IBGP versus EBGP
When you read about BGP, often you see EBGP or IBGP mentioned. These are both BGP routing, but BGP used in different roles. Exterior BGP (EBGP) involves packets crossing multiple autonomous systems (ASes) where interior BGP (IBGP) involves packets that stay within a single AS. For example the AS_PATH attribute is only useful for EBGP where routes pass through multiple ASes.
These two modes are important because some features of BGP are only used for one of EBGP or IBGP. For example confederations are used in EBGP, and route reflectors are only used in IBGP. Also routes learned from IBGP have priority over EBGP learned routes.
FortiGate units have some commands specific to EBGP. These include:
• automatically resetting the session information to external peers if the connection goes down — set fast-external-failover {enable | disable}
• setting an administrative distance for all routes learned from external peers (must also configure local and internal distances if this is set) — set distance-external <distance_integer>
• enforcing EBGP multihops and their TTL (number of hops) — set ebgp-enforce-multihop {enable | disable} and set ebgp-multihop-ttl <seconds_integer>