Routers & WAN Bandwidth

Unanswered Question
Nov 15th, 2008

Cisco specifies WAN Bandwidth limits for its ISR routers starting from 2 Mbps for the 1841 to 45 Mbps for 3845.What is/are the factor(s) that dictate the maximum WAN bandwidth that can be handled by a Router whereas there is no limit(other than the interface speed)for routing between two LAN networks that terminate on the integrated LAN interfaces?

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Overall Rating: 3.2 (5 ratings)
Giuseppe Larosa Sat, 11/15/2008 - 09:10

Hello Sumesh,

these estimates are conservative and are made taking in account the usage of QoS in the form of outbound scheduler with congestion avoidance and possibly security features like ip inspect.

You could have also some form of encryption but I don't think they are included.

On lan interfaces usually routers may simply forward frames without any QoS feature and no acl applied.

To be correct before the interface wire speeds the key parameter is the pps = packet per second that can be processed and this on ISR routers is directly related to the cpu speed and power.

So you cannot expect even a C3845 to be able to use a GE port at wire speed with the smallest packets:

full duplex 1000 Mbps of 64 bytes frames means 1,489,000 frames sent and 1,489,000 received that is much more then the declared pps for C3845 (500,000 pps)

1841 is supposed to support 75,000 pps

3845 supports 500,000 pps

Some years ago I did performance tests on software based routers like c7200 and c7500 and every feature we enabled leaded to some performance reduction.

Hope to help


viyuan700 Sat, 11/15/2008 - 11:03

First, Cisco dont specify WAN bandwidth 2Mb/45M. This speed is defined by the Transmission Technoligies who carries data between 2 routers. Read SONET/SDH will clarify ur concern.

Second, Cisco trying to make product as per customer requirement. Will there be customers who will be willing to pay for more 2/45Mb ports on these routers. As the cost of 2/45 MB lease line is too much.

sumesh_un Sat, 11/15/2008 - 11:06

Please refer Q & A of ISR Routers on the website.Cisco does specify BW for ISR Routers

viyuan700 Sat, 11/15/2008 - 11:16

Ok let me ask you this if cisco make WAN port with 3Mbps, can any service provider give you that bandwidth? ask ur service provider what bandwidth they can provide you

Speed of 2mbps/45Mbps is defined by the technologies who carries this data and cisco have to comply with those standard.

viyuan700 Sun, 11/16/2008 - 09:10

service provider can give you 100Mbps on ethernet. Hope u r not expecting to have 10Gbps ethernet inteface in 1800 or 2800 series router.

Please check(taking from the same link you have provided me) the cost of 2801 supports 1 T1/E1 or 2851 which supports 6 T1/E1.

R u trying to figure out why a family car cannot give speed of a racing/sports car.

Joseph W. Doherty Sat, 11/15/2008 - 16:46

The limiting factor, for the software based routers, is the performance of the CPU. As the traffic load increases, so does the CPU load. When the CPU load approaches 100%, router is unable to continue to increase amount of traffic it can forward.

Beside just forwarding traffic which consumes CPU, additional configuration features will increase the load on the CPU for the same amount of overall traffic. Features such as ACLs or QoS are examples.

Software routers have an optimized path that processes most "normal" traffic. Some traffic, alone, or depending on the features being used by the router, will cause the router to process traffic in a much less optimal manner which is often about 10x or even more slower. This is known as processed switched traffic.

The WAN bandwidths you've seen listed are very conservative, but they do tend to insure you're unlikely to encounter a performance limitation if you go by them.

Some make the mistake when they see a small router with a FastEthernet or GigEthernet interface, that it's capable of routing LAN traffic at such speeds, but that's seldom true. For LAN routing, the L3 switches offer much, much more performance although usually with a loss of some of the more advanced WAN routing features. (E.g. LAN L3 switches QoS is often limited compared to small WAN routers.)

As a comparison, the 3845, the most powerful ISR is rated at 500 Kpps (good for about 335 Mbps Ethernet for 64 byte packets) but the little 8 port L3 switch, the Catalyst 3560-8PC, is rated at 2.7 Mpps (good for about 1.8 Gbps Ethernet for 64 byte packets).

marikakis Sun, 11/16/2008 - 11:24

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