BGP Additional Paths

(Rene Molenaar) #1

This topic is to discuss the following lesson:


(zain k) #2


This image below of the network topology has the same IP addresses for all links, which makes it a bit confusing if you didn’t read the configurations before hand.


(Rene Molenaar) #3

Hi Zain,

You are right, that doesn’t really help :smile: I just fixed this.



(Mohammad Hasanuz Zaman) #4

Hi Rene,
I have learnt lots of networking stuff in your plain way . God bless you . carry on dear .Thanks


1 Like

(Fabio P) #5

Hi Rene,

For eBGP additional paths is working by default, is this correct ?



(Rene Molenaar) #6

Hi Fabio,

I’m afraid not. eBGP only advertises a single best path to its neighbors.



(Thejohn N) #7

Dear Rene,
I am confused about this stuff:

  • neighbor neighbor-id additional-paths send/receive
  • bgp additional-paths select all
  • bgp additional-paths install
  • neighbor neighbor-id advertise additional-paths all

Could you please give me a short explanation?



(Rene Molenaar) #8

Hello Thejohn,

  • neighbor neighbor-id additional-paths send: We use this to configure the router so it sends multiple BGP paths to a neighbor.
  • neighbor neighbor-id additional-paths receive: If you have a neighbor that sends multiple paths, that’s nice but you still have to configure your local router that it wants to receive multiple paths.
  • bgp additional-paths select : you receive a bunch of paths from your neighbor but you can still configure your router which of these paths you actually want to use.
  • bgp additional-paths install: this tells the router to actually install a backup path that you selected with the “bgp additional-paths install” command.
  • neighbor neighbor-id advertise additional-paths: This configures your router which additional-paths you want to advertise to a neighbor. “all” means all additional-paths.

These options make more sense if you see them in action. Try to recreate the topology I did in my lesson. When you try each command, look at the BGP and CEF table to see what is actually advertised/received/installed on each device. That makes it much easier to understand all of this.

Hope this helps!



(Thejohn N) #9

Dear Rene,
If you want to send addtional-path for your neighbor:

  • First, you have to configure additional-paths send feature to send to your neighbor
  • Second, you chose which additional-paths which you want to advertise to the neighbor
    And your neighbor:
  • First, he has to configure additional-paths receive feature to receive from you
  • Second, he has to configure bgp additional-paths install to add which BGP path into his routing table
    I am right? Thanks

(Lazaros Agapides) #10

Hello Thejohn

Yes, that is absolutely right!



(Dominique R) #11

hi Rene and staff,
thanks for always replying to my posts

Just a simple question about what happens just behind configuration R1 to R6 before any consideration about additional paths
R4 and R5 learn Net6 ( via eBGP (next-hop=R6) = OK
R5 has another path to Net6 via R4, but R4 has not the symetric path to Net6 via R5
This is surprising because the topology is symetric

I think the reason is:
R2 as the reflector prefers R4 to go to Net6 (that is the best choice from its perspective)
So, to go to Net6, R2 (the reflector) only advertises the path via R4 to his clients R4 and R5: R5 gets this path (so R5 install a path to Net6 via R4 in its BGP table) and R4 gets also this path from R2, but in this path the next-hop is itself => so this is done
This is why, in R4, there is not a path to Net6 via R5
I just want to know if i am right ??

I will test this in a lab, but i suppose if i shut Gi0/1on R6 (or i shut Gi0/3 on R4) BGP will reconverge via R5 (so in this case, i will find in R4 the route to Net6 via R5)


(Lazaros Agapides) #12

Hello Dominique

That’s what we’re here for! :slight_smile: It’s always a pleasure.

Your explanation is correct. I suggest you try shutting down the ports you mentioned and allow BGP to reconverge. You should get the behaviour that you predicted.

I hope this has been helpful!