BGP dual-homing - can I advertise the same /24 with two different ASNs?
I currently have an Internet link to AT&T. I am leasing a Class C from AT&T (I'll call it A.B.C.0/24), receiving full Internet routes, and I am advertising their Class C using their ASN, yyyy.
I want to dual home this with a lower speed Verizon link. VZ said they will advertise the AT&T Class C and I will receive full Internet routes. The question I have is which ASN I will use to advertise A.B.C.0/24 to VZ.
To complicate matters, I have another pair of Internet links that I have dedicated for my company's eCommerce where I am dual homed to AT&T and Verizon and I have 4 Class Cs (X.Y.Z.0/22) and a ASN qqqqq from ARINs. These 4 Class Cs are different than the Class C above.
The issue is that VZ wants to advertise the single A.B.C.0/24 Class C using the ASN qqqqq associated with X.Y.Z.0/22 as opposed to the AT&T ASN yyyy.
Is this legal/possible and not violate Internet protocol?
Verizon should not originate the A.B.C.0/24 with one of their AS numbers. Asking to you to originate prefix A.B.C.0/24 with your own public AS number just moves the problem to you as you have noted.
You should find out what AS number is used by ATT to publish the prefix in the internet and also to check if it exists as a /24: if ATT uses a less specific aggregate over the internet the risk is that new link would become the primary link for return traffic (most specific route wins).
The only clean ways are:
you get a fifth public /24 and you advertise it to both ATT and VZ with your own AS number
you get a second public address block from Verizon and you make NAT part of the game as explained here
Re: BGP dual-homing - can I advertise the same /24 with two diff
I'm on the phone with VZ tech now. They will give me one of their ASNs (not 701) to use. I guess I'm not that big of a customer!
One thing, there must be a "split-horizon" or route-loop prevention type of issue when I used my own AS (qqqqq) to advertise these AT&T routes to VZ. The problem was that I could not see my own /22 range (that is on a different Internet link) unless I used the VZ ASN instead.
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