For connection to our ISP we used Linux-gateway and decided to use Cisco, firstly for QoS.
We installed 2651XM and connected 2 interfaces eth0/1 to ISP modem and eth0/0 to PIX, such connection was with Linux-gateway.
But we have bugs periodically interface 0/0 goes down, and this interface sometimes goes up again after some different periods of time, for example 30 minutes.
Interface reload by router reload or writing shut/noshut helps for some minutes.
So, our interface is up/down/up/down periodically.
We tried some variants: changed interfaces by places, wrote different parameters for interfaces speed, duplex, changed MTU, changed IOS versions nothing helps.
Where here may be the trouble?