Understanding The As-Path Attribute - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

JUNOSe 11.1.x BGP and MPLS Configuration Guide
Table 20: Origin and AS Path for Routes Viewed on Different Routers (continued)
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 121:
1.
2.
3.
4.
120
Selecting the Best Path
Route Router
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
Route 172.21.10.0/23 is injected into BGP by means of router London in AS 47.
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.
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.
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.
Origin AS Path
IGP empty
IGP 100
IGP empty
IGP empty
IGP empty
IGP 100