>"Michael Fountain" <[EMAIL PROTECTED]> wrote,
>If you have an ISP assigned address that you are using, can you
>advertise that address out to another ISP?? Or if you want
>redundancy do you have to get your own registered IP range and then
>BGP advertise that through two ISPs?
To coin a phrase, "it depends." One of the real-world zingers of
Internet routing is that it is much more than BGP alone. There is a
complex set of policies that need to be understood and agreed-to
among a wide range of AS, and there are administrative procedures
that may become involved,
Technically, it can and does work. You will have to get both ISPs
(AS2, AS3) to agree how it is to be done, because the ISP to which
the address is assigned will need to advertise your sub-block (AS1 or
a private AS number) in addition to their supernet, and the other ISP
will need to agree to advertise a "hole" of the other ISP's space.
Both ISPs need to register this policy in appropriate policy
registries, and/or manually coordinate, so other ISPs can generate
filters to match. Assume AS3, the one that the address space does not
come from, is a small local provider. Its upstream provider (AS5) or
providers (AS5,AS6) may not trust ISP2 sufficiently to accept any
route announcement that AS3 cares to send. AS5 might very well
filter the route advertisements such that if AS5 receives a route
from AS3 that offers a route in AS2's space (as assigned to AS1),
might block it. AS5 has to know that is a legitimate route.
You can extend this case to think of other cases
Due to other route policies, it is entirely possible that some
multihomed address blocks, especially small ones, may not propagate
everywhere in the Internet. When an enterprise is well-connected and
a clueful person designed the system, those destinations that are
marginally reachable through one ISP may be reachable through the
other. Again, if you want guaranteed service, contract for a VPN or
physical network.
>
>>
>>
>>See comments below:
>>
>> > I haven't worked with BGP yet, so someone please correct me if I'm wrong
>> > with this -
>> >
>> > My understanding is that even if you have internet connections to two
>> > differnet ISPs, you don't have to BGP. If you are using IP addresses they
>> > have assigned to you then you are ok.
>>It depends what "ok" means for you. If you need redundancy and backup,
>>then you are not "ok". Consider the scenario below :
>>
>>|-------| |-----------| |-------|
>>| ISP1 |---------------| Customer |-----------| ISP2 |
>>| | | | | |
>>| AS1 | | | | AS2 |
>>|-------| |-----------| |-------|
>>
>>The customer is multihomed to AS1 and AS2. The customer is not running BGP
>>in this scenario.
>>
>>Suppose ISP1 has assigned the customer 10.10.10.0/24 and ISP2 has
>>assigned the customer 10.10.11.0/24. Both ISPs are statically routing
>>those blocks to the customer. The customer has two default routes, one to
>>each ISP (he could prefer one ISP or the other for outbound traffic but
>>inbound traffic will come from ISP1 or ISP2 depending on the destination
>>ip address).
>>
>>Inbound traffic destined for hosts in 10.10.10.0/24 comes through ISP1 as
>>ISP1 is originating and advertising this block. Inbound traffic destined
>>for hosts in 10.10.11.0/24 comes through ISP2 as ISP2 is originating and
>>advertising this block.
>>
>>Now what happens if the connection between the customer and ISP1 goes down
>>for any reason ? Well you will see that hosts in 10.10.10.0/24 will be
>>unreachable from outside the customer's network. So redundancy is not
>>achieved properly as half of the customer's network will be down.
>>
>>
>> > The big reason to use BGP would be if you have a block of registered IP
>> > addresses and you want to advertise them to the internet via two different
>> > ISPs.
>>I would say that as soon as you are multihomed (to 2 different providers)
>>then using BGP is a Must. (Of course if you really want total redundancy
>>and the option to do intelligent routing, load-sharing..).
>>
>>
>> >
>> >
>> > >
>> > >Hellow group!!
>> > >According to cisco press book, when company has two connections active to
>> > >two different ISPs, BGP should be used.
>> > >Could anybody tell me what is the result of connecting two active
>> > >connections to two different ISPs?
>> > >I know that one connection should be used as a backup line only while the
>> > >other is active in order not to use BGP.
>> > >Thanks in advance.
>> > >
>> > >........................................................
>> > >iWon.com http://www.iwon.com why wouldn't you?
>> > >........................................................
>> > >
>> > >___________________________________
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>>
>>--
>>Contrary to popular belief, Unix IS user friendly. It just happens
>>to be very selective about who it decides to make friends with.
>>
>>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
>>Andre Riscalla Network Specialist - Internet
>>[EMAIL PROTECTED] 514-940-5664
>>Network Engineering AT&T Canada
>>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
>>
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