We have now a global idea about the Routing Engine, which is inside the Control Plane, let’s see what is the Packet Forwarding Engineer (PFE) which is inside the Forwarding Plane.
Juniper defines the PFE as the muscle of the device. Why the muscle of the device? Because it has the function to allow mainly the transit packets to pass. The PFE use a complete other hardware than in the Routing engine, and in some Juniper models it uses ASICs ship which is considered a very fast ship when it comes to processing the traffic, and this makes sense as PFE has its main function to allow the transit traffic to pass.
If you look in the PFE, you see that there is a Forwarding Table (FT). This FT is used to route the transit packets correctly. It is an identical copy of the Primary Forwarding Table that is in the Routing engine, meaning it has only the active routes. Using the internal link, the Forwarding table in the routing engine sends a copy of its active routes to the forwarding table in the Packet Forwarding Engine. Now in case any route goes down, then the primary FT in the routing engine sends only routing changes to the FT in the packet forwarding engine using the internal link, meaning that it doesn’t send the complete routing table for the routing update.
Other function that the PFE is responsible for are:
- Rate limiting and Class of Service (which means Quality of Services)
- Stateless Firewall
Finally, I want to give an example so you can understand more the difference between the RE and PFE. Let’s say that we want to run a debug on the command line (named traceoptions on Juniper) to be able to check what is happening on your router. Using debug command on many other vendors will take a lot of loads on the router device and it can lead to crash the router and not being able to do its normal function anymore. While on Juniper this doesn’t happen, that’s because the debug command line is running on the RE while the traffic is flowing on the PFE. So, in case the debug froze our CLI, then we shouldn’t worry a lot because the traffic will remain working without anymore. I wanted to give you this example so you can understand more the difference between the RE and the PFE.
That’s all what you need to know about the Packet Forwarding Engine in the Forwarding plane.