[c-nsp] ISP Dual AS

[Reuben Farrelly]

On 6/12/2012 10:54 PM, mert ozkul wrote:
> Hi All,
> I have query about ISP Design.
> Why some ISP`s (Ex: BT) using dual AS`es on their network?
> What are the advantages of using more than one AS in the ISP network?What can be achieved if you use more than one AS?
> Thanks,Best Regards,
> -Mert

I operate a network where we have two AS's.  We inherited the second AS 
when we took over the second network.

Why haven't we merged this into one big AS?  Well the two AS's have two 
completely separate routing policies.  One of those AS's supplies only 
schools as downstream users, and the upstream connectivity to them is 
via a specific supplier (a regional educational/research network) whose 
rules of engagement are that they only supply to education/non corporate 
clients.

Our other AS is a bit more relaxed, and there is no such restriction 
with any suppliers to that network.

The two AS's peer with each other though so it doesn't cost anything for 
data to transit between them anyway.

It does pose some fairly minor complications in terms of training and 
one-bgp-as-per-IOS but given we have full control over both AS's and all 
the equipment in them, it means on the other hand that we get to work 
with some slightly more exciting things like inter-AS MPLS that you 
don't get to experiment with when it's all one vanilla system ;-)

Just one reason why an ISP may have more than one AS.

Reuben