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Author Topic: much delayed ARP response - DLink 2750u AP connected to DLink DIR-825ACG router  (Read 426 times)

knilduser

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I have a DLink 2750 AP set up as an access point (with SSID A) and connected via Ethernet to a DLink DIR-825ACG router (with SSID B), which in turn is connected to my ISP. DHCP relaying is enabled on the 2750, pointing to the IP address of the DIR-825ACG. And on the DIR-825ACG, an untagged VLAN is set up spanning all 4 Ethernet ports and the 2.4 GHz & 5 GHz WiFi ports.

When my client connects via Ethernet or to SSID B, things work just fine. However, when it connects to SSID A, it takes a few minutes for there to be connectivity after the association has happened. A WireShark sniff shows that my client is repeatedly sending ARPs for the gateway’s address (i.e., the IP address of the DIR-825ACG) and these don't elicit any response for a few minutes. Then suddenly after few minutes, there is a response to the ARP and connectivity is established.

I’ve scratched my head over this for a while and poked around, but I haven't been able to figure out what the issue is. I don't even know if the problem is with the setup of the 2750 AP or the DIR-825ACG.

I would appreciate any inputs on what the issue might be and/or how I can investigate/debug this. I do hope that it's not the case that my setup is "non-standard and so won't work".

Thanks.
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FurryNutz

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    • Router Troubleshooting

Does the system work if you turn OFF the 2570u AP?
I presume the 2570u was a dsl modem?
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"Nothing Funny about It...." We are not here to Impress anyone! You have a be a COMPETENT user first to under stand COMPETENT help!

knilduser

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Yes, the 2750u is a DSL modem-cum-router. But it is now just used as an AP, without an DSL/phone connection.

As I wrote, things work just fine when I connect directly to SSID B that is broadcast by the 825ACG router. In fact, the problem of ARP responses being delayed by minutes happens only when I switch from SSID B to SSID A (the one broadcast by the 2750u). Once I get connected to A (after a delay of a few minutes), things work fine. I can even disconnect from A and reconnect to A without suffering an delay. I can even switch from A to an unrelated network X (e.g., my phone's hotspot), and then reconnect to A, without suffering any delay. However, if I go from A to B, then when I switch back to A, I suffer a delay of minutes because there is no response for a while to the ARP requests from my client This happens with different clients (Windows laptop, iPad, etc.).

Pictorially: A = 2750u, B= 825ACG, C = client, I = Internet, X = phone hotspot; note that the 2750u is connected to the 825ACG via Ethernet

C --> B --> I (works, no delay)
C --> B-->I    ---------------------> C --> A --> B -->I (works but WITH DELAY of a few minutes)
C --> A --> B --I  --------------------> C --> A --> B -->I (i.e., client disconnects from and then reconnects to A) (works, no delay)
C --> A --> B --I  --------------------> C --> X -->I --------------------> C --> A --> B -->I  (works, no delay)
C --> A --> B --I  --------------------> C --> B -->I --------------------> C --> A --> B -->I (works but WITH DELAY of a few minutes)

I don't understand what it is about connecting directly to B (825ACG) that stops it from responding to ARP requests for minutes when the client then connects to A.

Thanks.
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knilduser

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Just one other update. After further fiddling, I found the following: after disconnecting from the 825ACG router (which broadcasts SSID B), if I reboot (power off/on) the 2750u AP (which broadcasts SSID A), and then try to connect to A, the connection happens without any delay.

I did a telnet to the 2750u and ran "arp show", both before and after rebooting it. I didn’t see any difference in the ARP table entry for the router IP -- it's present with the same HW type and flag before the reboot (when connecting to A doesn't quite work) and after the reboot (when connecting to A works just fine).   

IP address       HW type     Flags       HW address            Mask     Device
<router IP>         0x1          0x2        <router MAC>            *          br0

So, rebooting the 2750u AP seems to reset/refresh some state that fixes the problem (i.e., the client's ARP request receives an immediate response from the 825ACG router) but it doesn't seem to be the ARP table state on the 2750u AP.

If you have thoughts on what the issue might be and how I can fix it without having to reboot the 2750u AP each time, please let me know.

Thanks.
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