How to PXE boot clients via external (ISC) dhcpd

Important are next-server (the SCCM PXE-Server) and filename. And you have to escape the Windows-specific backslash.

subnet 10.10.16.0 netmask 255.255.255.0 {
 ...
  next-server 10.10.27.51;
  filename "smsboot\\x86\\wdsnbp.com";
...
}

Here the complete subnet

subnet 10.10.16.0 netmask 255.255.255.0 {
  default-lease-time 3600;
  max-lease-time 86400;
  option subnet-mask 255.255.255.0;
  option broadcast-address 10.10.16.255;
  option routers 10.10.16.1;
  option domain-name-servers 10.10.18.150, 10.10.8.18;
  option domain-name "sccm.service.foo.com";
  #option vendor-class-identifier "PXEClient";
  #option vendor-encapsulated-options 3a:02:aa:08:ff;
  #option tftp-server-name "pxe.entw.sccm.service.foo.com";
  #option bootfile-name "smsboot\\x86\\wdsnbp.com";
  next-server 10.10.27.51;
  filename "smsboot\\x86\\wdsnbp.com";

  host pc00150 { hardware ethernet 04:25:2f:cd:da:c3; fixed-address 10.10.16.100; }
  host pc00151 { hardware ethernet 00:20:4b:89:c4:79; fixed-address 10.10.16.101; }
  host pc00152 { hardware ethernet 04:25:2f:c9:1b:36; fixed-address 10.10.16.102; }
  host pc00153 { hardware ethernet 0c:2b:e5:20:e1:44; fixed-address 10.10.16.103; }
}