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Difficulty: Very hardHigh Availability and Redundancy Concepts

An enterprise network infrastructure utilizes dual core switches configured with Virtual Router Redundancy Protocol (VRRP) to provide default gateway redundancy for multiple VLANs. Downstream access switches connect to both core switches via multi-chassis trunking. During traffic baseline audits, network engineers notice significant asymmetric traffic flow and sub-optimal hairpinning (tromboning) across the inter-switch link between the core switches. Which of the following configuration changes and architectural adjustments will optimize Layer 2/Layer 3 path symmetry and eliminate unnecessary inter-switch link transit? (Select TWO.)

  1. Align the VRRP Master router assignment for each VLAN with the Spanning Tree Protocol (STP) Root Bridge priority for that same VLAN.Answer
  2. Implement Multi-Chassis Link Aggregation (MC-LAG) with active-active local forwarding enabled on both core switches.Answer
  3. C
    Configure end-host default gateways to use the physical interface IP address of the VRRP Master router instead of the virtual IP address.
  4. D
    Set the STP bridge priority on the VRRP Backup router to a higher numerical integer value than the VRRP Master router.
  5. E
    Configure host ARP tables statically using the burn-in physical MAC address of the primary core switch interface.

Answer

To resolve asymmetric routing and inter-switch link hairpinning, the network design must ensure that the VRRP Master router priority and the STP Root Bridge priority are aligned on the same switch per VLAN, and deploy Multi-Chassis Link Aggregation with local active-active Layer 3 forwarding capabilities.
Aligning the VRRP Master role with the STP Root Bridge role for a given VLAN ensures that the shortest Layer 2 spanning-tree path directly coincides with the active Layer 3 default gateway interface. Additionally, implementing Multi-Chassis Link Aggregation with local forwarding enables both core switches to process and route ingress traffic locally on their physical ports, avoiding suboptimal transit over the inter-switch peer link.

Step-by-Step Solution

1
Analyze the cause of Layer 2/Layer 3 path asymmetry in dual-homed core topologies.
Identified that when the active STP root path terminates on Switch 1 while the VRRP active gateway resides on Switch 2, frames must cross the inter-switch link to reach their gateway.
Aligning STP Root Bridge election with VRRP Master status forces Layer 2 frames to flow directly to the active Layer 3 interface.
2
Evaluate link aggregation mechanisms across core switches.
Determined that using MC-LAG with active-active local forwarding allows both core switches to terminate Layer 2 LAG trunks and perform local Layer 3 routing.
Local forwarding eliminates the requirement to send packets across the inter-switch peer link prior to routing.

Key Concept

FHRP and STP Alignment with MC-LAG Active-Active Local Forwarding
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Topics

High Availability and Redundancy Concepts

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