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5.1.3. NEXT_HOP

5.1.3. 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 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 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.

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