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In one of the Discords that I’m in there was a user with a complex network consisting of a mix of DMVPN, BGP over MPLS VPN circuits, and SD-WAN. For some prefixes, the path via the private MPLS is preferred, for others, the SD-WAN path. Now, if a prefix is available in two different protocols, BGP vs Overlay Management Protocol (OMP), there is nothing we can do in BGP or OMP to modify which one gets installed into the Routing Information Base (RIB). This is no different than if EIGRP and OSPF were competing to install a prefix into the RIB, the protocol with the lower Administrative Distance (AD) would have its route installed.
The default AD values used on a Cisco device for these protocols are:
- eBGP – 20
- iBGP – 200
- OMP – 251
Based on the AD, OMP will always lose out. It is of course possible to change the AD of BGP, but that would have an effect of all prefixes and we lose the ability to have some prefixes preferred via BGP and others via OMP. I had never changed the AD of a specific BGP prefix before, so I turned to Twitter to see Continue reading