Examine the EBGP over IBGP Selection
Purpose
To examine a route to determine if EBGP is selected over IBGP as the selection criteria for the single, active path.
Action
To examine a route to determine if EBGP is selected over IBGP as the selection criteria for the single, active path, enter the following Junos OS CLI operational mode command:
Sample Output
user@R4> show route 100.100.3.0 detail inet.0: 20 destinations, 24 routes (20 active, 0 holddown, 0 hidden)
100.100.3.0/24 (2 entries, 1 announced)
*BGP Preference: 170/-101
Source: 10.1.45.2
Next hop: 10.1.45.2 via so-0/0/2.0, selected
State: <Active Ext>
Local AS: 65002 Peer AS: 65001
Age: 5d 0:31:25
Task: BGP_65001.10.1.45.2+179
Announcement bits (3): 0-KRT 3-BGP.0.0.0.0+179 4-Resolve inet.0
AS path: 65001 I
Localpref: 100
Router ID: 10.0.0.5
BGP Preference: 170/-101
Source: 10.0.0.2
Next hop: 10.1.24.1 via so-0/0/3.0, selected
Protocol next hop: 10.0.0.2 Indirect next hop: 8644000 277
State: <NotBest Int Ext>
Inactive reason: Interior > Exterior > Exterior via Interior
Local AS: 65002 Peer AS: 65002
Age: 2:48:18 Metric2: 10
Task: BGP_65002.10.0.0.2+179
AS path: 65001 I
Localpref: 100
Router ID: 10.0.0.2
Meaning
The sample output shows that R4 received two instances of the 100.100.3.0 route: one from 10.1.45.2 (R5) and one from 10.0.0.2 (R2). R4 selected the path from R5 as its active path, as indicated by the asterisk (*). The selection is based on a preference for routes learned from an EBGP peer over routes learned from an IBGP. R5 is an EBGP peer.
You can determine if a path is received from an EBGP or IBGP peer by examining the Local As and Peer As fields. For example, the route from R5 shows the local AS is 65002 and the peer AS is 65001, indicating that the route is received from an EBGP peer. The route from R2 shows that both the local and peer AS is 65002, indicating that it is received from an IBGP peer.
The reason that the inactive path is not selected is displayed in the Inactive reason field, in this case, Interior > Exterior > Exterior via Interior. The wording of this reason shows the order of preferences applied when the same route is received from two routers. The route received from a strictly internal source (IGP) is preferred first, the route received from an external source (EBGP) is preferred next, and any route which comes from an external source and is received internally (IBGP) is preferred last.
