Controlling DIS Election

Problem

You want to force IS-IS to use a specific router as the designated router.

Solution

Increase the priority on the desired router:

	[edit protocols isis]
	aviva@RouterH# set interface fe-0/0/1 level 2 priority 65

 

Discussion

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, RouterG 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 RouterH):

	aviva@RouterG> show isis adjacency
	Interface System L State Hold (secs) SNPA
	fe-0/0/1.0 RouterH 2 Up 7 0:5:85:c1:d1:d1
	fe-1/0/0.0 Captain-Caveman1 Up 7 0:5:85:ca:ca:70
	
	aviva@RouterH> show isis adjacency
	Interface System L State Hold (secs) SNPA
	fe-0/0/1.0 RouterG 2 Up 20 0:5:85:c2:2e:d1
	fe-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 RouterG is the DR for the Level 2 area:

	aviva@RouterG>  
show isis interface detail
	IS-IS interface database:
	fe-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 RouterG.02 (us)
	fe-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 RouterH to become the DR, its priority needs to be greater than 64. After changing the value, you see that RouterH has become the Level 2 DR:

	aviva@RouterG> show isis interface
	 
IS-IS interface database:
	Interface L CirID Level 1 DR Level 2 DR L1/L2 Metric
	fe-0/0/1.0 3 0x2 RouterG.02 RouterH.02 10/10
	fe-1/0/0.0 1 0x3 RouterA.02 
Disabled 10/10
	lo0.0 0 0x1 Passive Passive 0/0

Looking at RouterH, you see it has a DR priority of 65:

	aviva@RouterH> show isis interface fe-0/0/1 detail
	 
IS-IS interface database:
	fe-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 RouterH.02 (us)