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Difficulty: HardHigh Availability and Redundancy Concepts

An organization is deploying a pair of enterprise edge switches configured with multi-chassis link aggregation (mLAG) connected to two core routers configured with Virtual Router Redundancy Protocol (VRRP). Server host interfaces are configured in an LACP IEEE 802.3ad dynamic bond. During failure simulation testing, host traffic experiences intermittent packet loss and unidirectional asymmetry upon primary router failure. Which TWO of the following administrative actions or verification steps will resolve these specific high availability operational defects?

  1. Verify that the VRRP virtual MAC address is correctly populated in the source address field of gratuitous ARP packets sent upon master state transitions.Answer
  2. B
    Configure static ARP entries on the client workstations mapping the VRRP virtual IP directly to the physical interface MAC address of the primary router.
  3. Ensure that the LACP system identifier and operational mode on both mLAG switches are synchronized to present a single logical switch system to the host bond.Answer
  4. D
    Set the Spanning Tree Protocol (STP) priority value on the backup VRRP router to 65535 to ensure it assumes the STP root bridge role during interface state changes.
  5. E
    Change the server host bonding driver configuration from dynamic LACP (802.3ad) mode to active-backup mode while keeping LACP configured on the switch ports.

Answer

The issues are resolved by verifying that VRRP transmits gratuitous ARP frames containing the virtual MAC address upon master state transitions, and ensuring the mLAG switch pair shares a synchronized LACP system identifier to maintain host link aggregation stability.
High availability designs utilizing both FHRP (VRRP) and multi-chassis link aggregation (mLAG) depend on seamless Layer 2 transition and unified control protocol appearance. Verifying that gratuitous ARP (GARP) broadcasts the VRRP virtual MAC allows adjacent switches to immediately update their MAC address tables upon gateway failover. Synchronizing the LACP system ID across mLAG peers allows host dual-homed NIC teams operating in IEEE 802.3ad mode to treat both physical switch connections as a single logical bundle without dropping member ports.

Step-by-Step Solution

1
Analyze the layer 2 network topology and failover mechanisms
Identify that host NIC teaming uses LACP across two switches (requiring mLAG), while default gateway redundancy relies on VRRP.
Traffic asymmetry and intermittent loss after a failover typically indicate MAC table staleness (missing or unacknowledged GARP) or link aggregation protocol mismatches across multi-chassis nodes.
2
Evaluate default gateway ARP updates during failover
Confirm that VRRP state changes require gratuitous ARP (GARP) frames with the virtual MAC address to refresh switch forwarding tables.
Without GARP updating the switches' CAM tables, traffic destined for the Virtual IP will continue to be sent toward the failed master's interface port.
3
Evaluate link aggregation parameters across multi-chassis nodes
Confirm that the mLAG switch pair must synchronize its LACP system ID.
If the LACP system ID differs between the two switches in an mLAG pair, the host NIC team will split the aggregate group or disable links due to perceived switch mismatch.

Key Concept

High Availability Integration of FHRP (VRRP) and Multi-Chassis Link Aggregation (mLAG/LACP)
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Topics

High Availability and Redundancy Concepts

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