Ifgrps adds some intelligence to IP's source address selection and outbound
interface selection to allow IP traffic to flow on any of the interfaces.

The Solaris IP implementation adds a cache entry per destination IP
address, which can be viewed with the netstat command:

% netstat -ra | grep UHA
Ifgrps come into play when creating these cache entries.

Use the ndd command to enable/disable ifgrps, as follows:
  ndd -set /dev/ip ip_enable_group_ifs 0 


When a cache entry is created with ip_enable_group_ifs set to 1, a source
address for this entry is selected and the outgoing interface is assigned 
based on this source address. This behavior is new for 2.6, and is part of
the ifgrps functionality.  

The algorithm is:

  1. If there's a cache entry, use it, and send traffic out that
     interface regardless of source address.

  2. If there's no cache entry, check if there's a source address
     already in the datagram.  If so, create a cache entry with 
     the interface corresponding to the source address.  (NEW IN 2.6)

  3. With no entry and no source address, round-robin to one of
     the interfaces in the interface group that routing says to
     use. (NEW IN 2.6)

If ip_enable_group_ifs is set to 0, the machine reverts to pre-2.6 behavior: 

  1. If there's a cache entry, use it, and send traffic out that
     interface regardless of source address.

  2. If there's no cache entry, use the interface routing indicates
     (it'll always be just one) and set the cache entry to that.

Note: There are two important limitations to what is described above when
      ifgrps is enabled:
  1.For logical interfaces (le0:1, le0:2 etc) the load balancing does 
    not take place and only the physical interface associated with the 
    logical interfaces le0 is listed in the interface group list. 

  2.For systems with a number of physical interfaces and some logical 
    interfaces, the load balancing takes place only on the physical  
    interfaces and not on the logical interfaces. 



From patch 105786:
------------------
Synopsis: SunOS 5.6: /kernel/drv/ip patch

BugID 4077132: ip_enable_group_ifs=0 doesn't pick default hostname
The ip patch fix restores the correct behaviour of interface groups. The new
code has the ip_enable_group_ifs disabled by default, and so it restores the old
behaviour and the first physical interface acts as the *primary interface*.