Hi Pradeep,
This article explains what happens during an ExpressRoute circuit maintenance and provide actions you should take to minimize service outage affected by a planned or an unplanned maintenance.
Also there is a notifications that you can turn on so you get enough days to prepare for this routing mainenance, so detailed information is in the article please review - https://video2.skills-academy.com/en-us/azure/expressroute/planned-maintenance
Also you can use Azure Monitor and implement proactive link status.
Hope this helps.
JS
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