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Understanding the Origin Attribute

BGP uses the origin attribute to describe how a route was learned at the origin—the point where the route was injected into BGP. The origin of the route can be one of three values:

IGP—Indicates that the route was learned by means of an IGP and, therefore, is internal to the originating AS. All routes advertised by the network command have an origin of IGP.
EGP—Indicates that the route was learned by means of an EGP.
Incomplete—Indicates that the origin of the route is unknown—that is, learned from something other than IGP or EGP. All routes advertised by the redistribute command—such as static routes—have an origin of Incomplete. An origin of Incomplete occurs when a route is redistributed into BGP.
Figure 33: The Origin Attribute

Consider the sample topology shown in Figure 33. Because routers Albany and Boston are not directly connected, they learn the path to each other by means of an IGP (not illustrated).

The following commands configure router Boston:



host1(config)#ip route 172.31.125.100 255.255.255.252 

host1(config)#router bgp 100 

host1(config-router)#neighbor 10.2.25.1 remote-as
100 

host1(config-router)#neighbor 10.4.4.1 remote-as
100 

host1(config-router)#neighbor 10.3.3.1 remote-as
300 

host1(config-router)#network 172.19.0.0 

host1(config-router)#redistribute static

The following commands configure router NY:



host2(config)#router bgp 100 

host2(config-router)#neighbor 10.4.4.1 remote-as
100 

host2(config-router)#neighbor 10.2.25.2 remote-as
100 

host2(config-router)#network 172.28.8.0 mask
255.255.248.0

The following commands configure router Albany:



host3(config)#router bgp 100 

host3(config-router)#neighbor 10.4.4.2 remote-as
100 

host3(config-router)#neighbor 10.2.25.2 remote-as
100 

host3(config-router)#network 192.168.33.0
mask 255.255.255.0

The following commands configure router LA:



host4(config)#router bgp 300 

host4(config-router)#neighbor 10.3.3.2 remote-as
100 

host4(config-router)#network 192.168.204.0
mask 255.255.252.0 

host4(config-router)#redistribute isis

Consider how route 172.21.10.0/23 is passed along to the routers in Figure 33:

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.
Router Boston learns about route 172.21.10.0/23 by means of EBGP from router LA.
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 21 shows this for all the routes known to routers NY and LA.

Table 21: Origin and AS Path for Routes Viewed on Different Routers

Route	Router	Origin	AS Path
192.168.204.0/22	Albany	IGP	300
192.168.204.0/22	Boston	IGP	300
192.168.204.0/22	NY	IGP	300
192.168.204.0/22	LA	IGP	empty
172.21.10.0/23	Albany	Incomplete	300
172.21.10.0/23	Boston	Incomplete	300
172.21.10.0/23	NY	Incomplete	300
172.21.10.0/23	LA	Incomplete	empty
172.28.8.0/21	Albany	IGP	empty
172.28.8.0/21	Boston	IGP	empty
172.28.8.0/21	NY	IGP	empty
172.28.8.0/21	LA	IGP	100
172.31.125.100	Albany	Incomplete	empty
172.31.125.100	Boston	Incomplete	empty
172.31.125.100	NY	Incomplete	empty
172.31.125.100	LA	Incomplete	100
172.19.0.0/16	Albany	IGP	empty
172.19.0.0/16	Boston	IGP	empty
172.19.0.0/16	NY	IGP	empty
172.19.0.0/16	LA	IGP	100
192.168.330/24	Albany	IGP	empty
192.168.330/24	Boston	IGP	empty
192.168.330/24	NY	IGP	empty
192.168.330/24	LA	IGP	100

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.

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