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Juniper Controlling DIS Election
 
You want to force IS-IS to use a specific router as the designated router.
Increase the priority on the desired router:
 
[edit protocols isis]
source@RouterC# set interface ge-0/0/1 level 2 priority 65
 
With the IS-IS configuration we have shown in previous recipes, all IS-IS routers have the default priority value (64), which IS-IS uses to elect the DR in each area. In this situation, the router with the higher MAC address is elected as the DR.
 
In the previous recipes, RouterA is elected DR because it has a higher MAC address (0:5:85:c2:2e:d1, compared to 0:5:85:c1:d1:d1 for RouterC):
 
source@RouterA> show isis adjacency
Interface             System         L State Hold (secs) SNPA
ge-0/0/1.0            RouterC        2 Up             7 0:5:85:c1:d1:d1
ge-1/0/0.0            Captain-Caveman1 Up             7 0:5:85:ca:ca:70
 
source@RouterC> show isis adjacency
Interface             System         L State Hold (secs) SNPA
ge-0/0/1.0            RouterA        2 Up            20 0:5:85:c2:2e:d1
ge-1/0/1.0            RouterA        1 Up             7 0:5:85:ca:e7:d0
 
Looking at the IS-IS interfaces, you see the default priority value of 64 for all interfaces at both Level 1 and Level 2 and that RouterA is the DR for the Level 2 area:
 
source@RouterA> show isis interface detail
IS-IS interface database:
ge-0/0/1.0
  Index: 64, State: 0x6, Circuit id: 0x2, Circuit type: 3
  LSP interval: 100 ms, CSNP interval: 10 s
  Level Adjacencies Priority Metric Hello (s) Hold (s) Designated Router
    1             0       64     10     9.000       27
    2             1       64     10     3.000        9 RouterA.02 (us)
ge-1/0/0.0
  Index: 68, State: 0x6, Circuit id: 0x3, Circuit type: 1
  LSP interval: 100 ms, CSNP interval: 10 s
  Level Adjacencies Priority Metric Hello (s) Hold (s) Designated Router
    1             1       64     10     9.000       27 RouterA.02
     (not us)
lo0.0
  Index: 70, State: 0x6, Circuit id: 0x1, Circuit type: 0
  LSP interval: 100 ms, CSNP interval: disabled
 
  Level Adjacencies Priority Metric Hello (s) Hold (s) Designated Router
    1             0       64      0 Passive
    2             0       64      0 Passive
 
Unlike OSPF, which has a " sticky" DR, in IS-IS, if a new router with a higher priority than the existing DR becomes active, or if the new router has an equal priority and a higher MAC address, it becomes the new DR. In this recipe, for RouterC to become the DR, its priority needs to be greater than 64. After changing the value, you see that RouterC has become the Level 2 DR:
 
source@RouterA> show isis interface
 
IS-IS interface database:
Interface             L CirID Level 1 DR        Level 2 DR   L1/L2 Metric
ge-0/0/1.0            3   0x2 RouterA.02        RouterC.02        10/10
ge-1/0/0.0            1   0x3 RouterA.02  
Disabled                 10/10
lo0.0                 0   0x1 Passive Passive                      0/0
 
Looking at RouterC, you see it has a DR priority of 65:
 
source@RouterC> show isis interface ge-0/0/1 detail
 
IS-IS interface database:
ge-0/0/1.0
  Index: 67, State: 0x6, Circuit id: 0x2, Circuit type: 3
  LSP interval: 100 ms, CSNP interval: 10 s
  Level Adjacencies Priority Metric Hello (s) Hold (s) Designated Router
    1             0       64     10     9.000       27
    2             1       65     10     3.000        9 RouterC.02 (us)
 
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