Propogated route list from ER vs UDR.

Hello @secui-se-team ,

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

Please find the answers to your queries below:

Q1. Which one has higher priority?

1. dst on-prim(1.1.1.0/24) next hop VNG (propagated by ER)
2. dst on-prim(1.1.1.0/24) next hop VM1'ip (user defined)

Answer : dst on-prim(1.1.1.0/24) next hop VM1'ip (user defined) has higher priority.

If multiple routes contain the same address prefix, Azure selects the route type, based on the following priority: User-defined route > BGP route > System route.
Refer : https://video2.skills-academy.com/en-us/azure/virtual-network/virtual-networks-udr-overview#how-azure-selects-a-route

Q2. If UDR has higher priority, the longest prefix match rule is higher than it? Which one has higher priority?

1. dst on-prim(1.1.1.0/24) next hop VNG (propagated by ER)
2. dst on-prim(1.1.0.0/24) next hop VM1'ip (user defined)

Answer : Yes, Azure always selects a route based on LPM (longest prefix match) algorithm. Only when multiple routes contain the same address prefix, UDR will take precendence. I believe the address prefixes that you shared in this question do not overlap. But for example:

1. dst on-prim(1.1.1.0/24) next hop VNG (propagated by ER)
2. dst on-prim(1.1.1.0/16) next hop VM1'ip (user defined)

In this case, the BGP route advertised from ExpressRoute will be preferred.

Refer : https://video2.skills-academy.com/en-us/azure/virtual-network/virtual-networks-udr-overview#how-azure-selects-a-route

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.

So, in conclusion the Azure routing would be as below:
LPM --> Always takes priority.
Only when multiple routes contain the same address prefix, Azure selects the route type as follows : User-defined route > BGP route > System route.

Kindly let us know if the above helped or you need further assistance on this issue.

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