As Path; Origin - Juniper J2300 User Manual

NOTE: In contrast to almost every other metric associated with dynamic routing
protocols, the local preference gives higher precedence to the larger value.

AS Path

Routes advertised by BGP maintain a list of the ASs through which the route travels.
This information is stored in the route advertisement as the AS path, and it is one of
the primary criteria that a local router uses to evaluate BGP routes for inclusion in
its forwarding table. Figure 69 shows how BGP creates an AS path.
Figure 69: BGP AS Path
A
In the network shown in Figure 69, the route from host A to host B travels through
two intermediate ASs. As the route advertisement is propagated through the BGP
network, it accumulates an AS path number each time it exits one AS and enters
another. Each AS number is prepended to the AS path, which is stored as part of
the route advertisement. When the route advertisement first leaves host B's AS,
the AS path is
AS number
advertisement exits the third AS, the AS path becomes
shortest AS path is preferred for inclusion into the BGP forwarding table.

Origin

The BGP router that first advertises a route assigns it of the following values to
identify its origin. During route selection, the lowest origin value is preferred.
AS 4
AS path = 4 7 17 AS path = 7 17
. When the route is advertised between intermediate ASs, the
17
is prepended to the AS path, which becomes
7
—The router originally learned the route through an IGP (OSPF, IS-IS, or
0
a static route).
—The router originally learned the route through an EGP (most likely BGP).
1
—The route's origin is unknown.
2
AS 7
AS path = 17
. When the route
7 17
. The route with the
4 7 17
BGP Overview
Routing Overview
AS 17
B
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