Increased latency for static route

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rcraig114
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Joined: 08 Jul 2019
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PostPosted: Mon Jul 08, 2019 17:11    Post subject: Increased latency for static route Reply with quote
I have a secondary router behind my DD-WRT (WRT1900AC V1). For the subnet behind that secondary router, I did a simple static route in the DD-WRT interface. While the route works (PC on DD-WRT LAN can ping other subnet and vice versa), there is about 16ms of added latency when bouncing between subnets. I can confirm the DD-WRT is causing the latency because if I have an OSPF relationship between two routers underneath the DD-WRT (thereby eliminating the need to hop through the DD-WRT), the latency goes down to 1ms (as expected on a wired network). But if the packet has to traverse through the DD-WRT, it adds 16ms of latency. Any ideas on what would be causing this?

Robert
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ttowling
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Joined: 01 Mar 2019
Posts: 74

PostPosted: Mon Jul 08, 2019 18:05    Post subject: Re: Increased latency for static route Reply with quote
rcraig114 wrote:
I have a secondary router behind my DD-WRT (WRT1900AC V1). For the subnet behind that secondary router, I did a simple static route in the DD-WRT interface. While the route works (PC on DD-WRT LAN can ping other subnet and vice versa), there is about 16ms of added latency when bouncing between subnets. I can confirm the DD-WRT is causing the latency because if I have an OSPF relationship between two routers underneath the DD-WRT (thereby eliminating the need to hop through the DD-WRT), the latency goes down to 1ms (as expected on a wired network). But if the packet has to traverse through the DD-WRT, it adds 16ms of latency. Any ideas on what would be causing this?

Robert


See my comment in the "fed up frustrated" thread below. It's something to do with AMSDU I think.
On these builds of open wrt it has been disabled for the 1900 models.
https://dc502wrt.org/

I have found latency is normal on 2.4GHz and adds c.20ms on 5GHz
rcraig114
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Joined: 08 Jul 2019
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PostPosted: Mon Jul 08, 2019 19:24    Post subject: Re: Reply with quote
OK, so this is normal for this hardware?

Robert
ttowling
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Joined: 01 Mar 2019
Posts: 74

PostPosted: Mon Jul 08, 2019 20:08    Post subject: Re: Reply with quote
rcraig114 wrote:
OK, so this is normal for this hardware?

Robert


All I know is that it has this info on the download page

"5. **Custom** Wifi has the AMSDU option disabled. This keeps latency low for the 1200ac/1900acx models. The 3200acm model is unaffected"

.. And that the latency is poor on 5GHz for me
rcraig114
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Joined: 08 Jul 2019
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PostPosted: Mon Jul 08, 2019 23:56    Post subject: RE Reply with quote
Ok, that may be the case for WIFI, but all of my testing has been on Wired. So I’m really lost here.
ttowling
DD-WRT User


Joined: 01 Mar 2019
Posts: 74

PostPosted: Tue Jul 09, 2019 4:59    Post subject: Re: RE Reply with quote
rcraig114 wrote:
Ok, that may be the case for WIFI, but all of my testing has been on Wired. So I’m really lost here.


16ms over ethernet is a lot. Are you sure it's not a duff cable? That's almost exactly the latency I'm getting over WLAN though.
rcraig114
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Joined: 08 Jul 2019
Posts: 4

PostPosted: Tue Jul 09, 2019 11:52    Post subject: RE Reply with quote
Oh yeah, it’s a good cable. I can ping something on the same subnet using the same path and it’s 1ms. I know it’s a software problem because if I have a ping going and hit save/apply on the router for something, for a few brief seconds the latency drops back down to 1ms. But you can tell that as soon as the processor finishes doing whatever, it goes back up to 16ms.

Robert
Per Yngve Berg
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Joined: 13 Aug 2013
Posts: 5131
Location: Akershus, Norway

PostPosted: Tue Jul 09, 2019 16:29    Post subject: Reply with quote
What is happening here is:

The PC connected to the LAN of the dd-wrt router tries to connect to a node behind the secondary router.

As the PC have the dd-wrt router as default gateway, it will try to send the packet to it. The dd-wrt router will reply back. I am not the router to forward this packet. You have to send it to the secondary router. The PC will then send the packet to the secondary router which will forward it to the correct sub-net.
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