Understanding The As-Path Attribute - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

Understanding the AS-Path Attribute

Copyright © 2010, Juniper Networks, Inc.
Table 20: Origin and AS Path for Routes Viewed on Different Routers
(continued)
Route Router
172.19.0.0/16 Albany
172.19.0.0/16 Boston
172.19.0.0/16 NY
172.19.0.0/16 LA
192.168.330/24 Albany
192.168.330/24 Boston
192.168.330/24 NY
192.168.330/24 LA
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.
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:
Route 172.21.10.0/23 is injected into BGP by means of router London in AS 47.
1.
Suppose router London advertises that route to router Paris in AS 621. As received
2.
by router Paris, the AS-path attribute for route 172.21.10.0/23 is 47.
Router Paris advertises the route to router Berlin in AS 11. As received by router Berlin,
3.
the AS-path attribute for route 172.21.10.0/23 is 621 47.
Router Berlin advertises the route to router London in AS 47. As received by router
4.
London, the AS-path attribute for route 172.21.10.0/23 is 11 621 47.
Chapter 1: Configuring BGP Routing
Origin AS Path
IGP empty
IGP empty
IGP empty
IGP 100
IGP empty
IGP empty
IGP empty
IGP 100
117