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400-007 · Question #122

You are designing a large-scale DMVPN network with more than 500 spokes using EIGRP as the IGP protocol. Which design option eliminates potential tunnel down events on the spoke routers due to the…

The correct answer is C. Increase the hold queue on the tunnel interface of the hub router. In large DMVPN deployments with EIGRP, the hub tunnel interface can drop hello packets under high spoke-count load, causing hold timer expiration on spokes; increasing the hub tunnel interface hold queue prevents this.

Designing Network Infrastructure

Question

You are designing a large-scale DMVPN network with more than 500 spokes using EIGRP as the IGP protocol. Which design option eliminates potential tunnel down events on the spoke routers due to the holding time expiration?

Options

  • AIncrease the hold queue on the physical interface of the hub router.
  • BIncrease the hold queue on the tunnel interface of the spoke routers
  • CIncrease the hold queue on the tunnel interface of the hub router
  • DApply QoS for pak_priority class
  • EIncrease the hold queue on the physical interface of the spoke routers.

How the community answered

(37 responses)
  • A
    22% (8)
  • B
    3% (1)
  • C
    65% (24)
  • D
    8% (3)
  • E
    3% (1)

Why each option

In large DMVPN deployments with EIGRP, the hub tunnel interface can drop hello packets under high spoke-count load, causing hold timer expiration on spokes; increasing the hub tunnel interface hold queue prevents this.

AIncrease the hold queue on the physical interface of the hub router.

The physical interface hold queue on the hub is not the bottleneck - EIGRP hello congestion occurs at the tunnel interface where all spoke sessions converge.

BIncrease the hold queue on the tunnel interface of the spoke routers

Increasing the spoke tunnel hold queue does not resolve hub-side congestion; packets are dropped at the hub before the spoke can process them.

CIncrease the hold queue on the tunnel interface of the hub routerCorrect

With 500+ spokes, the hub tunnel interface receives a very high volume of EIGRP hellos simultaneously; increasing the hold queue depth on the hub tunnel interface prevents queue drops that would cause EIGRP adjacency loss and trigger spoke tunnel down events.

DApply QoS for pak_priority class

QoS pak_priority prioritizes control plane traffic but does not increase the hub tunnel interface queue depth, so drops can still occur under high spoke-count load.

EIncrease the hold queue on the physical interface of the spoke routers.

The spoke physical interface hold queue is not the bottleneck - the problem originates at the hub tunnel interface where all spoke EIGRP traffic is processed together.

Concept tested: DMVPN hub tunnel interface hold queue scaling for EIGRP

Source: https://www.cisco.com/c/en/us/support/docs/security-vpn/dynamic-multipoint-vpn-dmvpn/116381-technote-dmvpn-00.html

Topics

#DMVPN#EIGRP#hold queue#hub-and-spoke scalability

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