Cisco Router Gigabit interface VS WAN throughput/access speed - why ?

[SJ K]

Hi all,

Been reading up abit on the wan throughput for several cisco routers e.g 1941, 2911, 2921 etc.

Can some gurus point to me why do the routers have gigabit interfaces but support only wan throughput of even less then 1/2 ?

e.g. 1 gigabit interface but WAN throughput for the router is only 25Mbps or 50Mbps ?

What am I missing ?

Regards,

Noob

[Reza Sharifi · Accepted solution]

Hi,

Can some gurus point to me why do the routers have gigabit interfaces but support only wan throughput of even less then 1/2 ?

Its a combination of marketing, keeping the price low and not always needing a whole lot of throughput.  For example; if your provider gives you a 20 or 30Mbps Ethernet connection for accessing the Internet over a gig interface than all you really need a device that can handle that much throughput and the 2900 series would be a good choice.  Now, if you are purchasing a gig interface from your provider and you need to have up to 1gig of throughput, than you are not going to use a 2900 or a 3900 series, you need to move to a device that can handle that throughput (e.g an ASR). This is not just Cisco, as other vendors do the same thing. You have to know your requirement before purchasing anything.

HTH

[SJ K]

Hi Reza,,

Thanks for your reply.  But that precisely my point, the specifications for the Router's WAN through for 2901 is 25Mbps. So what's the point of having a 1Gb interface when the router can only 25Mbps of throughput ?  (why not just have a 100Mbps interface , wouldn't that lower the cost further ?)

Regards,
Noob

[Karsten Iwen]

Keep in mind that with each setup that can handle more than 100 Mbit, you would have a bottleneck without Gig-Interfaces. As Reza mentioned, If you need more throughput, you probably have a bigger box. But in scenarios where you don't need services like ACLs, NAT, encryption, QoS and so on, even a 1900 can handle much more than 100 Mbit.

The 25 MBit speed for the 2901 is also marketing, but a positive marketing-value. Regardless what you configure, with 25 MBit on the WAN, the device will not slow you down.

[SJ K]

Hi Karsten,

Thanks for your reply.

Do you mean if I am not using any features (just some plain acl + maybe even just static routing),  can the router handle throughput as much as its 1Gb interface bandwidth  ?

I don't know, i know i have just look at it the wrong way.... but i just can't see it right ->

- having a router with 1Gb interface but only 25Mbps throughput (when all features on) is just like

- having a high speed engine but it can only speed up to 25KM/hrs

Regards,
Noob

[Karsten Iwen]

- having a router with 1Gb interface but only 25Mbps throughput (when all features on) is just like having a high speed engine but it can only speed up to 25KM/hrs

But the high-speed engine won't help if the street is not designed for high speed ...

The 1941 can go far beyond 100 MBit, but with complex services it will be much slower. And the customers who don't use all services, shouldn't be limited to 100 MBit.

[Karsten Iwen]

Another document that shows how different the performance of a router can be:

http://www.cisco.com/c/dam/en/us/products/collateral/routers/3900-series-integrated-services-routers-isr/performance_2013.pdf

[Joseph W. Doherty]

Even when the router doesn't support anywhere near gig sustained throughput, the gig interface will accept a packet at gig (transmit them too at that rate), the router might even be able to accept a short burst of gig packets (where they might be queued for forwarding).

For transmission, perhaps you have multiple interfaces receiving traffic, at the about the same time, a gig interface may reduce the delay transmitting those packets.

Remember, often the routers can forward some traffic at a much higher rate than its recommended for.

For example, a 1941 or 2901 are both recommend for 25 Mbps, but the former is documented as forwarding up to 2.932 Gbps and the latter as forwarding up to 3.114 Gbps.

In other words, a gig interface allows better advantage of some "burst" situations.

Another reason these routers might offer gig interfaces, they might all be using commodity chips to support the interfaces.  It probably easier, and perhaps even cheaper, to manufacture different ISR with, internally, the same interfaces.  (Sort of like putting the same tires on different models of cars, even if some have more powerful engines.)

[SJ K]

Hi all,

Sorry to the late reply.

Thanks for all the feedbacks! . great insights and make me think twice (thrice or more actually )

Reason for this question is actually I have got a telco quoting me a 2921 series CPE router for around 50Mbps bandwidth but the router is managed by them and I don't see much features other then

- routing (ospf or bgp)

- acl (minimum i guess)

- qos (doubt so) didnt buy this service

- hsrp

Just thinking if i can go 2901 (just 1U) instead of 2921 (2U) as i am running of of rack space.

Regards,
Noob

[Reza Sharifi]

Hi Noob,

You should be fine with a 2901 as long as the hand off is copper.  The 2901 and the 2911 do not have any fiber ports. Also 2 10/100/1000 copper vs 3.

Good Luck

[Karsten Iwen]

Reason for this question is actually I have got a telco quoting me a 2921 series CPE router for around 50Mbps bandwidth but the router is managed by them and I don't see much features

There is another possible reason in addition to the fiber-port mentioned by Reza. This could be "normal" business behavior of an ISP, by using a faster router as standard device, a customer could increase the bandwidth or book additional services  without a change of the equipment.

[Reza Sharifi · Accepted solution]

Noob,

As I said before, part of that is marketing.  Cisco does not want to be the only company building routers that support 10/100 only, even if the box can only Fast/CEF switch between 167 and 296mb (see link and look under ISRG2).  Also, most other switches or routers in the market are moving away from fast Ethernet and more and more you see 1G/10G/40G interfaces but that doesn't mean they support these speeds at line-rate. Also, these boxes provide other functionalists like NAT, GRE tunnels, VoIP gateway, EHWAC switch port support , etc.. that other devices may not support. Cost is another factor.

https://www.cisco.com/web/partners/downloads/765/tools/quickreference/routerperformance.pdf

HTH