3745 dual ISP performance problems

Unanswered Question
May 14th, 2008

We have a 3745 that's connected to a T-1 and a cable modem. Heavy inbound/outbound traffic to either ISP is slowing traffic through the router which in turn affects browsing, Citrix, etc. Ping times go from 50ms to 300-400ms or greater. The odd thing is that the router CPU is not busy (<10%, usually 2-3%), the interfaces show no errors, and there's no obvious signs (to me) that the router is overworked at all. I've gone through Performance Tuning Basics (doc 12809) with no improvement.

The router terminates about 15 VPN's with very little traffic on them. We have NAT setup using multiple pools and route-maps.

I can post relevant parts of the config here if I know what would be helpful.

What am I missing??

I have this problem too.
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Paolo Bevilacqua Wed, 05/14/2008 - 08:51

Seems like you're not using frame-relay for output at all for output, while ethernet has some in/out load.

Suggest you try to understand why serial is not being used, then change "load-internetva" under interface to 30 sec, take the show interface again when things go slow.

bmiszuk Wed, 05/14/2008 - 09:04

Serial is used by servers for uploading/downloading. One server is downloading updates now which explains the T-1 inbound. Outbound T-1 is generally SMTP which is very little.

Ethernet is used for web browsing, Citrix and users uploading files (pictures) to remote web sites. We want most of the traffic to be on the higher-speed cable modem.

What I can't explain is why a big upload or download to either interface seems to slow everybody down no matter what interface they go out of. But the router never looks busy.

See attached.

Paolo Bevilacqua Wed, 05/14/2008 - 09:14

Well that is very normal. T1 is 1.5 mbps (may be less as you have fram-relay, can have a CIR on it), and cable is perhaps 4 mbps down / 512 up.

These speeds are easily saturated by large transfers. Moreover, the true speed can be much less that what the ISP advertises, do a speed test with dslreports.com for example.

bmiszuk Wed, 05/14/2008 - 09:31

If one ISP is saturated would you expect performance to suffer on the other connection?

Paolo Bevilacqua Wed, 05/14/2008 - 09:42

Not really, on the other hand in the output you produced, only one circuit is being used.

Note that in practice aT1 is likely to give you much better performances than cable, being designed for professional use.


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