can you help me? Why vios4 dont change next-hop(ebgp) when it send to vios5? On vios5 i see network with next-hop

enter image description here

  • You will need to tell us a lot more about your network, start with the router details (make, model) and their (sanitized) configs. Check the help center for how to ask a good question. – Zac67 Dec 23 '20 at 12:30
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    His question is good. The diagram contains the information needed to understand it. If you don't have a lot of experience with BGP it could be tough to parse, though. – Jeff Wheeler Dec 23 '20 at 13:23

You would need to configure a routing policy to rewrite that next-hop if that's the behavior you want. BGP does not automatically change them when propagating routes in this scenario because of the common LAN.

Since there is a common LAN among the three routers, it actually removes a hop to keep the original next-hop.

Related trivia: on most Internet Exchange / Peering Points, there is a policy requiring members to rewrite next-hop, so they do not sell next-hop to others on the exchange.

The specification is RFC4271 §5.1.3 NEXT_HOP Path Attribute rule 2.2, which reads as follows:

Otherwise, if the route being announced was learned from an external peer, the speaker can use an IP address of any adjacent router (known from the received NEXT_HOP attribute) that the speaker itself uses for local route calculation in the NEXT_HOP attribute, provided that peer X shares a common subnet with this address. This is a second form of "third party" NEXT_HOP attribute.

  • ok, thank you, can you get me RFC on this question? – quber Dec 23 '20 at 14:23
  • Sure, I added a reference to the spec to my answer. – Jeff Wheeler Dec 23 '20 at 15:27

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