Messages and path attributes

The five message types

Message	Purpose
OPEN	First message after TCP comes up: version, ASN, hold time, router ID, capabilities.
KEEPALIVE	Confirms an OPEN and keeps the session alive when idle.
UPDATE	Advertises new paths (NLRI + path attributes) and/or withdraws old ones.
NOTIFICATION	Reports an error and closes the session.
ROUTE-REFRESH	Asks the peer to resend its routes so new inbound policy can be applied without a reset (RFC 2918).

Path attributes

An UPDATE carries a set of path attributes that describe the route. These are what policy matches on and what the best-path algorithm compares.

Attribute	Category	What it does
ORIGIN	well-known mandatory	How the route entered BGP: i IGP (network statement), e EGP (historic), ? incomplete (redistributed).
AS_PATH	well-known mandatory	The ordered list of ASes the route has traversed. Used for loop prevention and as a path-length metric.
NEXT_HOP	well-known mandatory	The IP address to forward traffic to for this prefix.
LOCAL_PREF	well-known discretionary	iBGP only. Higher wins. The primary tool for choosing your outbound exit.
MED	optional non-transitive	"Multi-Exit Discriminator". A hint to a neighbouring AS about which of your entry points to use. Lower wins.
COMMUNITY	optional transitive	32-bit tags (ASN:value) used to signal policy, e.g. "do not export" or "prepend toward peers". Large communities (RFC 8092) extend this for 32-bit ASNs.

AS_PATH is the loop-prevention mechanism. When a router receives an UPDATE whose AS_PATH already contains its own ASN, it discards it. Follow a prefix across the internet below.