Step 2: Examine a Route

Purpose

You can determine the cost associated with a route and the selection of a route. In this step, the path from router R5 to R3 is examined in two situations. In the first example, all metrics in the network are set to the default of 10; in the second example, all metrics on a transit router (R6) are set to 5.

Action

To examine a route in an IS-IS network, enter one or all of the following CLI commands:

user@host> show route destination-prefix

user@host> show route detail destination-prefix

user@host> show isis route destination-prefix

Sample Output 1

The following sample output shows the route from R5 to R3 when all metrics across interfaces are set to the default of 10:

user@R5> show route 10.0.0.3 

inet.0: 28 destinations, 28 routes (28 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

10.0.0.3/32        *[IS-IS/18] 00:02:00, metric 30

                      to 10.1.56.2 via so-0/0/0.0

                    > to 10.1.15.1 via so-0/0/1.0

user@R5> show route detail 10.0.0.3 

inet.0: 28 destinations, 28 routes (28 active, 0 holddown, 0 hidden)

10.0.0.3/32 (1 entry, 1 announced)

        *IS-IS  Preference: 18

                Level: 2

                Next hop: 10.1.56.2 via so-0/0/0.0

                Next hop: 10.1.15.1 via so-0/0/1.0, selected

                State: <Active Int>

                Age: 34:29      Metric: 30 

                Task: IS-IS

                Announcement bits (1): 0-KRT 

                AS path: I

user@R5> show isis route 10.0.0.3 

 IS-IS routing table                 Current version: L1: 241 L2: 243

Prefix             L Version Metric Type Interface    Via

10.0.0.3/32        2     243     30 int  so-0/0/0.0   R6                

                                         so-0/0/1.0   R1                

The following sample output shows the IS-IS configuration for transit router R6 with the metric on so-0/0/2.0 changed from the default of 10 to 5:

[edit protocols isis]

user@R6# show 

level 1 disable;

interface so-0/0/0.0 {

level 2 metric 10;

}

interface so-0/0/2.0 {

    level 2 metric 5;

}

interface fxp0.0 {

    disable;

}

interface lo0.0;

Sample Output 2

The following sample output shows the route from R5 to R3 after the metric on R6 is changed from the default of 10 to 5:

user@R5> show route 10.0.0.3    

inet.0: 28 destinations, 28 routes (28 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

10.0.0.3/32        *[IS-IS/18] 00:00:10, metric 25

                    > to 10.1.56.2 via so-0/0/0.0

user@R5> show route detail 10.0.0.3 

inet.0: 28 destinations, 28 routes (28 active, 0 holddown, 0 hidden)

10.0.0.3/32 (1 entry, 1 announced)

        *IS-IS  Preference: 18

                Level: 2

                Next hop: 10.1.56.2 via so-0/0/0.0, selected

                State: <Active Int>

                Age: 4:57       Metric: 25 

                Task: IS-IS

                Announcement bits (1): 0-KRT 

                AS path: I

user@R5> show isis route detail 10.0.0.3

 IS-IS routing table                 Current version: L1: 250 L2: 257

Prefix             L Version Metric Type Interface    Via

10.0.0.3/32        2     257     25 int  so-0/0/0.0   R6                

What It Means

Sample output 1 shows the cost for each route and the preferred next hop. In this example, there are two next hops, one through R1 and the other through R6. Both have an equal cost (30) to the destination. The cost is indicated in the Metric field. The preferred next hop is randomly chosen. In this case, the preferred next hop is through R1, interface so-0/0/1.0. In the output for the show route command, the selected next hop is indicated by a forward arrow (>). With the show route detail command, the next hop is indicated by the key word selected. The output for the show isis route command shows the selected interface and indicates that the IS-IS protocol is building the correct routing table from the link-state database.

After the metric on R6 is changed to a lower value, sample output 2 shows a different cost and next hop. With IS-IS, routes with lower total path metrics are preferred over those with higher path metrics. The path through R6 (so-0/0/0.0) is now lower (25) than the path through R1 (so-0/0/1.0) (30). In the output for the show route command, the lower cost (25) is indicated in the Metric field, and the preferred path is indicated by the forward arrow (>) and the keyword selected. The output for the show isis route command shows the selected interface and indicates that the IS-IS protocol is building the correct routing table from the link-state database.

In general, the output for the show route commands shows all active entries in the routing table. The information displayed includes the name of the routing table (inet.0), the number of destinations for which there are routes in the routing table (28), how the route was learned, and the route preference value, such as [IS-IS/18]. In addition, any metric associated with the route (metric 30), and the name of the interface through which the route was learned are displayed.