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Date:      Thu, 29 Jul 1999 20:51:49 -0700
From:      "David Schwartz" <>
To:        "Joe Gleason" <>, "Jeffrey J. Libman" <>, <freebsd-stable@FreeBSD.ORG>
Subject:   RE: routing over a dual t1 connection (fwd)
Message-ID:  <000001beda3e$da10bc70$>
In-Reply-To: <008301beda3a$76f94ac0$>

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> I suggest getting a Cisco 2500 or 2600 series router.  They can
> do the load
> balancing quite easily and 2 T1's can be attached to either.  I
> know this is
> a non-freebsd solution, but I have always found that a hardware
> router with
> no drives and few moving parts is quite reliable.  I know this is
> not where
> I should be peddling my wares, but I have a 2500 and a 2600 for sale. ;-)
> Joe Gleason
> Tasam

	There are several problems I have had with that type of setup:

	1) The router has a problem separating inbound traffic from outbound
traffic. It can start to move outbound traffic away from an interface just
because its inbound traffic level is high. Or, for shared media, it can
continue to route large amounts of outbound traffic to an interface, causing
the inbound traffic to collide to hell.

	2) While you can control the way your outbound traffic splits, you can't
control the inbound traffic. If the other end isn't configured just right,
your inbound traffic will be badly split and there isn't a thing you can do
about it.

	3) Recovery from a single link failure is often very bad. This isn't quite
so bad for serial links because the router can usually discover a link
failure pretty rapidly. But if it can't, if something fails at a higher
level, it's very difficult to get good failover. You can try to implement
failover at a higher level with IP routing protocols, but if this is a link
to an ISP, they are unlikely to be willing to do this.

	4) The only splits possible are even or none. You can't allow a little
traffic, or just overflow traffic to take one link. This problem is worst
for shared media (like Ethernet), where inbound traffic and outbound traffic
can collide badly, even if both directions are well split.

	5) The load is measured over too long a period of time and commitments are
made to route connections over one link or another which the router can't
quickly change. Dramatically varying traffic patterns are very badly split.

	Don't draw the wrong conclusion. I'm not saying you should never do this.
Sometimes it is the best solution. But it can have some really serious
drawbacks. The biggest advantages of this setup are:

	1) It can be very easy to get working.

	2) It doesn't cause too many out-of-order packets to be received.

	3) It's fairly inexpensive, especially if you already have a Cisco router
with a spare fast serial port.

	4) Cisco routers are very solid and can have many months of uptime without
a problem.


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