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Juniper JN0-664 Exam - Topic 5 Question 49 Discussion
Topic #: 5
In IS-IS, which two statements are correct about the designated intermediate system (DIS) on a multi-access network segment? (Choose two)
Option A (Correct):
In IS-IS, theDesignated Intermediate System (DIS)is elected based on thehighest configured priority(as defined in Junos OS).
If priorities are equal, the router with thehighest MAC addressbecomes the DIS.
A priority value of10will always override a lower priority (e.g., 1).
Option C (Correct):
On a multi-access network (e.g., Ethernet),all IS-IS routers form adjacencies with every other routeron the segment.
Unlike OSPF, IS-IS does not restrict adjacencies to only the DIS.
The DIS is responsible for creating apseudonode LSPto represent the broadcast network, but full mesh adjacencies are maintained.
Why Other Options Are Incorrect:
Option B:Incorrect. Higher priority always wins the DIS election. A priority of 1 cannot override a priority of 10.
Option D:Incorrect. IS-IS routers form adjacencies withall neighbors, not just the DIS.
Key Takeaways:
DIS Election:Prioritizes highest numerical value (e.g., 10 > 1).
Adjacency Behavior:Full mesh adjacencies are maintained, unlike OSPF.
DIS Role:Primarily for generating pseudonode LSPs and optimizing flooding, not adjacency restriction.
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