Forwarding Capacity

Unanswered Question
Apr 13th, 2011

Hi,

      What is Forwarding Capacity ? and i need to know the Forwarding Capacity of ISRG2 i.e 2900 and 3900 series routers in Kpps ?

regards

Neo

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cisco@learn Thu, 04/14/2011 - 00:32

Hi Milan,

               I had already gone through the PDF attached. It only gives values of Fast/CEF switching , which i think is router's internal switching capacity , it is NOT the forwarding capacity .

Through my research i have digged out that

pps = Mbps / (  64bytes * 8bits/byte )

and Recomended WAN Access Speed of a 2901 router is 25 Mbps


so can i safely conclude that Forwarding Capacity of the router is 48.8 Kpps.

i am assuming "Recomended WAN Access Speed " is Forwarding Capacity of the Router.

Let me know if i am right or wrong ?

regards

Neo

milan.kulik Thu, 04/14/2011 - 00:52

Hi Neo,

no, IMHO,  Fast/CEF switching means the router forwarding capacity when CEF is used.

"Recomended WAN Access Speed " is not the Forwarding Capacity of the Router, as the router can connect multiple WAN lines.

But it's recommended not to use hihger speeds on particular WAN lines.

HTH,

Milan

hobbe Thu, 04/14/2011 - 02:03

Hi


"Recomended WAN Access Speed "

To me this is the recomendation wich tells me what cisco believes a "normal setup" with this router will be able to comfortable handle.

Yes they can have several WAN connections, but this is the sum of all of them combined.

So what is a "normal setup"

This is a value that is calculated so that cisco can stand behind it and say yes most implementations with this max speed will work just fine.

They have to have some value of what is reasonable to expect from the router.

Otherwise they would have to say, we do not know until you tell us what you want to do with the router and what type of traffic it will encounter.

so if you use the recomended WAN speed as a measurement you will most likely not be dissapointed.

The routers of today have about 4000 features i read, but we use normaly only 4-6 of them. The more features we use the slower the router becomes, the less features we use the faster the router becomes.

Good luck

HTH

hobbe Thu, 04/14/2011 - 02:18

cisco@learn wrote:

Hi Milan,

               I had already gone through the PDF attached. It only gives values of Fast/CEF switching , which i think is router's internal switching capacity , it is NOT the forwarding capacity .

Through my research i have digged out that

pps = Mbps / (  64bytes * 8bits/byte )

and Recomended WAN Access Speed of a 2901 router is 25 Mbps


so can i safely conclude that Forwarding Capacity of the router is 48.8 Kpps.

i am assuming "Recomended WAN Access Speed " is Forwarding Capacity of the Router.

Let me know if i am right or wrong ?

regards

Neo

No you can not safely conclude that the forwarding capacity is 48.8 Kpps.

the 2901 has a forwarding capacity of  327 Kpps when routing.

Everytime you add a feature such as Firewall or Nat or QoS. you add load to the router and the more it has to inspect the packages.

so recomended would mean that cisco thinks that if you are a "normal" general user the router will be working comfortably with the featuresets that normaly would get chosen at 25 Mbit. if my memory does not let me down comfortable = with a processing power of less than 75% and no dropped packets.

Good luck

HTH

hobbe Thu, 04/14/2011 - 01:39

This is not as easy as one would think at first glance.

we measure a router in PPS, packets per second, ie how many packets can that router handle

but a packet can be different sizes.

the different sizes makes the router send through more or less traffic.

and the router can do extra functions not only routing for those who wants those functions.

This makes it impossible to tell you exactly what or how much a specific router will give you in regards to performance at a specific place.

it all depends on packetsizes and what services you are using and how much.

cisco will give you the worst case scenario in their lists wich is a good place to start.

This is how it works. The very Simplified version.

Throughput can be looked at as a cars and busses over a toll booth.

The tollbooth can handle x amount of traffic (x cars/busses) per second.

This is the Packets Per Second value. ie PPS.

Then we have the throughput ie how much is going through.

Lets define that as how many passengers is going through.

this is Mbit/s ie throughput per second

a buss would be a packet with a big size such as fx 1500bytes which is the "standard max" unit

a small sportscar would be a 64bytes packet wich is the smallest unit that we can send through.

so now how do we make sense out of this.

well lets say that there are no busses and the tollbooth operates at maximum speed and only the small sportscar comes through

to calculate the throughput we would calculate PPS x size of sportscars x 8 (bytes to bits)

This gives us in the case of fx 1941.

299000x64x8= 153088000 bits. which translates to roughly 153 Mbit.

The throughput is 153 Mbit.

now we do the same with the biggest packets. ie the busses.

299000 x 1500 x 8 = 3588000000 wich roughly translates to 3588 Mbit.

This is the ideal scenario, only routing and the biggest packets will give you 3.5 Gbit.

The truth is of course most often somewhere in the middle. It is very seldom we find a bridge with only sportscars or busses but normally that mixes.

there are some known mixes that is calculated to be the average real world traffic, that is called IMIX traffic, InternetMIX.

Now cisco, the maker of this imaginary tollbooth will have to declare the smallest value otherwise people will feal that they have been ripped off, right ?

so that is one reason they give us the worst case scenario when it comes to routing alone.

but cisco knows that just letting traffic through will not be enough, people wants to do more.

how about we can stop unwanted traffic ? we add firewall services.

That will slow down the tollbooth since now it does not only have to do the tollbooth functions (ie routing) but also security functions

is that enough for us ? No! we want more !

How about we let the busses that shuttles the workers to work trough faster than everybody else, we add Quality of Service.

now the tollbooth not only has to do its tollbooth duties but it also handles the Security and QoS

Also we have to calculate that the tollbooth must handle traffic in many directions. to the city, from the city and to different suburbs near the city.

so the the more extra features we add to the router the harder it has to work and since the resources are limited that gives us lower throughput.

hope this helps someone, somewhere.

Good luck

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