Troubleshooting WAN issues

Hello all,

I have recently joined a NOC team where we mainly focus on monitoring networks and provide WAN support. We have full access to all of our customer's routers, so my question would be after I have done the initial and basic stuff like line checks, power cuts on site, reset router, exchange, dslam etc, what troubleshooting can I do on the actual router's (juniper and cisco mainly) to verify or find issues? I have used the show interface " " to verify bandwidth connections, show process cpu, to verify utilization, but I'm guessing to check the connection something like BGP would get involved which I'm not too confident as yet.

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Thanks
Viktor