MULTI_EXIT_DESC (MED)
BGP AS systems can have one or more routers that connect them to other ASes. For ASes with more than one connecting router, the Multi-Exit Discriminator (MED) lists which router is best to use when leaving the AS. The MED is based on attributes such as delay. It is a recommendation only, as some networks may have different priorities.
BGP updates advertise the best path to a destination network. When the FortiGate unit receives a BGP update, the FortiGate unit examines the Multi-Exit Discriminator (MED) attribute of potential routes to determine the best path to a destination network before recording the path in the local FortiGate unit routing table.
FortiGate units have the option to treat any routes without an MED attribute as the worst possible routing choice. This can be useful because a lack of MED information is a lack of routing information which can be suspicious — possibly a hacking attempt or an attack on the network. At best it signifies an unreliable route to select.
The BGP commands related to MED include:
config router bgp
set always-compare-med {enable | disable}
set bestpath-med-confed {enable | disable}
set bestpath-med-missing-as-worst {enable | disable}
set deterministic-med {enable | disable}
config neighbor
set attribute-unchanged [as-path] [med] [next-hop]
end
end