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 PathOriginRouterRoute
300IGPAlbany192.168.204.0/22
300IGPBoston192.168.204.0/22
300IGPNY192.168.204.0/22
emptyIGPLA192.168.204.0/22
300IncompleteAlbany172.21.10.0/23
300IncompleteBoston172.21.10.0/23
300IncompleteNY172.21.10.0/23
emptyIncompleteLA172.21.10.0/23
emptyIGPAlbany172.28.8.0/21
emptyIGPBoston172.28.8.0/21
emptyIGPNY172.28.8.0/21
100IGPLA172.28.8.0/21
emptyIncompleteAlbany172.31.125.100
emptyIncompleteBoston172.31.125.100
emptyIncompleteNY172.31.125.100
100IncompleteLA172.31.125.100
emptyIGPAlbany172.19.0.0/16
emptyIGPBoston172.19.0.0/16
Selecting the Best Path â– 119
Chapter 1: Configuring BGP Routing
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