[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: pf and two ADSL links

On Mon, Dec 19, 2005 at 05:57:58PM +0000, Stuart Henderson wrote:
> > > If you need redundancy, try a DSL and a cable TV broadband or leased
> > line.
> > 
> > Ensuring that the leased line goes through different exchanges to the
> > ADSL circuit....
> ...and that they don't all cross the country on the same fibre route.
> (though, you probably won't be able to determine this in the case of
> these consumer-grade connections).

Which is why multi DSL is not a HA solution. BT do not offer a SLA on
any ADSL service, and all UK ADSL is operated by them, with the minor
exception of LLU. At work, we do LLU SDSL, not ADSL, but most of our
DSLAMS reside in BT exchanges, and hook into backhauls from varoius
providers. BT still operates the copper from the exchange to the EU
building with LLU. Small ISPs don't have the ability to lay cables
though central business districts.

> Multiple ADSL, even on copper from just one telco, can easily have:
> 1. different kit terminating PPP sessions
> 2. different modem/router at your end
> 3. different interconnect point with the telco

No, copper from one address always runs to the same exchange.

> 4. in areas with unbundled connections, different dslams.

AFAIK there is only one UK operator unbundling for ADSL, in some southern
exchanges (eg London & there abouts). Many ISPs unbundle for SDSL in areas
that they operate in, but no-one does it nationally. But the ADSL and SDSL
will still run down the same copper bundle under the street to the same
exchange, so there is no physical redundancy.

I've seen it often enough where a firm has both ADSL and SDSL into their
HQ, and a JCB has dug though the footpath and taken the lot out. I've
had rats chew though leased lines on the Forth Road Bridge and organised
the cops to stop traffic so that Telewest can patch the cable.

We run leased lines from 1meg up to 100meg, and some firms think that
one line is enough, until it goes down.

Look to different media alltogether for HA.

> In the case of the UK using BT, putting them on different contention
> ratios is meant to help too (aaisp mention this, istr). These measures
> don't always help (e.g. in the case of a telco using radius proxies
> which are malfunctioning) but probably are worthwhile for some users.
> The problems I personally have seen the most of are 1 and 2, which
> are solved quite nicely by natting a connection with a source address
> of whichever of two providers is functional (or tunnelling to a 3rd
> point on a highly-reliable network if you want to use real addresses).

This is all fine for messing about at home or in a small style, no SLA
business. You need multiple routes for HA, and Telewest don't do static
IPs on consumer cable "blueyonder". If you contact TW for a business
connection, and you can't afford a LL, they will resell BT's ADSL with
a static IP. Different ISPs, but same media, so no good.

ADSL and blueyonder is a good cheap SOHO outbound solution (dynamic
IPs). Each is cheap enough so that it doesn't matter if one or the other
is down for a week. And the chances of both going down at the same time
is good enough for SOHO situations.

When an ADSL is faulted to BT via eCo once a fault has been detected
though Woosh, the GPMS case will sit in the diagnostics queue for 48
hours before it is even looked at. Then resolution will typically
take another 3-5 days.

SDSL is a bit quicker, with turn arounds in about 2 days.

If you want to offer your customers an SLA, go colo and manage your
boxes via ADSL, ISDN, cable, whatever. In London, you can get a U
for #500 PA, while one SDSL will cost #200 per month, and be less

Just my 2p after supporting ASDL, SDSL, Leased Lines, colo space, etc.


Visit your host, monkey.org