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Version: current

Nexthop Selection

RouterOS does not perform strict nexthop checks on BGP input; instead, it relies on filter rules created by the user.

Actions performed on input:

  • Read the nexthop from the BGP NEXT_HOP attribute.
  • For multi-protocol NLRI, check whether the nexthop has a valid length:
    • Send an Update error notification and exit if the length is invalid.
    • Store the nexthop value.
    • If a link-local nexthop appears in the update message:
      • Determine the connection interface.
      • Store the link-local address and interface.
  • Apply nexthop actions from input filter rules.
  • Send the route to the main calc process, where nexthop reachability is determined.

BGP Output​

  • If nexthop-choice is not force-self or the route AFI is not IPv4 or IPv6:
    • If the peer is a route reflector, or nexthop-choice is propagate, or the peer is not eBGP:
      • Check the nexthop AFI and try to set the outgoing link-local nexthop.
      • Check the nexthop AFI and try to set the outgoing nexthop.
    • If the outgoing nexthop is still unset and the BGP peer is not multihop:
      • Loop through available immediate nexthops and try to set the outgoing nexthop and the LL nexthop.
    • If the outgoing nexthop equals the remote router-id, unset the outgoing nexthop.
    • If the outgoing nexthop equals the remote peer's address, unset the outgoing nexthop.
    • If the interface of the peer's address is not equal to the LL nexthop's interface, unset the outgoing LL nexthop.
    • If the outgoing LL nexthop equals the remote peer's address, unset the outgoing LL nexthop.
  • If the outgoing nexthop is still not set, check the AFI and set the peer's local-address.
  • If the outgoing nexthop is still unset and the route AFI is IPv6 or L2VPN:
    • If the peer's local address is IPv4, set the outgoing nexthop to the IPv6-mapped peer's local address.