Step 3: Examine the EBGP over IBGP Selection

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 CLI operational mode command:

user@host> show route destination-prefix < detail >

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

What It Means

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.