Interface Groups (for Solaris 2.6)
----------------------------------
To turn on interface groups:
ndd -set /dev/ip ip_enable_group_ifs 1

How can I check what the setting is?
ndd -get /dev/ip ip_enable_group_ifs



What do interface groups help me with?
--------------------------------------
How is the outgoing IP address selected when more than one interface is configured? 
	OR
Why doesn't Solaris respond back on same address that the packet arrived on? 
	OR
Why are incoming packets going to both hme0 and hme1 but all outgoing packets
    leaving the server via hme0 only?



Answers to maddening questions from above
-----------------------------------------
IP routing on Solaris is strictly destination based and Solaris does not 
necessarily respond back out on the same interface.  Solaris uses standard 
IP routing and will respond back out on first match in the routing table. 

Without interface groups and with multiple interfaces having the same network
route, the outgoing interface selected is the one that comes first in the 
logical order (check with "route get <IP#>" and netstat -rvan) 

The primary (lowest interface instance) is usually selected as the first 
interface in the routing list that corresponds to that subnet. 



How Interface Groups can help
-----------------------------
When a connection is made to the server, an IP cache entry is made which contains 
the interface that the client connected to.  All packets going to that client 
will leave the server via that interface.  The cache entry is valid for the 
length of the ARP entry for that host.

If the same client now connects to the other interface of the server, the 
return packets will leave via the interface in the cache, not via the 
interface they arrived on.

Another client which connects to the server's second interface will have 
its return packets leave via the second interface.  This will give good load
balancing over a number of interfaces.

If one interface breaks (unplugged), then any client server sessions that
go through that interface will hang. If the client has a session through the
interface that is still working, then it won't hang (unless the server is 
dependent on a service that is using the failed interface). Once the cache
entry expires, a new connection from the client to the server through the
remaining interface will create a new cache entry for this host. 

If a session was hung because the packets back to the client were leaving 
via the broken interface (but were arriving via the working interface)
then the new cache entry will make the packets leave via the working
interface instead.

Of course any sessions where the packets arrived on the broken interface
will still be hung as the client can't contact the interface (but it could
start a new connection via the interface still working).






NOTE:  Make sure you have installed patch 105786 that is supposed to fix Bug ID 4077132 
       (ip_enable_group_ifs=0 doesn't pick default hostname)