Juniper JUNOSE, Configuration Manual

Page: 155 / 738

Consider how route 172.21.10.0/23 is passed along to the routers in Figure 33 on
page 118:
1.
IS-IS injects route 172.21.10.0/23 from router Chicago into BGP on router LA.
BGP sets the origin attribute to Incomplete (because it is a redistributed route)
to indicate how BGP originally became aware of the route.
2.
Router Boston learns about route 172.21.10.0/23 by means of EBGP from router
LA.
3.
Router NY learns about route 172.21.10.0/23 by means of IBGP from router
Boston.
The value of the origin attribute for a given route remains the same, regardless of
where you examine it. Table 20 on page 119 shows this for all the routes known to
routers NY and LA.
Table 20: Origin and AS Path for Routes Viewed on Different Routers
AS Path Origin Router Route
300 IGP Albany 192.168.204.0/22
300 IGP Boston 192.168.204.0/22
300 IGP NY 192.168.204.0/22
empty IGP LA 192.168.204.0/22
300 Incomplete Albany 172.21.10.0/23
300 Incomplete Boston 172.21.10.0/23
300 Incomplete NY 172.21.10.0/23
empty Incomplete LA 172.21.10.0/23
empty IGP Albany 172.28.8.0/21
empty IGP Boston 172.28.8.0/21
empty IGP NY 172.28.8.0/21
100 IGP LA 172.28.8.0/21
empty Incomplete Albany 172.31.125.100
empty Incomplete Boston 172.31.125.100
empty Incomplete NY 172.31.125.100
100 Incomplete LA 172.31.125.100
empty IGP Albany 172.19.0.0/16
empty IGP Boston 172.19.0.0/16
Selecting the Best Path
■ 119
Chapter 1: Configuring BGP Routing