Jónatan Þór Jónasson took the time to implement Wake-on-LAN functionality using UDP support introduced in Cisco IOS Tcl in release 15.1(1)T. He found a TCL/TK example of a magic packet being sent, used that as a base, and with small modifications got it to work on his router. Here‘s his code (it’s obviously a proof-of-concept, but you need just a few more lines to get a working Tclsh script):

proc WakeOnLan {broadcastAddr macAddr} {
     set net [binary format H* [join [split $macAddr -:] ""]]
     set pkt [binary format c* {0xff 0xff 0xff 0xff 0xff 0xff}]

     for {set i 0} {$i < 16} {incr i} {
        append pkt $net
     }

     # Open UDP and Send the Magic Paket.
     set udpSock [udp_open]
     fconfigure $udpSock -translation binary \
          -remote [list $broadcastAddr 4580] \
          -broadcast 1
     puts $udpSock $pkt
     flush $udpSock;
     close $udpSock
}

For whatever reason, he wasn’t able to generate subnet-specific broadcasts; sending WOL packet to subnet-specific broadcast address didn’t work, while sending it to local broadcast address (255.255.255.255) did (obviously sending the packet through all router interfaces).

Remote WOL

Obviously you could use subnet broadcast address of a remote LAN as a unicast destination address if you’d want to wake a remote PC, but that would require directed broadcast support on the remote router, which is not always a good idea. It’s way better to configure a static ARP entry for a real unicast IP address on remote LAN with broadcast MAC address. In both cases, you shouldn’t specify the ‘-broadcast 1’ flag in Tcl (as you’re sending a unicast UDP packet).