BGP Configuration Guide - Juniper Networks

BGP Configuration Guide
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BGP Messages Overview
6
The following information describes the path:
• AS path, which is a list of numbers of the ASs that a route passes through to reach the
local router. The first number in the path is that of the last AS in the path—the AS
closest to the local router. The last number in the path is the AS farthest from the local
router, which is generally the origin of the path.
• Path attributes, which contain additional information about the AS path that is used
in routing policy.
BGP peers advertise routes to each other in update messages.
BGP stores its routes in the Junos OS routing table (inet.0). The routing table stores the
following information about BGP routes:
• Routing information learned from update messages received from peers
• Local routing information that BGP applies to routes because of local policies
• Information that BGP advertises to BGP peers in update messages
For each prefix in the routing table, the routing protocol process selects a single best
path, called the active path. Unless you configure BGP to advertise multiple paths to the
same destination, BGP advertises only the active path.
The BGP router that first advertises a route assigns it one of the following values to
identify its origin. During route selection, the lowest origin value is preferred.
• 0—The router originally learned the route through an IGP (OSPF, IS-IS, or a static route).
• 1—The router originally learned the route through an EGP (most likely BGP).
• 2—The route's origin is unknown.
•
Understanding BGP Path Selection on page 8
• Example: Advertising Multiple Paths in BGP on page 254
All BGP messages have the same fixed-size header, which contains a marker field that
is used for both synchronization and authentication, a length field that indicates the
length of the packet, and a type field that indicates the message type (for example, open,
update, notification, keepalive, and so on).
This section discusses the following topics:
• Open Messages on page 7
• Update Messages on page 7
• Keepalive Messages on page 8
• Notification Messages on page 8
Copyright © 2012, Juniper Networks, Inc.