# Lab3 Backup Path Load **lab3.init.cfg** ``` #R1, R6 configure replace unix:lab3.init.cfg #R2-R5 configure load bootflash:lab3.init.cfg commit replace y ```

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 ``` #R5 router bgp 65000 add ipv4 unicast advertise best-external add ipv6 unicast advertise best-external #R2, R3, R4 route-policy INSTALL_BACKUP set path-selection backup 1 install end-policy ! router bgp 65000 address-family ipv4 unicast additional-paths selection route-policy INSTALL_BACKUP ! address-family ipv6 unicast additional-paths selection route-policy INSTALL_BACKUP ``` ## 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 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. ``` #R5 router bgp 65000 add ipv4 unicast advertise best-external add ipv6 unicast 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. 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. ``` RP/0/0/CPU0:R3(config-bgp-af)#additional-paths ? receive Additional paths Receive capability selection Additional paths selection send Additional paths Send capability ``` We must create an RPL that matches which routes to select and the action to take (”install” or “advertise” or both “install advertise”). ``` RP/0/0/CPU0:R3(config-rpl)#set path-selection ? all BGP all paths backup BGP backup path best-path BGP best path group-best BGP group best path multipath BGP multipath RP/0/0/CPU0:R3(config-rpl)#set path-selection backup 1 ? advertise Advertise the path install Install the path ``` If desired we can both advertise *and* install in one command: ``` RP/0/0/CPU0:R3(config-rpl)#set path-selection backup 1 install ? advertise Advertise the path multipath-protect Mutlipath Protect ``` However, in this lab, we are not advertising additional paths. We are simply installing a backup route. ``` #R2, R3, R4 route-policy INSTALL_BACKUP set path-selection backup 1 install end-policy ! router bgp 65000 address-family ipv4 unicast additional-paths selection route-policy INSTALL_BACKUP ! address-family ipv6 unicast additional-paths selection route-policy INSTALL_BACKUP ``` 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://ccie-sp.gitbook.io/ccie-spv5.1-labs/labs/bgp-multi-homing-xr/lab3-backup-path.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.