nanog mailing list archives
RE: CIsco 7206VXR w/NPE-G1 Question
From: Lincoln Dale <ltd () interlink com au>
Date: Sat, 31 Jan 2004 16:13:33 +1100
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 basedon 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 levelnow, 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.
Current thread:
- RE: CIsco 7206VXR w/NPE-G1 Question, (continued)
- RE: CIsco 7206VXR w/NPE-G1 Question Luke Starrett (Jan 31)
- RE: CIsco 7206VXR w/NPE-G1 Question sthaug (Jan 31)
- RE: CIsco 7206VXR w/NPE-G1 Question David Luyer (Jan 31)
- RE: CIsco 7206VXR w/NPE-G1 Question Simon Hamilton-Wilkes (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question Jack.W.Parks (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question alex (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question sthaug (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question alex (Jan 30)
- Re: CIsco 7206VXR w/NPE-G1 Question matt (Jan 30)
- Re: CIsco 7206VXR w/NPE-G1 Question Rubens Kuhl Jr. (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question alex (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question Lincoln Dale (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question alex (Jan 30)
- Re: CIsco 7206VXR w/NPE-G1 Question Petri Helenius (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question Alex Yuriev (Jan 30)
- Re: CIsco 7206VXR w/NPE-G1 Question Petri Helenius (Jan 30)
- RE: CIsco 7206VXR w/NPE-G1 Question jlewis (Jan 30)
- Re: CIsco 7206VXR w/NPE-G1 Question Chris Adams (Jan 30)