[M/T/MX] How to change the preference value of Level 2 internal routes in Level 1 area

Description

This article demonstrates how the preference values of Level 2 internal routes can be changed in Level 1 with the help of an example, and indicates that this may sometimes be necessary to reduce the dependency of the Level 1 internal route on the default route generated by the L2 router.

Symptoms

The IS-IS preference values are, by default, set at 15 (for Level 1 internal routes), 18 (for Level 2 internal routes), 160 (for Level 1 external routes), and 165 (for Level 2 external routes). With these default values, the L1 internal route uses only the default route generated by the L2 router.

With the default preference values, we will see the following output:

root# run show isis overview
Instance: master
  Router ID: 210.183.28.11
  Adjacency holddown: enabled
  Maximum Areas: 3
  LSP life time: 1200
  Attached bit evaluation: enabled
  SPF delay: 200 msec, SPF holddown: 5000 msec, SPF rapid runs: 3
  IPv4 is enabled, IPv6 is enabled
  Traffic engineering: enabled
  Restart: Disabled
    Helper mode: Enabled
  Level 1
    Internal route preference: 15
    External route preference: 160
    Wide metrics are enabled, Narrow metrics are enabled
  Level 2
    Internal route preference: 18
    External route preference: 165
    Wide metrics are enabled, Narrow metrics are enabled

Solution

However, sometimes, users may want to change the preference value of all routes from Level 2 area to be the same as Level 1. Or they may want the preference value of Level 2 internal routes to have a special value in the Level 1 area.

If only one protocol is used, then the preference value will have no meaning. But if there are several protocols to calculate routes to the same destination and you want to use/select a particular IS-IS route, the configuration example in this article will be helpful.

Note: If the preference knob is not used, the preference value of Level 2 internal routes in Level 1 area will be always 18.

There is a command option for the preference knob, which can be used to modify preference values. In addition to modifying preference values using this knob, users will also need to allow route leaks so that the IS-IS L2 route information can be input in the Level 1 area, which will reduce the dependency of the Level 1 internal route on the default route generated by the L2 route.

How the route leaks can be enabled is demonstrated with the following example topology :

Topology

On vMX 1

set interfaces ge-0/0/1 unit 0 family inet address 13.0.0.1/24
set interfaces ge-0/0/1 unit 0 family iso
set interfaces ge-0/0/2 unit 0 family inet address 14.0.0.1/24
set interfaces ge-0/0/2 unit 0 family iso
set interfaces ge-0/0/4 unit 0 family inet address 12.0.0.1/24
 set interfaces ge-0/0/4 unit 0 family iso
set interfaces lo0 unit 0 family inet address 210.183.28.11/32
set interfaces lo0 unit 0 family iso address 49.9410.2101.8302.8011.00
set routing-options router-id 210.183.28.11
set protocols isis export ISIS-L2-to-L1       <<<<<< Use the policy to make a route leak.
set protocols isis interface ge-0/0/1.0 level 2 metric 5
set protocols isis interface ge-0/0/1.0 level 1 metric 5
set protocols isis interface ge-0/0/2.0 level 1 metric 10
set protocols isis interface ge-0/0/2.0 level 2 disable
set protocols isis interface ge-0/0/4.0 level 1 disable
set protocols isis interface ge-0/0/4.0 level 2 metric 100
set protocols isis interface lo0.0
set policy-options policy-statement ISIS-L2-to-L1 term 10 from protocol isis
set policy-options policy-statement ISIS-L2-to-L1 term 10 from level 2
set policy-options policy-statement ISIS-L2-to-L1 term 10 to protocol isis
set policy-options policy-statement ISIS-L2-to-L1 term 10 to level 1
set policy-options policy-statement ISIS-L2-to-L1 term 10 then accept

On vMX3

set interfaces ge-0/0/1 unit 0 family inet address 13.0.0.3/24
set interfaces ge-0/0/1 unit 0 family iso
set interfaces ge-0/0/2 unit 0 family inet address 23.0.0.3/24
set interfaces ge-0/0/2 unit 0 family iso
set interfaces ge-0/0/3 unit 0 family inet address 34.0.0.3/24
set interfaces ge-0/0/3 unit 0 family iso
set interfaces lo0 unit 0 family inet address 210.183.28.12/32
set interfaces lo0 unit 0 family iso address 49.9410.2101.8302.8012.00
set routing-options router-id 210.183.28.12
set protocols isis export ISIS-L2-to-L1    <<<<<< Use the policy to make a route leak.
set protocols isis interface ge-0/0/1.0 level 2 metric 5
set protocols isis interface ge-0/0/1.0 level 1 metric 5
set protocols isis interface ge-0/0/2.0 level 2 metric 100
set protocols isis interface ge-0/0/2.0 level 1 disable
set protocols isis interface ge-0/0/3.0 level 1 metric 10
set protocols isis interface ge-0/0/3.0 level 2 disable
set protocols isis interface lo0.0     
set policy-options policy-statement ISIS-L2-to-L1 term 10 from protocol isis
set policy-options policy-statement ISIS-L2-to-L1 term 10 from level 2
set policy-options policy-statement ISIS-L2-to-L1 term 10 to protocol isis
set policy-options policy-statement ISIS-L2-to-L1 term 10 to level 1
set policy-options policy-statement ISIS-L2-to-L1 term 10 then accept

On vMX2

set interfaces ge-0/0/2 unit 0 family inet address 23.0.0.2/24
set interfaces ge-0/0/2 unit 0 family iso
set interfaces ge-0/0/4 unit 0 family inet address 12.0.0.2/24
set interfaces ge-0/0/4 unit 0 family iso
set interfaces lo0 unit 0 family inet address 210.183.28.3/32
set interfaces lo0 unit 0 family iso address 49.0100.2101.8302.8003.00
set routing-options router-id 210.183.28.3   <<<<< Level 2 internal routes in Level 1 
set protocols isis level 1 disable
set protocols isis level 1 preference 66
set protocols isis level 2 preference 99
set protocols isis level 2 external-preference 99
set protocols isis interface ge-0/0/2.0 level 2 metric 120
set protocols isis interface ge-0/0/4.0 level 2 metric 120
set protocols isis interface lo0.0

On vMX4

set interfaces ge-0/0/2 unit 0 family inet address 14.0.0.4/24
set interfaces ge-0/0/2 unit 0 family iso
set interfaces ge-0/0/3 unit 0 family inet address 34.0.0.4/24
set interfaces ge-0/0/3 unit 0 family iso
set interfaces lo0 unit 0 family inet address 210.183.28.17/32
set interfaces lo0 unit 0 family iso address 49.9410.2101.8302.8017.00
set routing-options router-id 210.183.28.17
set protocols isis level 2 disable  
set protocols isis level 1 preference 63   
set protocols isis level 1 external-preference 63     
set protocols isis interface ge-0/0/2.0 level 1 metric 10
set protocols isis interface ge-0/0/2.0 level 2 disable
set protocols isis interface ge-0/0/3.0 level 1 metric 10
set protocols isis interface ge-0/0/3.0 level 2 disable
set protocols isis interface lo0.0 passive

However, with the default preference value configuration and the above route leaks, the preference will still be 18 unless you modify the preference value. For example, when you look at the router route L2/210.183.28.3 in the Level 1 area on vMX4, the preference is shown as 18, which is the IS-IS default preference value for Level 2 routes in Level 1 area.

lab@VMX4# run show route 

inet.0: 14 destinations, 14 routes (14 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

210.183.28.3/32    *[IS-IS/18] 00:06:25, metric 110  <<<<<<<<<<<<<<<<<<<<<
                    > to 14.0.0.1 via ge-0/0/2.0
                      to 34.0.0.3 via ge-0/0/3.0

Changing the Default Configuration

To change the default configuration and set a specific preference value for L2 internal routes in Level 1 area, the preference value must be modified using the preference knob. In the following example, we configure vMX4 to set the preference of Level 2 internal routes/210.183.28.3 to be 70.

protocols {
    isis {
        level 2 {
            disable;
            preference 70;
        }
        level 1 {
            preference 63;
            external-preference 63;

Output

root# run show route 210.183.28.3
 
inet.0: 11 destinations, 11 routes (11 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
 
210.183.28.3/32    *[IS-IS/70] 00:00:17, metric 73
                      to 14.0.0.1 via ge-0/0/2.0
                    > to 34.0.0.3 via ge-0/0/3.0

 
root# run show isis overview
Instance: master
  Router ID: 210.183.28.17
  Adjacency holddown: enabled
  Maximum Areas: 3
  LSP life time: 1200
  Attached bit evaluation: enabled
  SPF delay: 200 msec, SPF holddown: 5000 msec, SPF rapid runs: 3
  IPv4 is enabled, IPv6 is enabled
  Traffic engineering: enabled
  Restart: Disabled
    Helper mode: Enabled
  Level 1
    Internal route preference: 63
    External route preference: 63
    Wide metrics are enabled, Narrow metrics are enabled
  Level 2
    Internal route preference: 70
    External route preference: 165
    Wide metrics are enabled, Narrow metrics are enabled