RFC 4861 describes the use of the (R) or ‟Router” flag in NA messages as follows:
A node capable of routing IPv6 packets must reply to NS messages with NA messages where the R flag is set (R=1).
Hosts must reply with NA messages where R=0.
The use of the ‟R” flag in NA messages impacts how the hosts select their default gateways when sending packets ‟off-link”. Therefore, the proxy-ND function on the 7750 SR, 7450 ESS, or 7950 XRS does one of the following, depending on whether it can provide the appropriate R flag information:
Proxy-ND provides the appropriate R flag information in proxy-ND NA replies
If proxy-ND cannot provide the appropriate R flag when replying, it floods the received NA messages.
Because of the ‟R” flag, the procedure for learning proxy-ND entries and replying to NS messages differs from the procedures for proxy-ARP in IPv4: the router or host flag is added to each entry, and that determines the flag to use when responding to a NS.