At 03:51 AM 31/01/2004, alex@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@cisco.com, but i work in Fibre Channel not IP these days.