Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

Page: 153 / 742

Table 20: Origin and AS Path for Routes Viewed on Different Routers
(continued)
AS Path Origin Router Route
empty IGP Albany 172.19.0.0/16
empty IGP Boston 172.19.0.0/16
empty IGP NY 172.19.0.0/16
100 IGP LA 172.19.0.0/16
empty IGP Albany 192.168.330/24
empty IGP Boston 192.168.330/24
empty IGP NY 192.168.330/24
100 IGP LA 192.168.330/24
As a matter of routing policy, you can specify an origin for a route with a set origin clause
in a redistribution route map. Changing the origin enables you to influence which of several
routes for the same destination prefix is selected as the best route. In practice, changing
the origin is rarely done.
Understanding the AS-Path Attribute
The AS-path attribute is a list of the ASs through which a route has passed. Whenever
a route enters an AS, BGP prepends the AS number to the AS-path attribute. This feature
enables network operators to track routes, but it also enables the detection and prevention
of routing loops.
Consider the following sequence of events for the routers shown in Figure 34 on page 118:
1.
Route 172.21.10.0/23 is injected into BGP by means of router London in AS 47.
2.
Suppose router London advertises that route to router Paris in AS 621. As received
by router Paris, the AS-path attribute for route 172.21.10.0/23 is 47.
3.
Router Paris advertises the route to router Berlin in AS 11. As received by router Berlin,
the AS-path attribute for route 172.21.10.0/23 is 621 47.
4.
Router Berlin advertises the route to router London in AS 47. As received by router
London, the AS-path attribute for route 172.21.10.0/23 is 11 621 47.
117 Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring BGP Routing