BGP Multipath load sharing iBGP and eBGP

Hello James

You are deploying various network features and capabilities in the same topology (HA DMVPN, BGP multipath, NHRP), and this can complicate the required configuration as well as the troubleshooting process. So I don’t have a clear cut solution for you, but I can share with you some principles you can use in your troubleshooting.

First of all, the mi status code in the BGP table indicates a multipath, which means BGP has chosen multiple best paths. In your case, the second router isn’t setting the second route to mi because it’s not considering it as a best path. This is the aspect you must focus your troubleshooting on.

Why could this be? It could be due to several reasons including:

  • Differing BGP attributes. Multipath will only work if the weight, local preference, AS path, origin type, MED, and IGP metric are the same.
  • The configuration of maximum-paths. If maximum-paths is not configured or is set to a value less than the number of paths available, then only one path will be chosen as the best path.
  • Consider how NHRP is interacting with BGP multipath:
    • If NHRP shortcuts (spoke-to-spoke tunnels) are not correctly established, traffic might be forced to take a less optimal path via the hub(s), impacting the effectiveness of BGP multipath.
    • Next Hop Resolution - BGP relies on correct next-hop information for routing decisions. If NHRP is not correctly resolving these next hops, especially in a setup with multiple hubs, BGP might not make the optimal path selections.
    • NHRP and BGP Synchronization - In a dynamic environment like DMVPN, the synchronization between NHRP and BGP is vital. Delays or mismatches in updates between these two can lead to routing inconsistencies.

To resolve this issue, you can start by checking the BGP attributes of the routes on the second router. If they are the same, then check the router’s configuration for maximum-paths. If it’s not configured or is set to 1, then you should configure it to a value equal to or greater than the number of paths available.

If the problem still persists, the next thing you should check is how NHRP interacts with BGP and the multipath feature by examining NHRP mappings and BGP next hop addresses to see how NHRP is leading BGP to route traffic.

Hopefully, this information will help you get started in your troubleshooting. Let us know how you get along!

I hope this has been helpful!