Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
Can you please elaborate your architecture in a more detailed manner.
- You said "new provider will only advertise A traffic to new ER with AS-path prepend"
- But for the testing, you said "We tested the above, Without As-path prepend for A network alone On-prem to azure"
- Are you using AS Path Prepend or not?
With your architecture,
- I take it that A,B,C Networks are the OnPrem Network Ranges
- But please correct me if I am wrong.
In that case ,
- The control is totally up to you.
- You can advertise A network via NewCircuit and B,C Network via OldCircuit.
- If you are advertising A network only via NewCircuit, there is no need for AS Path Prepending.
- AS Path Prepending only comes into picture when both the NewCircuit and OldCircuit advertise A's address prefix.
- I understand that the latter is your case
- In that case, AS Path prepend is used to reduce the preference. The lengthier the AS Path, the less it is preferred.
- So, ideally, for A's traffic to use the new Circuit, you must advertise AS Path prefix for A in the older Circuit (so it is less preferred)
- See : AS PATH prepending
Note
The maximum number of circuits in the same peering location that can be linked to a sinle virtual network is 4
Cheers,
Kapil