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352-001 · Question #230

Four routers running IS-IS are connected to a single Ethernet link. Then, a fifth router is connected, which has a priority higher than any of the other routers connected to the network. What will…

The correct answer is B. The new router will become the DIS without causing a temporary disruption in traffic through the link. In IS-IS, DIS election is preemptive - a higher-priority router immediately takes over as DIS - but unlike OSPF, this does not disrupt traffic because all routers on the segment maintain full adjacencies with each other, not only with the DIS.

Layer 3 Control Plane

Question

Four routers running IS-IS are connected to a single Ethernet link. Then, a fifth router is connected, which has a priority higher than any of the other routers connected to the network. What will happen?

Options

  • AThe new router will become the DIS and cause a temporary disruption in traffic through the link.
  • BThe new router will become the DIS without causing a temporary disruption in traffic through the link.
  • CThe new router will not be elected DIS unless the current DIS fails.
  • DThe new router will not be elected DIS unless it has the lowest NET ID.

How the community answered

(31 responses)
  • A
    3% (1)
  • B
    71% (22)
  • C
    19% (6)
  • D
    6% (2)

Why each option

In IS-IS, DIS election is preemptive - a higher-priority router immediately takes over as DIS - but unlike OSPF, this does not disrupt traffic because all routers on the segment maintain full adjacencies with each other, not only with the DIS.

AThe new router will become the DIS and cause a temporary disruption in traffic through the link.

While the new router does become DIS (preemptive election is correct), the claim of traffic disruption is false because IS-IS adjacencies are maintained between all routers on the segment and are not dependent solely on the DIS.

BThe new router will become the DIS without causing a temporary disruption in traffic through the link.Correct

IS-IS DIS election is preemptive, meaning a router with a higher interface priority immediately becomes the new DIS without waiting for the current DIS to fail. Unlike OSPF's DR/BDR model, IS-IS routers on a multi-access segment form adjacencies with all neighbors, not just the DIS, so the DIS change does not tear down adjacencies or disrupt traffic forwarding.

CThe new router will not be elected DIS unless the current DIS fails.

IS-IS DIS election is explicitly preemptive, unlike OSPF where the DR does not yield to a higher-priority router that joins later - a higher-priority IS-IS router takes over immediately.

DThe new router will not be elected DIS unless it has the lowest NET ID.

DIS election is determined by interface priority (highest wins) with MAC address as a tiebreaker, not by the NET ID (Network Entity Title).

Concept tested: IS-IS DIS preemptive election on multi-access networks

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-16/iri-xe-16-book/iri-overview.html

Topics

#IS-IS#DIS election#DIS preemption#broadcast network

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