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### Instruction:
What is the purpose of the 'bgp router-id' command in IOS XR BGP configuration?
### Response:
The 'bgp router-id' command is used to manually configure the BGP router ID. By default, the BGP router ID is chosen as the highest IP address configured on a loopback interface. If no loopback interface is configured, the highest IP address from the physical interfaces is chosen. Manually configuring the router ID provides stability in case of interface flaps.
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