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RE: CIsco 7206VXR w/NPE-G1 Question

  • From: Lincoln Dale
  • Date: Sat Jan 31 00:16:12 2004 
At 03:51 AM 31/01/2004, [email protected] 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 [email protected], but i work in Fibre Channel not IP these days.