Autoroute Announce Backup Path

Load mpls.te.base.config.with.isis.cfg

#IOS-XE
config replace flash:mpls.te.base.config.with.isis.cfg

#IOS-XR
configure
load bootflash:mpls.te.base.config.with.isis.cfg
commit replace

Configure a TE tunnel from R8 to R10 going through R3. Use autoroute so that this TE tunnel is only used in the event that the R9-R10 and R6-R10 links go down.

Answer

#R8
ip explicit-path name VIA_R3
 next-address loose 3.3.3.3
!
int tun10
 tunnel dest 10.10.10.10
 ip unn lo0
 tunnel mode mpls traffic-eng
 tunnel mpls traffic-eng path-option 1 explicit name VIA_R3
 tunnel mpls traffic-eng autoroute metric 21
 tunnel mpls traffic-eng autoroute announce

Explanation

By default, a TE tunnel uses the IGP cost of the best IGP path to the destination. In our case, if we don’t change the autoroute metric on the TE tunnel, the cost will be 20 even though the TE path itself goes R8-R1-R3-R10.

For example, remove the autoroute metric and notice that the TE tunnel replaces the IGP paths to R10:

We’ll now add the metric back in, and the TE tunnel should no longer be used:

What we have now is essentially a backup TE tunnel path. If we lose the two shortest IGP paths to R10, we force traffic to go via R3 instead of via XR12. To simulate failure, we’ll shut down the R9-R10 and R6-R10 links:

#R9
int GigabitEthernet2.590
 shut

#R6
int GigabitEthernet2.560
 shut

R8 now uses the TE path to R10. Due to the metric of 21, it is better than other remaining paths which have a metric of 30.