At 11:57 PM 3/20/02, John Green wrote: >""the routing decision consumes the bulk of the CPU >bandwidth, shovelling the rest of the packet through >is low-overhead."" > >say a router connects a between ethernet and Frame >Relay or between two dissimilar Layer2 networks. Then >the router would be stripping off one networks' layer2 >frame and replace it with the layer2 frame of the >other network where the packet is to be sent. Would >you call this low-overhead as well ? >I guess your example would be if the router were to >connect between same Layer2 networks ie say both >networks are ethernet. right ? just want to make >sure...
The Layer 2 header changes whenever a router forwards a packet. For one thing, the Layer-2 destination address changes. The frame goes to the next hop. The router strips the Layer 2 header on the incoming packet, figures out where to forward the frame from a routing table or cache, and re-encapsulates the frame into a new Layer 2 header. The amount of processing required to strip an Ethernet header, figure out the destination port and encapsulation, and re-encapsulate into Frame Relay is essentially the same as the amount of overhead required to strip an Ethernet header, figure out the destination port and encapsulation, and re-encapsulate into an Ethernet header. Marc's point was that the amount of overhead is also the same regardless of the packet size. The job must be done whether it's a 46-byte or 1500-byte packet. And I like the way he said that "shovelling the rest of the packet through is low overhead." That's true. Keep in mind, however, that the packets-per-second ratings are just vendor marketing departments trying to "one up" their competitors. So, they post the results of testing with 64 byte packets because that makes the number higher. More packets are coming in to get processed. Long packets take longer, not because of extra processing, but simply because of serialization delay. It's like a relay in a train-switching system. The relay doesn't have to do more work for long trains with many cars. But it still takes longer to get a long train through the relay than it does to get a short train through it. Priscilla >--- Marc Thach Xuan Ky >wrote: > > Sam, > > I think the question is: what is your average packet > > size? Using > > process or fast switching I should think that the > > packet size is almost > > irrelevant to the router. I have benchmarked many > > PCs and NICs running > > certain routing software. On a PCI bus PC the pps > > difference between 64 > > and 1518 octet frames was in the order of ten to > > twenty percent, i.e. > > the routing decision consumes the bulk of the CPU > > bandwidth, shovelling > > the rest of the packet through is low-overhead. > > Marc > > > > sam sneed wrote: > > > > > > I noticed Cisco uses pps when they give their > > specs for routers, firewalls, > > > etc. What is the assumed packet size when they > > come up with these specs? > > I'm > > > planning on using 2 2621's in HSRP mode (getting > > default routes via BGP) > > and > > > need to be able to support a constant 10 Mb/sec > > and would like know if > > these > > > routers will do the trick. > > > thanks >[EMAIL PROTECTED] > > >__________________________________________________ >Do You Yahoo!? >Yahoo! Movies - coverage of the 74th Academy Awards. >http://movies.yahoo.com/ ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=39068&t=38956 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
