Im sure some people have seen big routers and ever wondered what, if anything, the linecards have on them. They can look kind of unassuming, with just a bunch of ports.
Some cards are more complex than others, and these are definitely in the complex pile... e.g. IIRC the Cisco Catalyst 4500 linecards are quite simple, with basically a backplane connection to the Sup where all switching is done, not really a lot onboard. 6500 linecards get a bit more complex, with more supporting hardware for higher performance and other features.
Attached is a photo of an FPC3 from a Juniper T640 router, and I can have a photo of an FPC4 for a T1600 within the next couple of days if anyone is interested (though it'll probably look much the same)
FPC3's can take up to 4 PICs, which are modules that contain the physical port(s), and control the physical media - basically just layer 1 functionality. In this FPC there are 4 x 10GE PICs on the right with XENPAK optics.
On the left are where things get a little more interesting. There are 3 mezzanine boards:
* Top and bottom: packet forwarding engines
* Middle: processor module
Yep, each line card has its own little computer onboard to control it. It runs a PPC processor, has its own RAM, and talks with the host processor modules via internal ethernet links. Im not 100% certain of its function, but Im sure its got something to do with controlling the PFEs and other things related to the particular linecard its on.
The PFEs take packets coming in from the PICs, do various things with them, and interface the linecard to the switching boards installed at the rear of the chassis to move the packet between incoming and outgoing interfaces. They'll also take packets coming in from the switching boards and direct them out to the appropriate PIC. On these linecards, each PFE only serves two PICs, so even on a single linecard there is some redundancy.
If you stick an FPC3 from a T640 and an FPC3 from a T320 side by side, they will look almost identical, except for the fact two ports and a PFE are shaved off (also the T320 linecard is physically smaller.) A T320 really almost is 1/2 a T640.
The FPC4's for the T1600 are the same size as the T640 linecards, except that they only take two PICs, but the PICs can have more ports. For example, an FPC4 can have up to 8 x 10GE ports in a single slot, instead of 4 for an FPC3 (or two on a T320), providing 160Gbps full duplex per slot, and 1.28Tbps per chassis.
So if youve ever wondered why these things have such a high price tag, theres really a lot of brains involved. But with the PICs being relatively "dumb" sometimes you have to wonder why they are so damned expensive...