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Modem in Bridge mode is not working properly

Hi everybody,

I have face an interesting problem, I have a ADSL modem and I configured it in bridge mode. And then I configured the Cisco UC520 with a PPPOE connection and it is connected to the internet.
I also use the Cisco UC520 as a NAT device to internal Lan users.
I checked the internet from one of internal clients and I found that it is not good at all. the web sites are not brows and the ping is fluctuating between 200 and 1400 ms.
When I configure the PPPOE on the modem  and use the IP address of the modem as a default gateway of all lan devices, it will work fine.

I am wondering why modem in bridge mode is not working properly, I prefere to get the public IP address in my UC device. now I have to use the modem as the NAT device.

any idea.

Voice Over IPRoutersBroadband

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8/22/2022 - Mon

Make sure both devices are using 100Mbit LAN ports. One of the devices may be set for 10Mbits.  Also make sure both devices are operating Full Duplex. I am sure they are but check the LED's on the ports as they will normally signal that.

Beyond that, get a cheap router and substitute it to see if there is some incompatability between your Cisco router and modem.

.... Thinkpads_User

Thanks for reply, I have done that. the port configuration in UC device is in auto speed, auto duplex. and for being sure that the problem is related to the UC or not. I connect my notebook to the modem and setup the PPPOE connection on my notebook. it didn't work again. I mean, it connected to the internet but very unstable browsing and PING.

meantime, I changed the modem and still have the same problem. it works well when I configure the PPPOE on the modem and it doesn't work when it is in bridge mode.
George Fendler

What is the make/model of the modem?
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Walt Forbes

Can you please confirm that you tried a different router. That was not clear to me above.

.... Thinkpads_User

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Matt V

Make sure when you put the modem in bridge mode, you set the MTU on the dialer interface to 1492 and then add the set the MSS adjustment on the internal LAN interface to 1452.  This should resolve the speed issues.

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Thanks for the feedback and the Points.    : )

>> ISPs never accept that they have problem

Ain't that the truth.   : /