Some nice points in this Introduction to BGP:
- [BGP] routing information is usually exchanged between competing business entities — Internet Service Providers (ISPs) — in an open, hostile environment (public Internet). BGP is thus very security-focused (for example, all adjacent routers have to be configured manually),
- All other routing protocols are concerned solely with finding the optimal path toward all known destinations. BGP cannot take this simplistic approach because the peering agreements between ISPs almost always result in complex routing policies.
- Local preference — the “internal cost” of a destination, used to ensure AS-wide consistency.
- Multi-exit discriminator — this attribute gives adjacent ISPs the ability to prefer one peering point over another.
- Communities — a set of generic tags that can be used to signal various administrative policies between BGP routers. – I guess this is not used just for this purpose anymore.