William Westfield (BILLW@MATHOM.CISCO.COM)
Wed 21 Sep 88 14:15:33-PDT
Routers from cisco Systems have supported load balancing among
equivalent routes for a long time by round robining the packets
among routes. We also support a "variance" command for specifying
a multiplier that may be applied to consider routes for load sharing.
A variance of 1 implies that we use only the best route, a variance
of 4 implies that we use the best route, and any other routes up to
4 times as bad as the best route. A route 4 times as bad as the
best route will get 1/4 of the packets...
This works just fine for relatively simple networks, but for very
large networks, there is not enough resolution in routing metrics,
and you tend to use routes that you really wish you hadn't for distant
hosts (this is true even with cisco's IGRP, which has considerably
better resolution than any other routing protocol). It still seems
to work for relatively small variance values (say, <4). (What it
boils down to is that it is probably reasonable to use a 200 mS path
instead of a 50 mS path, but not to use a 16 S path instead of a
4 S path.)
Setting the variance to greater than 1 is also a good way to find
bugs in reassembly algorithms, as packets WILL arrive in a different
order than they were sent.
This will not allow a host to pick between several gateways on an
ethernet (it only allows a gateway to pick between several paths).
But current wisdom is that hosts should not know anything about the
internals of routing issues anyway.
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