> Are you saying that if you have multiple interfaces on the same subnet,
 > they must be in an IPMP group, or the config is broken?

Yes -- though the issue is really with having multiple interfaces on the
same link, rather than the same subnet.

 > Or if you have multiple IP addresses (on the same subnet) on the same
 > interface, it is broken? The latter does not seem so, but want to make
 > sure

Right, that is fine -- in fact, it is an extremely common configuration.

 > > With IPMP, outbound interface selection is determined in a round-robin
 > > fashion the first time an application attempts to send to a particular
 > > destination that does not yet have an IRE cache entry.
 > > See illgrp_scheduler() for details.
 >
 > For servers expecting connections to be established from a client, will
 > the IRE entry be set on the initial incoming datagram? Or the first
 > outgoing? Tobias' setup suggests that some responses go out one
 > interface, and others a different one.

It will be set up when the server first needs to contact the client.  In
the case of a TCP connection, this will be done in the kernel as part of
the three-way handshake in tcp_conn_request() -- so, for TCP, it is
effectively a set on the initial incoming datagram (TCP SYN).

 > Is this due to the fact that there are, based my expectation of the
 > first part, multiple interfaces on the same subnet but not in an IPMP
 > group, therefore not associated correctly.

Sorry, what is "this" in "is this due to the fact"?

 > I still don't understand the IRE behavior differences when IPMP is
 > enable vs. when not. Even after having tried to make sense of it
 > multiple times since IPMP was delivered.

What differences are you referring to?  The primary difference is that we
round-robin across the available interfaces; this is necessary for
outbound load-spreading.  A consequence of this is that one cannot predict
what interface in the group packets will be sent on.  However, once an IRE
cache entry to a particular destination has been created, the outgoing
interface will remain constant until the IRE cache entry is removed.

--
meem

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