On Wed, 31 May 2000, Todd wrote:
> althought it is possible to get your netblock announced on both of the
> networks you're 'dual' homed on under the circumstances that he described,
> this doesn't really get you the reliability you should be asking for. the
> reason is that every international network you're not directly connected
> to will properly filter the announcement from the second network and will
> only keep the route to your netblock through the first provider. that
They'll only filter it if it's advertised as a part of an aggragate
announcement by the primary provider and they have a vested interest in
blocking it, if your provider passes your AS as the originating AS, then
it doesn't matter who "owns" the address space.
There's no "list of providers" that get injected into the global routing
tables, it's all based on aggragation and originating AS as well as
peering agreements. Most Tier-1's I've worked with tend to be more
concerned about filtering based on their own advertisements when dealing
with their peers than blocking invalid advertisements from their peers
(which is why backbone peering contracts are sticky subjects and customer
advertisement peering (including lower-tier ISPs) *is* filtered based on
what the leaf node is allowed to advertise.)
Any Tier-1 and most Tier-2 providers will do this right, it's a key part
of their business model. Also, it's perfectly possible for peers to
accept a route that is part of an aggragate as long as the other
announcing entity has the ability to transit for that network. For
instance, if your ISP is dual-homed, then both of it's upstream entities
must be able to transit traffic for your netblocks and accept routes from
one another for your network (so that problems in their backbones don't
cause issues for their other customers who wish to reach your network when
the best or only path is through their competetor.) Adding additional
networks into that mix isn't technically difficult for them to do (and
that's why I generally recommend going with Tier-1's for this process),
but hopefully someone's watching how many networks they're adding (though
routers can take a lot more memory these days and store their tables
internally in a better-aggragated format thankfully.)
> real dual homing with real portable address space means that you can get
> routes to your netblock announced at the NAPs and propagated across *all*
> (or most, when they're working right)of the international backbones. that
> means that when one route goes down, traffic still comes through the other
> one.
>
> better. a lot better.
"Portable address space" is a straw-man, and it just means that nobody's
already aggragating your address space in a larger advertisement (you
hope.)[Actually, come to think of it, BGP should prefer the smaller
advertisement over the larger aggragate one- so reverse seeding the
advertisement by chosing the secondary carrier as the one who's address
space you'll use is probably advantageous if there's a provider-owned
peering point that it's advantageous for the bulk of your traffic to
take.]
A "portable address space" acts just like a "non-portable address space"
in a BGP table, there's not a BGP attribute for "portability" and anyone
carrying the announcement will accept announcements for the network no
matter who "owns" it unless they've speficically filtered it. That's why
it's important to arrange for cross-advertisement with both providers,
they can deal with announcing and filtering just like they do for a
customer who "owns" their own space.
[Who generally isn't treated any differently in their tables than anyone
else they carry traffic for, and who still needs to ensure that the
provider understands that the address block is being advertised by
multiple providers.]
> btw: i'd rather have two carrier-redundant connections to a well-managed,
> dual-homed ISP than a two carrier-rudundant connections to an ISP managing
> its own national/international backbone any day. that's just me.
Personally, I'd rather have two carrier-redundant connections to two
Tier-1's, but then when you've got millions of people a day hitting a
site, peering points matter a heck of a lot more than links for the bulk
of your traffic.
If your company starts buying up international properties, doing deals
with foreign trading partners, gaining foreign market share, etc., then
you'll get plenty of oppertunities to test those advertisements courtesy
of Joe Backhoe Operator and his big yellow fiber eating monstrosity.
Paul
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Paul D. Robertson "My statements in this message are personal opinions
[EMAIL PROTECTED] which may have no basis whatsoever in fact."
PSB#9280
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