Step 4: Examine BGP Routes

Purpose

You can examine the BGP path selection process to determine the single, active path when BGP receives multiple routes to the same destination. In this step, we examine the reverse LSP R6-to-R1, making R6 the ingress router for that LSP.

Action

To examine BGP routes and route selection, enter the following JUNOS CLI operational mode command:

user@host> show route destination-prefix detail

Sample Output 1

user@R6> show route 100.100.1.1 detail 

inet.0: 30 destinations, 46 routes (29 active, 0 holddown, 1 hidden)

100.100.1.0/24 (1 entry, 1 announced)

        *BGP    Preference: 170/-101

                Source: 10.1.13.1

                Next hop: via so-0/0/3.0, selected

                Protocol next hop: 10.1.13.1 Indirect next hop: 8671594 304

                State: <Active Int Ext>

                Local AS: 65432 Peer AS: 65432

                Age: 4d 5:15:39         Metric2: 2 

                Task: BGP_65432.10.1.13.1+3048

                Announcement bits (2): 0-KRT 4-Resolve inet.0 

                AS path: I

                Localpref: 100

                Router ID: 10.0.0.1

Sample Output 2

user@R6> show route 100.100.1.1 detail 

inet.0: 30 destinations, 46 routes (29 active, 0 holddown, 1 hidden)

100.100.1.0/24 (1 entry, 1 announced)

        *BGP    Preference: 170/-101

                Source: 10.0.0.1

                Next hop: via so-0/0/3.0 weight 1, selected

                Label-switched-path R6-to-R1

                Label operation: Push 100000

                Protocol next hop: 10.0.0.1 Indirect next hop: 8671330 301

                State: <Active Int Ext>

                Local AS: 65432 Peer AS: 65432

                Age: 24:35      Metric2: 2 

                Task: BGP_65432.10.0.0.1+179

                Announcement bits (2): 0-KRT 4-Resolve inet.0 

                AS path: I

                Localpref: 100

                Router ID: 10.0.0.1

What It Means

Sample Output 1 shows that the BGP next hop (10.1.13.1) does not equal the LSP destination address (10.0.0.1) in the to statement at the [edit protocols mpls label-switched-path label-switched-path-name] hierarchy level when the BGP configuration of R6 and R1 is incorrect.

Sample Output 2, taken after the configurations on R1 and R6 are corrected, shows that the BGP next hop (10.0.0.1) and the LSP destination address (10.0.0.1) are the same, indicating that BGP can use the LSP to forward BGP traffic.