Table 20: Origin and AS Path for Routes Viewed on Different Routers
(continued)
AS Path
Origin
Router
Route
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 121:
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.
120
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Selecting the Best Path
JUNOSe 11.0.x BGP and MPLS Configuration Guide
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Страница 535: ...There is no no version See undebug ip mbgp Monitoring BGP MPLS VPNs 499 Chapter 5 Configuring BGP MPLS Applications...
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