Odd result and underlying mistake

Posting for other's benefit in case someone else does this.  I searched the web without finding an answer then discovered the issue.  What I saw in a tcpdump output (because things weren't working) was

Request who-has <target IP address> tell <target IP address>

Where <target IP address> was a local interface address, quite odd since the local interface should know its own MAC address.  The problem was that I had accidentally used the local interface IP address in 'ip route add default via <local interface IP address> dev <local interface>' instead of 'ip route add default via <gateway IP address accessible from local interface> dev <interface>'.