A few questions regarding distributed CEF

[Mitrixsen]

Hello, everyone.

I have 2 questions regarding dCEF and chassis devices as I just started learning about them.

1. I've read that there are Route Switch Processor cards in chassis devices which serve the same purpose as a CPU. My question is, what is a processor card? I imagine a CPU being something you plug into a CPU socket, I’ve never seen it as a card before. Does the card's function differ in any way from regular CPUs?

2. With Distributed CEF, there are CEF components (FIB, ADJ tables) on each linecard, correct? So the device has CEF distributed across all of its linecards which then perform forwarding decisions and thus relieve the route processor of that function.

How exactly can linecards perform forwarding decisions? Is there some sort of chip like an ASIC built into them?

Thank you.

[M02@rt37]

Hello @Mitrixsen 

In high-performance routers and switches, a processor card, often referred to as a RSP, is a modular card that contains the CPU and other necessary components for managing the device. Unlike general-purpose CPUs that are plugged into a motherboard socket, processor cards are designed to be easily inserted or replaced in a chassis, integrating the CPU with memory, storage, and network-specific processing circuitry optimized for high-speed packet processing and control plane operations. In the context of CEF and its dCEF variant, the packet-forwarding mechanism is optimized by distributing FIB and adjacency tables to each linecard within the router. These linecards, which often contain ASICs and sometimes NPUs, are specialized chips designed to handle specific network functions at high speed, processing packets at line rate.

By having a copy of the FIB and adjacency tables, each linecard can independently make forwarding decisions locally, without consulting the central processor card. This distributed architecture reduces the load on the central processor, improves overall performance, increases throughput, and reduces latency by enabling efficient parallel processing as packets are forwarded directly by the linecards.

[Joseph W. Doherty · Accepted solution]

"I've read that there are Route Switch Processor cards in chassis devices which serve the same purpose as a CPU."

Not exactly.   A general CPU might be a component on such a card, but the card likely has other components to support routing and switching.  A RSP is sort of the router equivalent of a L2/L3 switch supervisor card.  Also, even within Cisco, the same "functional" card, might use different terms (and the approach isn't new).  On the CRS, card was simply a RP (route processor) on the 12000 series, is was the GRP (gigabit route processor).  On the 7200 routers, you had NPE (network processing engine - component 9 is the CPU)  and NSE (network service engine).  

"My question is, what is a processor card? I imagine a CPU being something you plug into a CPU socket, I’ve never seen it as a card before."

Example

"With Distributed CEF, there are CEF components (FIB, ADJ tables) on each linecard, correct?"

Maybe, maybe not.  Depends on the capabilities of the linecard and also whether it's a standard feature of the line card or an option.

"So the device has CEF distributed across all of its linecards which then perform forwarding decisions and thus relieve the route processor of that function."

Yes, the purpose of distributed CEF is to offload the main RP, but since its often LC dependent, some LCs might support it, and some might not.

For example, on the 6500 series, dCEF was provide by a DFC daughter card, which would piggy back onto the line card.

"How exactly can linecards perform forwarding decisions? Is there some sort of chip like an ASIC built into them?"

Pretty much the same as the main RP, it just gets first crack.

If you're inexperienced with Cisco RP cards, and DFC kind of add-ons, are you familiar with PCs?  Ever add or swap a video card?  Can PC "operate" a video monitor without an add-on video card?  Much the same idea, a video/graphics cards boosts an aspect of the PC's performance much as some form of RP add-on would do.

[Joseph W. Doherty]

Oh, I might add, when the device supports dual main RPs, or supervisors, usually one is just a warm spare whereas a line card add-on is always active (although usually only for ingress traffic on that particular LC).  However, sometimes the standby main RP might contribute to overall device forwarding.

[M02@rt37]

Perfect explication, thanks @Joseph W. Doherty.

When discussing RSP cards in chassis devices, it's important to clarify that they serve a broader purpose than a typical CPU. While a general CPU might be one component of an RSP, these cards include other elements necessary for routing and switching tasks. RSPs are analogous to L2/L3 switch supervisor cards. For instance, in Cisco devices, these cards might be referred to as Route Processors in the CRS series or Network Processing Engines (NPE) and Network Service Engines (NSE) in the 7200 series. Each of these cards integrates a CPU, memory, and specialized hardware to manage control-plane functions such as routing protocol computations and network management.

Regarding dCEF, whether each line card has CEF components like FIB and adjacency tables can vary based on the line card's capabilities and whether it's a standard or optional feature. The primary purpose of dCEF is to offload forwarding decisions from the main RP to the line cards, thus enhancing performance. For example, in the Cisco 6500 series, dCEF functionality can be provided by Distributed Forwarding Cards called 'DFCs' (see picture), which are daughter cards that attach to the line cards. These line cards can perform forwarding decisions using ASICs, which are specialized chips designed for high-speed packet processing. The ASICs allow line cards to quickly make forwarding decisions, distributing the load and increasing the overall throughput and efficiency of the network device. This setup is similar to how a video card in a PC enhances graphics performance, where line cards or add-ons enhance the router or switch's forwarding capabilities.

Thanks again!

DFC:

old school lol

[Mitrixsen]

That's a fantastic response, thank you for taking your time to write this down plus provide the links!