Step 2: Examine the Multiple Exit Discriminator Route Selection

Action

To examine a route to determine if the MED is 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 100.100.2.0 detail 

inet.0: 20 destinations, 24 routes (20 active, 0 holddown, 0 hidden)

100.100.2.0/24 (2 entries, 1 announced)

        *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: <Active Int Ext>

                Local AS: 65002 Peer AS: 65002

                Age: 2:32:01    Metric: 5       Metric2: 10 

                Task: BGP_65002.10.0.0.2+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: 10.0.0.2

         BGP    Preference: 170/-101

                Source: 10.1.45.2

                Next hop: 10.1.45.2 via so-0/0/2.0, selected

                State: <NotBest Ext>

                Inactive reason: Not Best in its group

                Local AS: 65002 Peer AS: 65001

                Age: 2w0d 1:37:58       Metric: 10 

                Task: BGP_65001.10.1.45.2+179

                AS path: 65001 I

                Localpref: 100

                Router ID: 10.0.0.5

What It Means

The sample output shows that R4 received two instances of the 100.100.2.0 route: one from 10.0.0.2 (R2), and one from 10.1.45.2 (R5). R4 selected the path from R2 as its active route, as indicated by the asterisk (*). The selection is based on the MED value contained in the Metric: field. The path with the lowest MED value is preferred. In the example, the path with the lowest MED value (5) is the path from R2. Note that the two paths are from the same neighboring network: AS 65001.

The reason that the inactive path is not selected is displayed in the Inactive reason: field, in this case, Not Best in its group. The wording is used because the JUNOS software uses the process of deterministic MED selection, by default.