Routing

Hi everyone,

Is BP protocol next hop unique to Juniper? It is not listed as standard BGP attribute such as next hop.

https://datatracker.ietf.org/doc/html/rfc1771

Thanks !!

------------------------------
Be kind!!
------------------------------

Hi,

Check this: https://datatracker.ietf.org/doc/html/rfc4271

BR,

Andrei

For everyone so they do not need to dig into RFC:

NEXT_HOP

   The NEXT_HOP is a well-known mandatory attribute that defines the IP
   address of the router that SHOULD be used as the next hop to the
   destinations listed in the UPDATE message.  The NEXT_HOP attribute is
   calculated as follows:

      1) When sending a message to an internal peer, if the route is not
         locally originated, the BGP speaker SHOULD NOT modify the
         NEXT_HOP attribute unless it has been explicitly configured to
         announce its own IP address as the NEXT_HOP.  When announcing a



Rekhter, et al.             Standards Track                    [Page 26]

RFC 4271                         BGP-4                      January 2006


         locally-originated route to an internal peer, the BGP speaker
         SHOULD use the interface address of the router through which
         the announced network is reachable for the speaker as the
         NEXT_HOP.  If the route is directly connected to the speaker,
         or if the interface address of the router through which the
         announced network is reachable for the speaker is the internal
         peer's address, then the BGP speaker SHOULD use its own IP
         address for the NEXT_HOP attribute (the address of the
         interface that is used to reach the peer).

      2) When sending a message to an external peer, X, and the peer is
         one IP hop away from the speaker:

         - If the route being announced was learned from an internal
           peer or is locally originated, the BGP speaker can use an
           interface address of the internal peer router (or the
           internal router) through which the announced network is
           reachable for the speaker for the NEXT_HOP attribute,
           provided that peer X shares a common subnet with this
           address.  This is a form of "third party" NEXT_HOP attribute.

         - 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.

         - Otherwise, if the external peer to which the route is being
           advertised shares a common subnet with one of the interfaces
           of the announcing BGP speaker, the speaker MAY use the IP
           address associated with such an interface in the NEXT_HOP
           attribute.  This is known as a "first party" NEXT_HOP
           attribute.

         - By default (if none of the above conditions apply), the BGP
           speaker SHOULD use the IP address of the interface that the
           speaker uses to establish the BGP connection to peer X in the
           NEXT_HOP attribute.

      3) When sending a message to an external peer X, and the peer is
         multiple IP hops away from the speaker (aka "multihop EBGP"):

         - The speaker MAY be configured to propagate the NEXT_HOP
           attribute.  In this case, when advertising a route that the
           speaker learned from one of its peers, the NEXT_HOP attribute
           of the advertised route is exactly the same as the NEXT_HOP



Rekhter, et al.             Standards Track                    [Page 27]

RFC 4271                         BGP-4                      January 2006


           attribute of the learned route (the speaker does not modify
           the NEXT_HOP attribute).

         - By default, the BGP speaker SHOULD use the IP address of the
           interface that the speaker uses in the NEXT_HOP attribute to
           establish the BGP connection to peer X.

   Normally, the NEXT_HOP attribute is chosen such that the shortest
   available path will be taken.  A BGP speaker MUST be able to support
   the disabling advertisement of third party NEXT_HOP attributes in
   order to handle imperfectly bridged media.

   A route originated by a BGP speaker SHALL NOT be advertised to a peer
   using an address of that peer as NEXT_HOP.  A BGP speaker SHALL NOT
   install a route with itself as the next hop.

   The NEXT_HOP attribute is used by the BGP speaker to determine the
   actual outbound interface and immediate next-hop address that SHOULD
   be used to forward transit packets to the associated destinations.

   The immediate next-hop address is determined by performing a
   recursive route lookup operation for the IP address in the NEXT_HOP
   attribute, using the contents of the Routing Table, selecting one
   entry if multiple entries of equal cost exist.  The Routing Table
   entry that resolves the IP address in the NEXT_HOP attribute will
   always specify the outbound interface.  If the entry specifies an
   attached subnet, but does not specify a next-hop address, then the
   address in the NEXT_HOP attribute SHOULD be used as the immediate
   next-hop address.  If the entry also specifies the next-hop address,
   this address SHOULD be used as the immediate next-hop address for
   packet forwarding.

Hi,

The BGP Protocol next-hop is the well known  next-hop attribute transported in BGP packets. The next-hop shown in routing tables is the resolved next-hop. This is how the router is going to reach the protocol next-hop.

Regards,

Christophe

All true Christophe. Let me add to that: it's not a really selection attribute, it's more a condition that must be fulfilled before the real attributes are taken into account. this describes it real nice I think Understanding BGP Path Selection | Junos OS | Juniper Networks.