ER Circuit co-exist for same location but different provider

nirmal kumar 1 Reputation point

We have a customer whose request is to migrated a ER circuit from existing provider to a new provider.

He is not going to totally migrate all the traffic to new circuit. He is planning to implement it in a phase. For example A,B,C etc. is the network routes currently using Old provider ER circuit. He wanted to move A traffic alone to new ER circuit and B and c will still need to use Old ER circuit.

Is the above achievable?

We suggest them to use AS-path prepend. So new provider will only advertise A traffic to new ER with AS-path prepend and B&C routes will only be available in old ER. In this case will B&C use OLD ER to reach on-prem.

We tested the above, Without As-path prepend for A network alone On-prem to azure able to reach via new ER. But azure to on-prem the request is getting timedout at azure gateway level. Any specific reason for this? In this case old ER has A,B,C routes & New ER has A routes alone. So azure should load-balances on-premises bound traffic across all the ExpressRoute paths using Equal-cost multi-path (ECMP) routing.

Why in my case it's not happening. Do I need to add any additional config on azure side.

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  1. KapilAnanth-MSFT 34,766 Reputation points Microsoft Employee

    @nirmal kumar ,

    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


    The maximum number of circuits in the same peering location that can be linked to a sinle virtual network is 4