Hi,
Please refer to RFC4861, section 6.3.6 for a description of how the default router should be selected.
https://tools.ietf.org/html/rfc4861#section-6.3.6
Furthermore, RFC4191 introduces the configuration of a preference that can be used to select a default router over another.
https://tools.ietf.org/html/rfc4191
Regards,
Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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