SDWAN with cellular interface up when primary WAN down

[m.yost]

Going to be starting an IOS XE SDWAN deployment where the primary WAN connection is cablemodem and secondary is cell for remote sites.  With old school routing, we would use EEM to detect when the primary connection was down and admin up the cellular interface.  This way there were no additional cell carrier charges.

Is the same possible with SDWAN routers?  If so, is EEM still used or is it something else?  

[Dan Frey · Accepted solution]

There is a command for SDWAN called last-resort-circuit and will enable the interface once all other VPN0 transports are down.  Once a VPN0 transport comes back up the last-resort-circuit reverts back to down.

 interface GigabitEthernet6
  tunnel-interface
   encapsulation ipsec
   group                         100
   color lte
   last-resort-circuit

[m.yost]

Thanks for this.  Just did a little bit of reading on this and is sounds like the cell interface is still active, but there is just minimum traffic on it (BFDs hello's which can be lowered and PMTU discovers which can be disabled).  The customer is very cost conscious and I'm not sure they would even want that knowing them.

Is there a way to have it completely offline so there is no data transferred at all but have it come online when the primary is down?

[svemulap@cisco.com]

It is offline with last-resort-circuit option.
Interface will be in up/down status and will ONLY come up/up, when the last BFD session goes down on the primary

CCO Link with additional details:
https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/system-interface/vedge-20-x/systems-interfaces-book/configure-interfaces.html?dtid=osscdc000283

[m.yost]

Thanks, this is the confirmation I was looking for.  The other document I was reading seemed to indicate the interface still exchanged BFDs and PMTUD but the document you referenced says the radio will be down.

[svemulap@cisco.com]

n p. There won't be any BFDs and / or PMTUs exchanged when the interface is configured with "last-resort-circuit"
The whole idea that this interface acts like a 'true' back up.
It is similar to 'dialer'' back up from ISDN days :--)

HTH