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NikaDavitashvili-9971 avatar image
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NikaDavitashvili-9971 asked GitaraniSharmaMSFT-4262 commented

UDR Source Prefix

Can somebody please explain to my clueless self, why in the world is a prefix configured in a UDR (user-defined route) referring to "source".

I lived a thousand lives in the networking and have carpal tunnel syndrome to prove it. Static routes always consist of destination prefix, next hop and an optional metric/admin distance.

Please help!


FRY_S1RagAEmDTx


azure-virtual-network
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Hello @NikaDavitashvili-9971 ,

Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.

You are correct. The address prefix in a UDR contains the prefix where you want to route traffic to i.e. destination address.

I have reached out to the Product group team to fix this on the Azure portal UI.

Regards,
Gita

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NikaDavitashvili-9971 avatar image NikaDavitashvili-9971 GitaraniSharmaMSFT-4262 ·

Do I get bug bounty? :)

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Hello @NikaDavitashvili-9971 ,

Thank you for reporting this Azure Portal bug but the Product Group team confirmed that they already saw this when they were reviewing customer feedback last week and have created an internal work item to fix this.

Regards,
Gita

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1 Answer

soysoliscarlos avatar image
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soysoliscarlos answered soysoliscarlos published

Hi @NikaDavitashvili-9971

Thank you for asking this question on the Microsoft Q&A Platform.

I understand that you want to know what is the "address prefix source" for Azure, right?

I found this in the documentation:

Each route contains an address prefix and next hop type. When traffic leaving a subnet is sent to an IP address within the address prefix of a route, the route that contains the prefix is the route Azure uses.
When outbound traffic is sent from a subnet, Azure selects a route based on the destination IP address, using the longest prefix match algorithm. For example, a route table has two routes: One route specifies the 10.0.0.0/24 address prefix, while the other route specifies the 10.0.0.0/16 address prefix. Azure routes traffic destined for 10.0.0.5, to the next hop type specified in the route with the 10.0.0.0/24 address prefix, because 10.0.0.0/24 is a longer prefix than 10.0.0.0/16, even though 10.0.0.5 is within both address prefixes. Azure routes traffic destined to 10.0.1.5, to the next hop type specified in the route with the 10.0.0.0/16 address prefix, because 10.0.1.5 isn't included in the 10.0.0.0/24 address prefix, therefore the route with the 10.0.0.0/16 address prefix is the longest prefix that matches.

Hope this helps,
Carlos Solís Salazar


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