CIsco 7206VXR w/NPE-G1 Question

Sat Jan 31 05:13:33 UTC 2004

At 03:51 AM 31/01/2004, alex at pilosoft.com wrote:
>Keep in mind, 72xx is still flow-based

72xx NPE-xxx is NOT flow-based -- unless you explicitly configure it to be.
(i.e. disable CEF, enable flow switching).

CEF is prefix-based switching - where all possible prefixes (routes/RIB) 
are already programmed into the forwarding table (FIB).
anything not programmed into the FIB doesn't exist in the RIB, ergo there 
is no route therefore is dropped.

i believe the words you're looking for is "NPE-xxx is SOFTWARE-based 
forwarding".  this part is true enough - but a NPE-G1 has far more cpu 
cycles to switch/route than previous NPE-400/300/225/200/150 et al.
software-based forwarding isn't so bad -- it means that platforms such as 
the 7200 typically have lots of features.

this is different to the NSE-xxx which is part software-based forwarding 
and part PXE-based forwarding.
the exact features accelerated by PXE varies depending what code release is 
used.

your said:
         flow-based means router's performance is based on number of flows
         established, and first packet of each 'flow' is processed differently
         [slower] from all other within the flow, and things like nachi 
will kill
         it.

no, this isn't true.  (at ieast, it isn't unless you explicitly configure 
it that way).  for a service-provider, you wouldn't want to use it in any 
forwarding mode other than CEF, unless there is very good reason to.

to provide you with a summary of forwarding paths and their uses:
   CEF switching:
         prefix-based pre-populated FIB
   dCEF switching:
         distributed version of CEF - typically each linecard has its own
         FIB and therefore switching decisions are distributed per linecard
   Fast switching:
         destination-based demand switching.  a 'route cache' exists of
         destinations to be forwarded to.  the first packet to a destination
         is "process switched", which installs the route-cache entry.
         subsequent packets are switched in the "fast" (aka interrupt)
         path.
   Process switching:
         all packets received (at interrupt level) are queued for process-level
         to route.

then there's Flow Switching, whose definition has changed over time:
   Flow Switching:
         a variation on Fast-switching, but where a flow-entry is created based
         on a 5-tuple (srcip/dstip/proto/srcport/dstport/TOS).  first 
packet is process-
         switched, which installs the flow entry, subsequent packets are 
switched
         at interrupt level

now, Flow Switching has changed over time.  you can enable both CEF+Flow 
and Flow simply becomes an accounting method that is useful for netflow - 
but you continue to have packets switched using CEF.

as to the exact level of forwarding used for each packet, that varies --
if you enable a feature that isn't in the CEF path, then the packet is 
switched using the next-lower-layer that supports the 'feature'.
for service-provider type environments, there aren't too many features 
necessary for /most/ deployments that aren't already covered in CEF on 
7200, so you're mostly ok there.

this is just a brief description of how a 72xx works - and there are many 
permutations and differences between different platforms and boxes.
if you want the full rundown, Phil Harris normally gives a "Router 
Architecture" presentation at every Networkers i've ever attended, and it 
covers all this and more.

cheers,

lincoln.
disclosure: my other email address is ltd at cisco.com, but i work in Fibre 
Channel not IP these days.