InterVLan speed question

[Safar Safarov]

Hello,

I have 1841 router with Gigabit Ethernet HWIC with One SFP Slot installed in it and connected via SX Multimode cable to 2950 switch. I have 5 VLans in trunk. The question is what will be the speed rate between VLans - 1 Gbps or less?

The case:

Port 1 assigned to a VLan10 and Port 2 Assigned to a VLan20 on 2950 switch.

InterVLan routing is configured on 1841.

Computer A is connected to Port 1 and Computer B is connected to Port 2 on the switch.

What be a file transfer rate from Computer A to Computer B assuming that NICs on both computers are 1000Base-T.

Thanks.

[manju.cisco]

Hi,

You will not be able to achieve complete transfer rate of 1Gbps. If you check the transfer rate with some tools you will get the transfer rate less than 1Gbps, thats because each packets will have IP headers and vlan headers added to the actual data packet.

[Safar Safarov]

Hi,

The thing I was afraid of is that in Cisco Router Performance table is shown that 1841 router operates at 38.40 Mbps Fast/CEF switching. I assume that the rate shown applies only to LAN-to-WAN packets and not to LAN-to-LAN. I just want to be sure that the router will pass the rate speed I need. I expect transfer rate between VLans at smth around 800 Mbps.

Please correct me if I'm wrong.

Thanks.

[Kishore Chennupati]

Hi safar,

Just my thoughts here. When you have multiple vlans terminating on the router you create sub interfaces right?

Now for each destination in the LAN wil be a different exit interface (sub interface I mean). Hence for the router you are pulling from one sub interface but pushing it out through another sub interface. Although this is logical but still its part of cef switching and in my opinion still counts towards the throughput. So, you are right when you say its only 38.40Mbps.

Hence the router is the bottle neck. If you want line rate speeds then go for L3 switches You can get line rate speeds as everything is done in HW.

HTH

Kishore

[Safar Safarov]

Hi Kishore,

got you point.

I think I will do some speed testing between VLans. It's very strange that any router (I'm not speaking about 7600 series or ASR1000/12000) then would create a bottleneck in my opinion. Therefore it would mean that Cisco forces you to purchase L3 switch in any case for InterVlan routing.

hmmm

[Kishore Chennupati]

Definetly and also while you do that please monitor the CPU on the router as well I will giv you a command to generate 30-40mbps in couple of seconds.

ping repeat 1000000 size 1500 timeout 0.

This will flood  the link without waiting for reply and you can easily check the interface stats and CPU on the router etc. The moment the traffic hits around 30Mbps you should see the traffic on the CPU spike up

HTH

Kishore

[manju.cisco · Accepted solution]

Hi,

Apologies, the 38Mbps h/w capacity didnt hit my head .......haha

Definitely nothing can go more than the hardware limit.......

due to headers we still cannot obtain throughput of 38M.

[Safar Safarov]

Nice I hope it would not affect on IntraVLan speed at least

[Joseph W. Doherty]

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Posting

Most of the small router are oriented toward WAN usage, so they are feature rich but performance poor (although performance is sufficient for the WAN links they are designed to work with).

(Small) L3 switches are feature poor (although feature often sufficient for LAN usage) but performance rich.

Technical reason for this difference, small routers tend to do everything with just their CPU, which is what allows for the rich features but limits performance.  L3 switches have special hardware that deals with "routine" frame/packet forwarding, which is what provides their great performance bue poor feature support.

In the case of ISRs, some modules support various switching modules.  Some of these allow high speed L2 between their switch ports (of the module).  There's also a L3 switch module that supports high speed L3 between its switch ports (and to stacked 3750s connected to it).  (1841 is probabably limited in what Ethernet modules its supports.)

PS:

An 1841 might provide performance more than 38 Mbps, when dealing with packets not all minimum sized, but depending on features used, might also be unable to deliver even 38 Mbps.  Further the PPS applies to all packets transiting the routers, i.e. 38 Mbps would be applied to both in and out.