[c-nsp] Over/Undersubscription on certain12000 Line cards

[Drew Weaver]

        Hi there.

       Let us say you were stuck with a set of routers for a finite set of time which wasn’t adjustable no matter how much you cried about it. Let us say for example that these were 12000 series routers and these particular 12000 series routers could only use line cards which go up to Engine 3.

       Lets also say that the network these routers are servicing is very busy, but also pretty simple: no ACL, no services (MPLS, etc) the routers take full feeds (BGP) from multiple transit carriers and OSPF is running as an internal routing protocol. These routers basically just take traffic from transit and pass to distro and vice versa, nothing fancy.  Now, it occurs to me, looking at this chart:

       http://www.cisco.com/web/partners/downloads/765/tools/quickreference/routerperformance.pdf

       some of the interfaces that could be in said routers such as:

       3GE-GBIC-SC (engine 2)
       4OC12/POS-IR-SC-B(engine2)

       Could suffer from a pretty iffy oversubscription issue:

       So then I did some further research and noticed that these routers could be upgraded to the engine 3 versions of said cards:

       4GE-SFP-LC (engine 3)
       4OC12X/POS-I-SC-B (engine 3)

       But then I noticed that engine3 has the same performance restrictions (2.5Gbps [theory]) and 4Mpps as engine3

       Except that in the case of the Gig-e card oversubscription would be even more damaging if you weren’t careful.

       So I guess my question would be: In the case that it was absolutely impossible to upgrade to routers which support Engine 4/4+ would it make sense to jump from e2 to e3 at all? Or to save the money and do more when the “time comes”.

       Also, would it be a good idea to just spread the Gig-e interfaces out across more 3GE-GBIC-SC cards to completely avoid oversubscription? And if anyone has tested this, what is the actual performance limitation of the 3GE-GBIC-SC?

       Sorry, I realize this is long; but I’ve spent the last two days going over this ☺

       Thanks,
       -Drew