SUMMARY: multiple network interfaces on a host (on a subnet or not)

From: Jaewan Kim (
Date: Tue Apr 18 2000 - 16:22:14 CDT

I received a few recommendations, which evolved around
parameter of /dev/ip module.

I did some research myself in the meantime to find that:

If set to 1, and if you have multiple physical interfaces with different MAC
addresses on a single subnet, each connection will remember which interface
the incoming packet came in. It does that by enabling a cache per interface
to keep track of packets on layer 2, thus maintaining a sanity in routing
decisions. Otherwise, it will use only one interface for outbound packets.

In the case of each interface on separate subnet, you may have more than one
default routes in your /etc/defaultrouter file when system is booting up,
and the default routes are used in a round robin manner. And if one of them
fails, there are some timeout tunables to delete the route entry.
Sunsolve infodoc 17947 is a very good reference, and
the infodoc 12618 is a good reference on Solaris TCP/IP.

 On Sat, 15 Apr 2000, Jaewan Kim wrote:

> This kind of puzzles me.
> Case 1.
> two hme interfaces on a host (same subnet)
> Suppose these interfaces have IP addressed of
> A.B.C.1 and
> A.B.C.2
> on a subnet
> A.B.C.0/24
> The routeing table entries would be
> A.B.C.0 A.B.C.1 U hme0
> A.B.C.0 A.B.C.2 U hme1
> default A.B.C.100 UG
> if /etc/defaultroute has A.B.C.100 in it.
> This setup is proposed by a vendor with a load balancer in fron to this
> server in the same subnet, say A.B.C.5
> The idea is to have this load balancer to distribute the traffic to this
> host between the two and if one interface fails, the remaining one will keep
> working.
> However my belief is that this routing table is referenced in a sequencial
> manner, that every outbound packet would leave through hme0, and when it
> fails, it would not switch to hme1 unless you manually delete the first
> route to A.B.C.0 subnet.
> Is it a correct assessment?
> Case 2.
> An alternative setup is to have each of the interface on a separate subnet
> so that the routing table would look like
> A.B.C.0 A.B.C.1 U hme0
> D.E.F.0 D.E.F.1 U hme1
> default A.B.C.100 UG
> This implies that the default route is set to go out hme0.
> However I have seen systems that has more than one default route set, which
> would have another entry
> default D.E.F.100 UG
> In this case, how would a packet leaving the system to an unknown IP address
> react? Is it also sequentially referenced that the second default route
> would not be used at all?
> I have yet to find out about the load balancer if it is a proxy in that it
> rewrites the packet so that the packets coming from it has it's IP as source
> address or it only rewrites destination portion of IP header that the
> outbound packet never stops at the load balancer.
> Can anyone discuss it with me or point me to a right resources for this?
> ============================================
> Jaewan Kim
> 856-988-0062 x425 (
> 877-250-8919 ( fax/voicemail)

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