Lab3 Backup Path

Load unix:lab3.init.cfg

configure replace unix: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.

Answer

https://www.youtube.com/watch?v=WpG81qm2PHE&list=PL3Y9eZjZCcsejbVWD3wJIePqe3NiImqxB&index=4

#R5
router bgp 65000
 add ipv4
  bgp advertise-best-external
 add ipv6
  bgp advertise-best-external

#R2, R3, R4
router bgp 65000
 add ipv4
  bgp additional-paths select backup
  bgp additional-paths install
 add ipv6
  bgp additional-paths select backup
  bgp additional-paths install

Explanation/Verification

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 Add Path, 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 Add Path.

#R5
router bgp 65000
 add ipv4
  bgp advertise-best-external
 add ipv6
  bgp advertise-best-external

We now see both paths on R3.

We need to tell R3 to install additional paths as repair paths in the FIB. We use the command bgp additional-paths select backup to mark the backup path as an additional path, and then use bgp additional-paths install to install the additional paths as repair paths. (Note: upon going through this lab again, selecting the backup path does not appear necessary. This only seems needed if the router is going to advertise these as additional paths to another peer).

#R3
router bgp 65000
 add ipv4
  bgp additional-paths select backup
  bgp additional-paths install
 add ipv6
  bgp additional-paths select backup
  bgp additional-paths install

We now see that the backup path is marked in the BGP RIB:

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 in this lab we only use R4 alone, and only failover to using R5’s path if the path through R4 is lost.