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Re: [PLUG] doubling bandwidth and achieving network redundancy
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> Date: Fri, 18 Aug 2000 12:02:23 -0400
> From: Stephen Brown <steve@dataclarity.net>
>
> Son To wrote:
> >
> > I do not know much about routers so can someone tell me if it is possible
> > to achieve this using a Linux router.
> >
> > My bellatlantic ADSL is unreliable. It would work great for a week or so
> > then disconnects me. I have to restart pppoet.
> >
> > Suppose I get two ADSL line from two different ISP, can a Linux router
> > be configured so that traffic is load balance between the two lines? A TCP
> > data stream is sent/recieve using both lines. If one line goes down, my
> > internal network should not notices the broken connection.
>
> Yes, but it isn't as easy as it should be. The linux end of things
> will be trivial compared to the hassles in the real world.
>
> You have 2 basic options:
> The easiest is to get redundant connections from a single provider
> which may allow you to do link trunking across the 2 links, and keep
> a single IP block.
>
> The other one is what most ISPs and colo facilities do, but on a smaller
> scale. It will require you to get an ASN (Autonomous System Number) from
> ARIN (http://www.arin.net/), run BGP4 on the Linux router and either get
> a block of 'transportable' IP addresses from ARIN, or IP addresses from
> one ISP that are advertised to the rest of the Internet so you can reach
> your IPs through either upstream connection. The other drawback is that
> for a single TCP connection you really won't get a doubling in bandwidth
> because the routers in the core of the net will point all traffic to the
> closest ISP instead of sending half to one and half to the other.
>
This sounds good, but in reality it is seriously non-trivial.
On a good day, (all) the Internet Backbone providers can get this to work.
However, on a bad day -- which is when you need it -- it doesn't work,
period, somebody has to intervene manually, 9 failures out of 10.
Penn keeps going back to this well every 6 months or so, but we sill come up
empty. It all works in theory, but when one line fails, somebody always has
to go "tweak something," that wasn't configured right anymore because of
some change that got made to the IOS or routing tables or... "last week."
The rate-of-change factor here is just deadly.
It's not that it can't work, it's just that it doesn't. Something always
changes between the two times when you need it to work.
Link trunking only works when the routers at both ends are going to the
same places and know about each other. (And you do need a router at both
ends, in addition to the xDSL modem.) Across two different ISPs, forget
about it. It will give you the redundancy AND bandwidth increase if you
use say a Cisco router on both ends, much the same way 128K ISDN line
"doubles up." I don't know if any xDSL ISP is sufficiently sophisticated
enough in the sales end to have a clue about what you want to do, and
therefore how to answer your query. The techs, especially if they work with
Cisco gear are familiar with it. (Cisco is the one who pioneered using two
say 56K lines to equal one 128K line. It's really kind of neat how it just
works when you have the matching equipment and configurations.)
By the time you move up the food chain from "residential" to "commercial"
sales, they are used to working with multiple lines and line types, but the
prices go up accordingly.
However, unless I am very mistaken, your problem has to do not with xDSL
reliability but with PPPoE. I've had BA Infospeed since January, and aside
from the great ATM cloud problem in the winter, have not seen a single
outage.... but then I have static addresses from my ISP, DCAnet, and even
if the link did drop, I'd never know it. Unlike PPPoE, I don't have to
have a daemon running locally to have connectivity.
So before you blame your Bell Atlantic Infospeed xDSL connection, consider
getting a different ISP and loose PPPoE. I don't know how easy that is now
that Verizon exists -- and they dropped all references to any ISP except BA
from their web site... and since Verizon got out of the xDSL business last
month. Call DCA (or visit their web site) and ask them to get you switched
over. Don't forget that not much will happen until after the strike is over.
One last point -- "Perfect connectivity" (24x7) is NOT what xDSL, or even
Cable Modems are about. Both are simply "best effort" services. Even on the
backbone, nobody gives you service guarantees unless you pay big bucks for
them. And even then, the contracts have more weasel words in them than
Bill Clinton ever heard of! Because of its pricing xDSL is seriously on the
low end of the totem pole when it comes to "service."
To get more of an idea what I'm talking about, check out:
http://www.dslreports.com/shownews/125
--
www.tru64unix.compaq.com
www.tru64.org
comp.unix.tru64
T.T.F.N.
William H. Magill Senior Systems Administrator
Information Services and Computing (ISC) University of Pennsylvania
Internet: magill@isc.upenn.edu magill@acm.org
http://www.isc-net.upenn.edu/~magill/
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