Lab3 Backup Path
Last updated
Last updated
Load lab3.init.cfg
R5 is setting LP to 50 on routes received from R6. Configure the core so that R4 is always used as the primary path, but if that path is lost, the core will immediately re-route to using R5.
https://www.youtube.com/watch?v=WpG81qm2PHE&list=PL3Y9eZjZCcsejbVWD3wJIePqe3NiImqxB&index=4
Before making any changes, notice that R5 is hiding its external path. This is because its iBGP path via R4 is better than its eBGP path due to local preference. On R3, we only see R4’s path to R6’s prefixes.
First we must configure R5 to advertise its external path. We can either use PIC, or simply use the advertise best-external feature. This allows R5 to advertise its eBGP path even when the iBGP path is best, and does not require PIC.
We now see both paths on R3.
We need to tell R3 to install additional paths as repair paths in the FIB. On IOS-XR, when you use add-path you select and install the paths at the same time with an RPL. There is no separate “install” command as there is with IOS-XE.
We must create an RPL that matches which routes to select and the action to take (”install” or “advertise” or both “install advertise”).
If desired we can both advertise and install in one command:
However, in this lab, we are not advertising additional paths. We are simply installing a backup route.
We now see that the backup path is marked when showing information about each prefix.
The backup path has been installed as a repair route in the FIB:
We must do this on all other iBGP routers (R2 and R4) to ensure that they all install R5’s path as a repair path. Remember that this is different from the previous labs in which we used ECMP or UCMP. In those cases, we were load sharing between R4 and R5. But now we only use R4 alone, and only failover to using R5’s path if the path through R4 is lost.
Note that R5 also automatically installs the repair routes due to the advertise best-external feature being enabled.
