UDR Source Prefix

Question

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!

Answer 1

Hi @Nika Davitashvili

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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