It depends what you are looking to troubleshoot. BGP looking glasses provide you a view into the routing table (high level explanation) of the service provider.
This would allow you to perform some of the following troubleshooting:
- confirm that ISP is recieving your prefixes;
- confirm they are receiving/sending them with specific metrics (ie. Paths, PREF, Communities);
- What paths the ISP is using to reach your network;
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