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352-001 · Question #246

Three routers in a single broadcast domain are connected by means of a standard Ethernet switch. The only Layer 2 protocol running on this link is Spanning Tree Protocol. The only Layer 3 protocol run

The correct answer is C. The EIGRP dead timers will expire due to the absence of EIGRP traffic.. In a switched multi-access network, when a router shuts down its neighbors retain their own physical links to the switch, so EIGRP must rely on dead timer expiration to detect the lost neighbor.

Layer 3 Control Plane

Question

Three routers in a single broadcast domain are connected by means of a standard Ethernet switch. The only Layer 2 protocol running on this link is Spanning Tree Protocol. The only Layer 3 protocol running on this link is EIGRP, which uses a standard configuration. Then, one of the three routers is manually shut down. How will the other two routers discover the loss of this neighbor?

Options

  • AThe line protocol on the remaining two routers will be brought down as soon as the carrier is lost
  • BThe switch will bring down the carrier on all ports momentarily to force all the routers connected
  • CThe EIGRP dead timers will expire due to the absence of EIGRP traffic.
  • DThe switch will send a reverse ARP when the router disconnects from the switch.

How the community answered

(19 responses)
  • A
    5% (1)
  • B
    11% (2)
  • C
    74% (14)
  • D
    11% (2)

Why each option

In a switched multi-access network, when a router shuts down its neighbors retain their own physical links to the switch, so EIGRP must rely on dead timer expiration to detect the lost neighbor.

AThe line protocol on the remaining two routers will be brought down as soon as the carrier is lost

The line protocol on the remaining routers will not go down because their physical connections to the switch are still active - only the port of the downed router loses carrier.

BThe switch will bring down the carrier on all ports momentarily to force all the routers connected

Switches do not bring down carrier on all ports when one device disconnects - each port operates independently and only the affected port loses the signal.

CThe EIGRP dead timers will expire due to the absence of EIGRP traffic.Correct

When a router shuts down in a switched Ethernet environment, the remaining routers keep their own ports active and their line protocols stay up. EIGRP sends hello packets at a regular interval, and when no hellos arrive from a neighbor within the dead interval (three times the hello interval by default), the adjacency is torn down and the neighbor is declared unreachable.

DThe switch will send a reverse ARP when the router disconnects from the switch.

Reverse ARP is used by diskless clients to obtain an IP address and is not a mechanism by which switches notify connected routers of topology changes.

Concept tested: EIGRP neighbor loss detection via dead timer

Source: https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html

Topics

#EIGRP hello timers#neighbor loss detection#broadcast segment#EIGRP dead timer

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