2921 - Speed and throughput

[John Adams]

Hello.

I've got a 2921 used to connect to physical sites over a WAN connection (BT Fibre).

The WAN line is 100MB with the potential to be 1GB.

I was wondering if someone can tell me (or point me to) what speeds the 2921 can support in terms of throughput.

We do dynamic routing but we don't so any encryption or NAT.

Thank you.

[Leo Laohoo]

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

[Leo Laohoo]

If you want to push 1 Gbps, then an ASR 1002, RP1, ESP5 is required. 

[John Adams]

Thanks - looking at that link I'm strugging to understand the figures.

Does it say 2921 can support 245.76 mbps?

It seems to suggest the model for pushing 1gb can push almost 5gb?

I'm obviously missing something here in understanding the figures.

Thanks.

[Joseph W. Doherty]

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Posting

Does it say 2921 can support 245.76 mbps?

Yes it does, but that's total aggregate, assuming no services packet forwarding.  See the beginning of the document's text, and the third paragraph for how "bandwidth" is calculated.

For software based routers, your "mileage" can really vary.  The Cisco document I've attached, better explains ISR performance under different conditions.  You also find the 2921, in that document, is only recommended for 50 Mbps.  (BTW, using the older document, I would generally recommend sizing for one fourth of the bandwidth listed.)

From both documents, none of the ISRs are really sufficient for gig.  For such speeds either you need to jump up a series, such as into the ASRs Leo recommends (or the now EoL 7200s) or you need to consider whether a L3 switch might work.

However, recently Cicso provided another ISR to fill the gap between the 3945 and the ASRs, the 4451-X.  It too should be suitable for a gig link.

[John Adams]

Thank you. That has helped a lot.

How come it's so much less throughput for WAN than internal routing?

Our WAN connection plugs into a BT 21CN box and then uses their network.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

To an ISR, packet forwarding LAN or WAN is about the same (there is some small differences due to media differences).

If you've been using the ISR for internal LAN routing, between its gig ports, it will work, but it's a performance bottleneck.  However, if that has been NOT noticeable to you, you probably wouldn't notice an issue using a gig WAN link, but as WAN bandwidth is often much more costly than LAN bandwidth, generally we want to be able to take full advantage of it.

The routers Leo and I suggested, should insure you can actually use all of a gig WAN (or LAN) link.

[ปลาวาฬทราย RMUTT CPE IX]

What about wan 100Mbps, does it support? It just do the OSPF routing over the MPLS L2VPN link.

Thank you very much.

[Joseph W. Doherty]

Maybe, maybe not. See table 5 in the document referenced in my first post. (NB: for 100 Mbps WAN, you may need 200 Mbps performance [i.e. duplex doubles PPS needed].)

[apogeenet]

The 50Mbps recommendation is based on where the router hits 75% CPU utilization when services such as QoS, ACLs and NAT are enabled, 105Mbps "aggregate" throughput.  The aggregate part is the most important part, meaning upload + download + local hair pinning through the router if you have that.  Local hair pinning would occur when you terminate local site VLANs through the router on separate physcial or logical (subinterfaces) interfaces.  The sum of traffic being processes, even if LAN to LAN passing through, equals the aggregate load. A WAN or Internet circuit of 50Mbps upload + 50Mbps download concurrently with most common router services turned on would be near this 75% CPU driving max, where packet delivery performance falls off.  Running at the 75% CPU levels above that make you more prone to router crashes, high and varying latency (jitter), packet loss, and other user impacting issues.

So to answer your question, what about a 100Mbps link - yes, as long as the aggregate load does not push the router past the 75% mark.  If you are simply routing through, with no IPSec, QoS, ACLs, NetFlow, and NAT, you can exceed 400Mbps in a pure OSPF throughput.  That would fall under table 1 on page 2.  

Pete