From: Andre Teku (andre.teku@xxxxxxxxx)
Date: Wed Jul 24 2002 - 22:37:53 GMT-3
If the backup/second ISP is a RIR member, then they should know better than
advertising a longer prefix. He is not supposed to.
As I understand it the rules for ISP's are:
1. DO NOT ANNOUNCE upstream SUBNETS OF NETWORKS or SUBNETS of other ISP's
network blocks unless some exceptional circumstances/arrangements require
it. (If you do then you will attract and carry the other guys traffic for
free)
2. All RIR or "upstream ISP" assigned blocks are to be announced as
aggregate so as to attract incoming traffic properly.
Transit ISP's typically all carry the full Internet table into their BGP so
the second/backup ISP will have both the aggregate from the main ISP and
"your more specific". Consequently, if as a "stub" you announce your "sub
block" to 2 neighboring ISP's, your entry will get inserted in the
NEIGHBORING AS/ISP's respective BGP tables along with the main aggregate
from the main ISP. As long as the 2nd ISP does not announce your entry
forward, he will not attract your incoming traffic; rather, the main ISP
continues to get the inflow. And even if your two entries end up in the
neighbor's IBGP/EBGP table, using the 10 steps decision points, the BGP
process everywhere will then pick and install the entry with the shortest
AS_PATH which, under normal conditions, points to the main ISP (the one you
announced them). Therefore, as long as the main ISP is up, your incoming
traffic will be attracted by them no matter what. If they go down, then the
"shortest path" entry disappear from all neighboring BGP tables, their
"default" too disappears from your own routing tables, BGP at the
backup/second ISP then installs the next best entry and attract your
incoming traffic, - and their "default" attracts your outgoing traffic.
Conclusion, I still think that whether or not the your block is "provider
independent" or "provider aggregatable" the solution of accepting only
"defaults" colored with "unequal Local Preference" blablabla should still
work. Please correct me if I am wrong?
Thanks
Andre
-----Original Message-----
From: Robert Rech [mailto:rjrech@cgeykcsc.com]
Sent: Wednesday, July 24, 2002 2:52 PM
To: andre.teku@csosa.gov; tron@huapi.ba.ar
Cc: ccielab@groupstudy.com; unanswered@hotmail.com
Subject: RE: OT: Multihoming to two ISP's
Some thing such as who's address space this is may effect you and if
your primary ISP is aggregating your subnets into a larger supernet that
is advertised outside of that ISP. This could cause a second ISP to
advertise you space with a longer prefix in which case prepending would
not do you any good.
You should look at your advertisements from an internet looking glass
outside of either of your ISP's if all the BGP entires are the same
prefix length then perpending would help and you might get some ideal of
how many times you should prepend your AS to the route path.
try some of the links at these sites.
http://www.merit.edu/~ipma/tools/lookingglass.html
http://nitrous.digex.net/
http://neptune.dti.ad.jp/
The route servers will give you a router console to run bgp cmds to
look at your ip space from someone else's perspective
Robert Rech
Senior Network Engineer
Cap Gemini Ernst & Young
Kansas City Service Center
rjrech@cgeykcsc.com
phone (816) 459-4767
fax (816) 459-6767
>>> Andre Teku <andre.teku@csosa.gov> 07/24/02 10:19AM >>>
If you are a stub (as opposed to a transit) do this:
1. Announce your aggregate or block of addresses to both ISPs but,
2. On the backup link announce your AS with a longer path length (use
the
prepend command) to make it less desirable
3. Take default route announcements from both ISPs but
4. On the backup link entry point assign a lower "Local Preference"
value to
the default route to make it unattractive
5. Don't take full routes and all that unless you want to optimize
neighborhood routing
Good luck
Andre
-----Original Message-----
From: Carlos G Mendioroz [mailto:tron@huapi.ba.ar]
Sent: Wednesday, July 24, 2002 9:57 AM
To: Scooby Dooby
Cc: ccielab@groupstudy.com
Subject: Re: OT: Multihoming to two ISP's
What ?
Scooby Dooby wrote:
>
> A quick input:
>
> Wihtout running BGP and playing with the BGP attributes(Local
pref,Med and
> AS pre-pending), it would be unwise to advertise the same route
through
two
> different ISP's.
> It would cause routing loops ,
> as the world would have only one way to reach the main advertised
services
> like(Web,DNS,WEB) .Only ISP or company registered with IANA and
> RIR(regional registration authority)would be preferred to pass the
traffic
> to the end host.
>
> Cheers,
> OK.
>
> >From: Carlos G Mendioroz <tron@huapi.ba.ar>
> >Reply-To: Carlos G Mendioroz <tron@huapi.ba.ar>
> >To: ccielab@groupstudy.com
> >Subject: Re: OT: Multihoming to two ISP's
> >Date: Wed, 24 Jul 2002 07:22:17 -0300
> >
> >Conditional advertisement seems to be the only reliable way for
> >directing
> >all incomming traffic throw one link. And it also serves well the
health
> >of the core routers :-)
> >
> >
> >
> >roel.fonteyn@belgacom.be wrote:
> > >
> > > Did you try AS prepending, to reduce the chance other providers
taking
> >this route?
> > >
> > > Mvg/Rgrds,
> > >
> > > Roel
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