Lab8 MPLS + “Shadow RR”
Load lab8.init.cfg
#R1, R6
configure replace unix:lab8.init.cfg
#R2-R5, R7, R8
configure
load bootflash:lab8.init.cfg
commit replace
y
R8 has been added as an additional RR, and all iBGP sessions have been preconfigured. R5 is setting LP to 50 on the routes received from R6.
Configure R8 as a “shadow RR” to reflect the backup path to R2. On R2, install this as a backup path.
Note that this will not work for 6PE, but you can configure it anyways.
Answer
https://www.youtube.com/watch?v=GlBXdYHvWmg&list=PL3Y9eZjZCcsejbVWD3wJIePqe3NiImqxB&index=11
Explanation/Verification
R5 is hiding its external path again, because the LP is lower than the path via R4. To get around this we can use “advertise best-external” or add path. We use add path here, because we use “advertise best-external” in a previous lab.
R8 will now receive the path via R5. However, there is a bug with 6PE as we saw with IOS-XE. R5 does not seem to be advertising the additional 6PE path even though it is marking it as a backup path locally. This seems to be due to the way BGP will allocate a local MPLS label. It doesn’t appear to be properlly allocating a label for backup paths on either IOS-XE or IOS-XR.
R8 now needs to mark the backup path and advertise this as the only path to R2. On IOS-XR, we don’t have the option of “advertise diverse-path backup” as we do on IOS-XE. Our closest option is to use add path and restrict outbound routes to only the backup path.
On R2, we will now have both paths. We simply need to identify the backup path and install it as a repair route.
Remember that R2, as the ingress PE, controls which egress PE is used. When it loses the path via R4, it will immediately switchover to R5. R3, as a P router in the core, simply switches on the top transport label and has no idea about the BGP routes.
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