Data packets are chunks of data that transit the router as they are being forwarded from a source to a destination. When a router receives a data packet on an interface, it determines where to forward the packet by looking in the forwarding table for the best route to a destination. The router then forwards the data packet toward its destination through the appropriate interface.

The Packet Forwarding Engine, which is the router’s forwarding plane, handles the flow of data packets in and out of the router’s physical interfaces. Although the Packet Forwarding Engine contains Layer 3 and Layer 4 header information, it does not contain the packet data itself (the packet's payload).

Flow of Local Packets

Local packets are chunks of data that are destined for or sent by the router. Local packets usually contain routing protocol data, data for IP services such as Telnet or SSH, and data for administrative protocols such as the Internet Control Message Protocol (ICMP). When the Routing Engine receives a local packet, it forwards the packet to the appropriate process or to the kernel, which are both part of the Routing Engine, or to the Packet Forwarding Engine.

The Routing Engine handles the flow of local packets from the router’s physical interfaces and to the Routing Engine.

Interdependent Flows of Routing Information and Packets

Figure 1 illustrates the flow of data through a router. Although routing information flows and packet flows are very different from one another, they are also interdependent.

Figure 1: Flows of Routing Information and Packets

Routing policies determine which routes the Routing Engine places in the forwarding table. The forwarding table, in turn, has an integral role in determining the appropriate physical interface through which to forward a packet.