Azure ExpressRoute weight vs. as-prepend? What causes as-prepend to be priority?

Ted Roy 20

We have 2 main Azure regions, CUS and EU2. We also have 2 physical DC with 10G ExpressRoutes that are connected by 10G dedicated circuits all routing via iBGP.

I have just fixed my internal iBGP routing to Azure by assigning local preferences derived from the BGP communities assigned in Azure for each region. So, my routing to Azure is correct buy region from each of my DCs. CUS from my Massachusetts DC and EU2 via my Ashburn DC.

I have also prepended another AS from each DC to Azure for the networks that are local to the DC as advertised to Azure. This way Azure should see the routes local to the DCs as a lower cost than the "backup" route and route accordingly.

My problem is that I have already assigned weights in Azure for each of these ExpressRoutes. As I understand: Weights are preferred over as-path. Does this also include when the weights are of equal cost or only if they are left are the default which I believe is 100. I am looking to set the ER weights equal and then have the as-prepend be the discriminator for which ER is used for which received routes.

So when does the prepend take over from ER weight as a routing determinator?

Sai Prasanna Sinde 1,260 Reputation points Microsoft Vendor

2024-11-20T08:56:52.1466667+00:00
Hi @Ted Roy

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

Yes, your understanding is correct. Weights are preferred over AS-path.

In general, BGP path selection criteria include:

Highest weight (Cisco-specific)

Highest local preference (The default local preference is typically 100. Higher local preferences are more preferred.)

Locally originated routes

Shortest AS path

If the weights for both ExpressRoute connections are set equally, the next BGP decision process attribute, Local Preference, will be considered.

If Local preference is identical or set to default, the next factor will be path locally originated.

Any path that has been originated on a local router will be preferred over any paths learned from a BGP Peer. If no locally originated prefix exists in the BGP table for the paths being compared, it is moved to AS path.

To use AS path prepending as a discriminator for route selection between your two ExpressRoute connections, please follow these steps:

Set equal weights for both connections.

Ensure local preferences are equal or unset.

Check for Locally originated routes.

Then you can utilize AS path prepending to influence route selection. The route with the shorter AS path will be chosen.

Reference: https://learn.microsoft.com/en-us/azure/expressroute/expressroute-optimize-routing

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

Regards,

Sai Prasanna.
Ted Roy 20 Reputation points

2024-11-20T12:44:59.2166667+00:00

Thank you, Sai, for the response. Azure's BGP implementation is different that the Cisco's I am used to. I went through the optimized routing link previously, but it does not speak to having both a weight and an as-prepend in play at the same time. The examples were for one or the other but not both. I will set the Azure side ER weights to the same value and see if that allows the as-prepends to take effect.
Ted Roy 20 Reputation points

2024-11-20T12:49:37.48+00:00

Thank you for your response. Azure has their own twist on BGP as they do for networking, and the provided link, did not address an example of both weights and as-prepend in together. That is the point of my question. I agree with your steps to test the desired routing, except how do you verify steps 2-3 in Azure? I will try setting the weights equal and see if the as-path prepend take effect.
Sai Prasanna Sinde 1,260 Reputation points Microsoft Vendor

2024-11-22T10:11:50.44+00:00
Hi @Ted Roy,

Greetings!

For each prefix, we will always look at the connection weight before the AS Path length when deciding how to send traffic.

To answer your question,

For Local preference,

BGP utilizes a best path selection algorithm based on many factors including longest prefix match (LPM).

To ensure that traffic destined for Azure through Microsoft traverses the ExpressRoute path, you must implement the Local Preference attribute.

This setting ensures that the path is always preferred on ExpressRoute.

The default local preference is typically 100. Higher local preferences are more preferred.

If you set this to default or equal, it would go for the next step in the criteria.

Please refer to this document for setting local preference as an example

Refer: https://learn.microsoft.com/en-us/azure/expressroute/expressroute-optimize-routing#path-selection-for-microsoft-peering

For Step-3,

Any path that has been originated on a local router will be preferred over any paths learned from a BGP Peer. If no locally originated prefix exists in the BGP table for the paths being compared, it is moved to AS path.

The VNets don't know which ExpressRoute circuit is local and which one is remote. Since Equal-Cost-Multi-Path (ECMP) routing is used to load-balance inter-VNet traffic, some traffic flows take the longer path and get routed at the remote ExpressRoute circuit.

Unfortunately, we didn't find any documents setting both weights and as-prepend in together.

Based on the understanding from the above linked document, we have suggested you this routing method.

Like you mentioned, please try to test the routing process by setting the weights equal and see if the as-path prepend take effect and let us know if it works.

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

Regards,

Sai Prasanna.

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