2911 bandwidth limitations

[Patrick McHenry]

Hi all,

I've seen many discussions on this topic, but not much of a clear breakdown of router performance with different services turned on....For instance if running only QoS, BGP, and EIGRP.

On a router that connects a remote site to the WAN is a 2911 sufficent for a 100 Mbps circuit

From documentation, if all services are being used..VPN,QoS,NAT etc....the limit is 35 Mbps. We are only using QoS, BGP, EIGRP with slightly more than 200 routes. It appears that with only QoS running, the 2911 can handle 125 Mbps.

We have a 2911 at the site that needs a circuit increase to 100 Mbps and would like to know if the 2911 router will be able to handle it.

Thanks for any help.

[Joseph W. Doherty]

The attached Cisco paper is one of the best in documenting the performance of a 2811.

Personally, I would consider a 2911 undersized for 100 Mbps (duplex).

[Patrick McHenry]

Thanks for the feedback...

Why would you consider it undersized?

To me this document makes it seem like it should be fine.

Table 3 says the 2911 can handle 225Mbps and that is with encrypted tunnels.

Thank you

[Joseph W. Doherty]

Read Table 3 again.  The 225 is number of tunnels, not Mbps.  Encryption Mbps is in Table 2 and lists the 2911 at 170 Mbps and they are unclear on packet size.  However, Table 7 is a bit clearer about traffic mix, and there Mbps is listed as 61.  Again, half that for a duplex link.

Table 5 and Table 8 are probably reasonable performance guides (vs. the "WAN" table "and kitchen sink").  But for 100 Mbps (duplex), you want one of those tables to list 200 Mbps for the 2911, and neither does, which is why I believe it's undersized.

[Patrick McHenry]

After reading the doc closer, it appears there are all these caveats...

They explain how voice packets are much smaller and will effect the CPU more and so on....how is one to figure this out...? Do we need to perform our own tests? Or, maybe install the 2911 and watch the CPU?

Thanks

[Joseph W. Doherty]

Indeed, a router's performance can be very variable based on the nature of the traffic and how the device is configured.  (To avoid this confusion, the later 4K ISRs have performance limits set to usually insure you can also get x bandwidth forwarding, regardless of your traffic or configuration.  The downside with this approach, though, you need to buy a more "powerful" router then you really need.)

That said, your device with your traffic and your config is the only fully accurate model of how it will perform.

[Patrick McHenry]

Thanks Joseph - correct with the 4Ks.

That is what we are using now at most of our sites.

And the throughput license does simplify, but like you said makes you get a bigger router or throughput license than you might actually need.

Thanks again, Pat

[Patrick McHenry]

So WAN.png appears to show how a consumer should pick a router based on using QoS.....2911 125 Mbps.

Then WAN2.png shows how a consumer should pick a router based on an average use case...I assume?

Seems to be huge spec differences...

Thanks

[Joseph W. Doherty]

By "WAN", they mean duplex bandwidth.  All the other tables show aggregate bandwidth.

I.e. 35 Mbps would be 70 Mbps, or your table 2's 125 Mbps would support 62.5 Mbps of "WAN" bandwidth.  That's still quite a difference, but the 35 Mbps allows for almost any configuration, if you're sure you only will be doing similar to what's in table 2, you can go by that measurement.

[Patrick McHenry]

So by table 2s recommendation, it will only support 62.5 Mbps Full duplex? I didn't see a half duplex designation.

Regardless, if we were to use a 2911 for a 100 Mbps circuit, bandwidth would not be limited until the CPU was taxed, right?

What I am trying to ask is, if using a 2911 for WAN connectivity utilizing QoS, we wouldn't suddenly be governed to 62.5? or would we?

Or, would any limitations show up in CPU over utilization first?

Thank you, Pat

[Joseph W. Doherty]

On a 2911, the CPU is the overall performance limiter.

Depending on your traffic and config, you throughput may be CPU limited.

For example, years ago, I do my own testing with with a 2811 and at about 40 Mbps, would my typical config, CPU was 100% and it wouldn't forward beyond that.

In a production environment, you don't want to (routinely) hit 100% CPU.  Which is one reason the Cisco documents note performance at 75%.