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BGP Additional path and next hop self

AtulMehra54469
Level 1
Level 1

There is additional path command to advertise BGP additional path to neighbor 

neighbor advertise additional-paths

To advertise additional paths for a neighbor based on selection, use the neighbor advertise additional-paths command in address family configuration mode. To prevent the advertisement of additional paths for a neighbor based on selection, use the no form of the command.

 

For  example:

address-family ipv4 unicast
  bgp additional-paths send receive
  bgp additional-paths select all best 3 group-best
  neighbor 192.168.101.15 activate
  neighbor 192.168.101.15 route-map add_path1 out
  neighbor 192.168.101.15 advertise additional-paths best 2

 

Also there is next-hop-self command to configure a router as the next hop for a BGP-speaking neighbor or peer group.

 

What will happen if we use above two commands on same neighbor.

Will both (best and additional path) will be advertised as next-hop?

or only best path will be advertised?

And what should happen if next-hop-self was applied by route-map where additional path was configured on neighbor ?

 

 

3 Replies 3

next-hop self not same as additional path,
we know that there are Algorithm to select the best path, if there is multi path for same Prefix then by default it will select one but if there are more then these path also will send as update to other BGP neighbor, this make neighbor fast recover if one path is down.
next-hop self only make these multi path with same next-hop. 

Its correct that next-hop-self is different than additional-path but when we say next hop self then it mean that I telling next router to send all routes to me as I am next hop for all routes. If that is the case then there is no need to send multiple paths as all paths will results in next hop to me only.

Is that the behavior on cisco boxes ? 

Hello @AtulMehra54469 ,

I have the same impression next-hop self is likely to make additional paths not effective to the neighbor.

 

However, looking for the explanation of how BGP additional paths work I have found the following:

 

>>> The BGP Additional Paths feature is implemented by adding a path identifier to each path in the NLRI. The path identifier (ID) can be considered as something similar to a route distinguisher (RD) in VPNs, except that a path ID can apply to any address family. Path IDs are unique to a peering session and are generated for each network. The path identifier is used to prevent a route announcement from implicitly withdrawing the previous one. The Additional Paths feature allows the advertisement of more paths, in addition to the bestpath. The Additional Paths feature allows the advertisement of multiple paths for the same prefix, without the new paths implicitly replacing any previous paths. 

 

see

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/bgp-additional-paths.html?dtid=osscdc000283

 

If this is true the two features BGP additional paths and next-hop self may coexist because each path has its own Path-ID even if the BGP next-hop is the same.

 

Hope to help

Giuseppe

 

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