Default Route Preference

Load isis.inter.area.cfg

#IOS-XE
config replace flash:isis.inter.area.cfg

#IOS-XR
configure
load bootflash:isis.inter.area.cfg
commit replace
y

Configure area 49.1234 so that R2 prefers R3 for the default route, and only uses R4 upon loss of R3. Do not use metrics to achieve this.

Answer

#R3
route-map SET_L1
 set level level-1
!
router isis
 default-information originate route-map SET_L1

Explanation

ISIS allows us to originate a default route just like OSPF. An L1 router will prefer an explicit 0/0 route that is advertised in the area over using the ATT bit.

By default, when a router uses default-information originate, the default is only injected into L2. To set L1 only, or L1/L2, we must use a route-map.

#R3
route-map SET_L1
 set level level-1
!
router isis
 default-information originate route-map SET_L1

R2 now sees an explicit 0/0 route advertised from R3:

Due to this, R2 only uses R3 as its default route:

We can see the ISIS RIB still has both 0/0 routes generated from the ATT bit set on R3 and R4, but the explicit 0/0 is preferred and installed: