> 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
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.
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