My customer has multiple sites, each with an internet connection using a different ISP for each site.
They also have their own public IP address range and AS# that will be used.
- For discussion, let's assume the range they own is 100.100.0.0/16 and they own AS# 999
The would like to sub-divide the public IP range over multiple sites, but use the same AS# for each site.
Site 1 would advertise 100.100.0.0/27 via asn 999 to ISP "A"
Site 2 would advertise 100.100.8.0/27 via asn 999 to ISP "B"
Site 3 would advertise 100.100.16.0/27 via asn 999 to ISP "C"
In a failure case, Site 1 would advert Site2's subnets .. etc
No Site would ever advert the whole 100.00.0.0/16 class B
Is it 'legal' to use the same ASN like this?
You do not necessarily need to run iBGP in this scenario. Each router can run standalone and make its own routing decisions.
iBGP would only be required if the 3 routers needed to have a certain level of cohesion in making their routing decisions.
It is certainly "legal" to have three independant sites using the same ASN to advertise different prefixes. Here are a few questions for you.
1. Is there any need for inter-site communication?
2. Are any of these sites dual-homed?
3. In a case of failure, are you planning on the subnet for the other site to be announced automatically. It would probably be easier just to advertise the specific /27 from each site and then the full /16 (or an aggregate covering the 3 sites) as well from the 3 sites, which would provide full redundancy in case of failure.