If you add "option MULTICAST" to your kernel, routed starts to grow.
This is because it does not ignore the new interface flags.  The following
is a fix:

*** interface.h.orig	Fri Dec 10 15:41:58 1993
--- interface.h	Fri Dec 10 15:43:04 1993
***************
*** 80,85 ****
--- 80,87 ----
  #define	IFF_LOOPBACK	0x8		/* software loopback net */
  #define	IFF_POINTOPOINT	0x10		/* interface is point-to-point link */
  
+ #define	IFF_FLAGMASK	0x1f		/* mask for relevant int_flags bits */
+ 
  #define	IFF_SUBNET	0x1000		/* interface on subnetted network */
  #define	IFF_PASSIVE	0x2000		/* can't tell if up/down */
  #define	IFF_INTERFACE	0x4000		/* hardware interface */
*** startup.c.orig	Fri Dec 10 15:41:02 1993
--- startup.c	Fri Dec 10 15:46:59 1993
***************
*** 101,107 ****
  			    ifr->ifr_name);
                          continue;
                  }
! 		ifs.int_flags = ifreq.ifr_flags | IFF_INTERFACE;
  		if ((ifs.int_flags & IFF_UP) == 0 ||
  		    ifr->ifr_addr.sa_family == AF_UNSPEC) {
  			lookforinterfaces = 1;
--- 101,108 ----
  			    ifr->ifr_name);
                          continue;
                  }
! 		ifs.int_flags =
! 			(ifreq.ifr_flags & IFF_FLAGMASK) | IFF_INTERFACE;
  		if ((ifs.int_flags & IFF_UP) == 0 ||
  		    ifr->ifr_addr.sa_family == AF_UNSPEC) {
  			lookforinterfaces = 1;

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