Redistribute Default OMP Route to BGP on the Service Side

[JUANNN]

Hello,

I am labbing SD-WAN with BGP routing on the service side, and I encountered an interesting issue: when I redistribute OMP routes into BGP on the Service Profile, in the BGP routing feature parcel, the ONLY route that it does not get redistributed and therefore learned by peers downstream the service side is the OMP default route, learned by the WAN Edge via OMP.

Here is the configuration of the WAN Edge router that does the redistribution. Note that peer 10.103.52.13 is the downstream neighbor router, which I expect to learn a default route via BGP from the WAN Edge.

and here is what the VRF 1 (Service Side VPN) routing table looks like:

On the downstream neighbor router however, I see all the routes except the default route...

Anyone knows why is this behaviour? I know i can use "Originate" to tell the WAN Edge router to artificially advertise a default route. However, I want the downstream router to receive the "original" one with the complete AS-PATH.

[Royalty · Accepted solution]

Hi @JUANNN,

There is a default behaviour (a hidden rule) in BGP that disallows redistribution into BGP of any default route. You cannot redistribute a default route into BGP from another routing information source unless you override this default behaviour. The only way to inject a default route into BGP with the native behaviour is to use the 'network' command or to use the 'neighbor x.x.x.x default-originate'.

To allow redistribution of a default route into BGP you can however add a special command called 'default-information originate' which can be applied under the BGP address-family (in the case of SDWAN, the address-family VRF of your service side VPN). The 'default-information originate' command is available and used within non-SDWAN (autonomous mode) IOS configs, but it is also the exact command that is configured when you enable the 'Originate' option within the Service Side BGP template (or feature parcel if you're using SDWAN Configuration Groups). Therefore, the 'Originate' option should also include any attributes carried over from the source routing protocol.

For example, ticking the 'Originate' option in the SD-WAN configuration should add the command 'default-information originate' (not to be confused with default-information-originate in OSPF) under your BGP service-side VPN address-family and that should allow the redistribution of the default route from OMP to BGP. This inherently would carry/create certain attributes over from OMP, such as the BGP extended community for SoO which is attached when a route is redistributed from OMP to BGP. All your other routes that have been redistributed from OMP to BGP with the 'redistribute omp' command should also have an SoO of which the value should be the Site ID of the SD-WAN WAN Edge performing the redistribution, It is added to prevent redistribution loops and prefix flapping. The default route will also have this.

If redistributing from OMP to BGP there should not be a current AS path in the output.

From what you've described, the solution is using the Originate option as that will work fine for your use case and will advertise the "original" default route from OMP to BGP. Note that this also means if the OMP default route disappears then so does the BGP default route; enabling the 'Originate' option is not locally originating a default route out of thin air, it is enabling the redistribution of the default route if it is already present in the RIB from another routing information source (e.g. static, OSPF, OMP, etc.)

Sorry if that is confusing but hope it helps, let me know if there's any further questions, more than happy to answer

[JUANNN]

Thanks, you are right! By enabling "Originate" in the BGP Routing feature parcel, on the Service Profile of the Configuration Group, the default route that is learned via OMP is then redistributed to BGP as well. And by enabling Propagate AS-PATH on both the remote router doing BGP-to-OMP redistribution and the one doing OMP-to-BGP redistribution, the AS-PATH is preserved.

On the router doing OMP-to-BGP redistribution, noow it shows...