Transitioning to Expressroute from Meraki MX SiteToSite Routing question

azure-000 1

I have been running a Meraki Site To Site VPN setup using MX on prem and VMX in Azure to connect our on prem environment to Azure. This includes having some Route Tables setup using the Virtual appliance as the next hop with associated subnets.

We are moving to Expressroute and I am using an Expressroute circuit and Expressroute gateway attached to an ExpressRoute Subnet. We are configuring BGP for route advertisement.

Using Expressroute will I need to create new route tables, edit the exiting but change from appliance to gateway/and or Vnet or not use them at all considering the BGP advertisements?

I have created the appropriate VNET peerings for the Expressroute network/subnet.

I do not see anything in the expressroute documentation mentioning route tables.

Thanks in advance

1 answer

ChaitanyaNaykodi-MSFT 26,216 Reputation points Microsoft Employee

2024-06-10T19:12:20.4533333+00:00

@azure-000

Thank you for reaching out.

I understand you are replacing your Site-Site VPN with an Azure Express route connection and you wish to know about how routing will work with Azure Express Route.Azure ExpressRoute uses Border Gateway Protocol (BGP) to exchange routing information between a network on-premises, Azure instances, and Microsoft public addresses. Routing exchange is over eBGP protocol. EBGP sessions are established between the MSEEs and your routers.

You must use BGP to advertise on-premises routes to the Microsoft Edge router. You can't create user-defined routes to force traffic to the ExpressRoute virtual network gateway. You can use user-defined routes for forcing traffic from the Express Route to, for example, a Network Virtual Appliance.

Based on your statement above:

Using Expressroute will I need to create new route tables, edit the exiting but change from appliance to gateway/and or Vnet or not use them at all considering the BGP advertisements?

You can't specify a virtual network gateway created as type ExpressRoute in a user-defined route because with ExpressRoute, you must use BGP for custom routes. If you can follow this documentation to advertise default routes in Azure ExpressRoute.

References:

https://learn.microsoft.com/en-us/azure/virtual-network/virtual-networks-udr-overview#user-defined

Please let me know if you have any additional questions. Thank you!

Please "Accept the answer" if the information helped you. This will help us and others in the community as well.
Please sign in to rate this answer.
azure-000 1 Reputation point

2024-06-10T19:25:55.6366667+00:00

Understood.

So If I set the existing route tables to next hop = none they will be ignored correct? I would like to leave them there just in case we need to switch back to Meraki for site to site but want them ignored by BGP-Express Route Gateway.

Will setting them to none be adequate for this or do I need to delete them completely?

On a side note I have another question

We had attempted to configure Expressroute this past weekend and we got to a point where not all VNETS were being advertised back to on prem. The only VNET being advertised back from MS was the VNET that the ER Gateway is on.

All VNETS are currently peered but there is a setting that was not set that I believe caused the other VNETS to not advertise back from Microsoft to on prem.

The following (see below) was not checked on any of the spoke VNETS.

It seems that if I enable this the spoke VNETS will advertise their routes back to us on prem through the Gateway.

Can you verify that this is the reason that they were not advertising back to us?

Again the only VNET advertised our side was the VNET that the GatewaySubnet is on.

Thanks in advance

ChaitanyaNaykodi-MSFT 26,216 Reputation points Microsoft Employee

2024-06-10T23:26:27.1833333+00:00

@azure-000

Thank you for getting back.

Will setting them to none be adequate for this or do I need to delete them completely?

Azure selects a route based on the destination IP address, using the longest prefix match algorithm.

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

This is currently documented here. Setting the next hop to none will be adequate when it is less specific then the BGP route. If both the routes contain the same address, then UDR will take preference and traffic will be sent to none.

Can you verify that this is the reason that they were not advertising back to us?

Yes, your understanding is correct this option needs to enabled and will most likely fix the issue.More details regarding this option can be found here

I understand you have asked a new question for this issue here

https://learn.microsoft.com/en-us/answers/questions/1693731/expressroute-bgp-advertising-issue

I will make this comment there as well and lets continue our discussion there if this does not fix the issue.

Please let me know if you have any additional questions. Thank you!

Please "Accept the answer" if the information helped you. This will help us and others in the community as well.

azure-000 1 Reputation point

2024-06-10T23:34:46.48+00:00

Thanks for the information. So if I have a route table that I set to none for a given network of say 20.20.0.0/16 and I have the same route configured on my switch to advertise via BGP I should basically delete the user defined route as it sounds like you are saying the route would default to NONE due to the UDR order preference? Am I understanding correctly?

azure-000 1 Reputation point

2024-06-10T23:54:18.0133333+00:00

Aslo can I just delete the routes and leave the subnets for the Route Table or do I need to delete the entire route table. I just want to make sure it doesnt interfere with the BGP routes but want to delete only what is necessary in case I need to put it back.

ChaitanyaNaykodi-MSFT 26,216 Reputation points Microsoft Employee

2024-06-11T15:38:31.1766667+00:00

@azure-000

Thank you for getting back

route would default to NONE due to the UDR order preference? Am I understanding correctly?

Yes, UDR is preferred over BGP routes for the same prefix

Aslo can I just delete the routes and leave the subnets for the Route Table or do I need to delete the entire route table. I just want to make sure it doesnt interfere with the BGP routes but want to delete only what is necessary in case I need to put it back.

Yes, just remove the routes. Do not delete the route table.

If it helps you can back-up the routes as mentioned here as a JSON template.

Hope this helps! Thank you!
Sign in to comment

Use comments to ask for clarification, additional information, or improvements to the question.

Share via

Transitioning to Expressroute from Meraki MX SiteToSite Routing question

1 answer

Your answer