BGP Additional Paths

Lessons Discussion
27

Hello Unni

I’ve gone in and labbed this one up again to get a clearer picture of what is going on. Now having all the additional paths configured as in the lab, but without the application of Local Pref, we find the following:

  • There is a single entry in the BGP table in R4 for a destination of 6.6.6.6 as seen below:

    R4#show ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
*>   6.6.6.6/32       192.168.46.6             0             0 6 i

  • R5 however has two paths, one via eBGP and one via iBGP. You can see that it is the one via eBGP (the directly connected one) that is considered best:

    R5#show ip bgp

Network          Next Hop            Metric LocPrf Weight Path
* i  6.6.6.6/32       4.4.4.4                  0    100      0 6 i
*>                    192.168.56.6             0             0 6 i

  • Now from this information, R2, the route reflector, has two paths to 6.6.6.6, the best one via R4 and the additional path via R5.

    R2#show ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
* ia 6.6.6.6/32       5.5.5.5                  0    100      0 6 i
*>i                   4.4.4.4                  0    100      0 6 i

These two are learned from the best path in R4 and the best path in R5. Now if you apply LP as you state in your post, then the result is that the best path in R5 will be via R6 directly, and the best path in R4 will be via R5 as shown:

R4#show ip bgp

 Network          Next Hop            Metric LocPrf Weight Path
*>i  6.6.6.6/32       5.5.5.5                  0    400      0 6 i
*                     192.168.46.6             0             0 6 i

and

R5#show ip bgp

 Network          Next Hop            Metric LocPrf Weight Path
*>   6.6.6.6/32       192.168.56.6             0             0 6 i

Remember that both R4 and R5 share only the best path with the route reflector. R4 advertises that the best path is via R5, and R5 advertises that it is the best path. So there is only one path that is being advertised to 6.6.6.6 by both routers. Therefore, R2 will only receive that one single path, and it will never learn of the path via R4–>R6, since that is never advertised:

R2#show ip bgp

 Network          Next Hop            Metric LocPrf Weight Path
*>i  6.6.6.6/32       5.5.5.5                  0    400      0 6 i

Notice in the BGP table that the local pref has a value of 400. This is the value that I configured to differentiate the path via R5 rather than R4. R5 is sending this value along with its iBGP updates to the route reflector. But remember, because R4 and R5 are not configured to share additional paths, it is only their best path (the one configured with the higher LP) that is being advertised. In this topology, only R2 is configured to share additional paths, if it itself is informed of these paths. In this case, it is not, so it has no additional path to include in its table.

I hope this has been helpful!

Laz

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