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Zorluk: Çok zorHigh Availability and Redundancy Concepts

An enterprise organization deploys two Layer 3 core switches, Core-A and Core-B, to provide default gateway redundancy for VLAN 10 (192.168.10.0/24192.168.10.0/24) using VRRPv3. Core-A is configured as the VRRP Master with a priority of 110, while Core-B acts as the Backup with a priority of 100. Preemption is enabled on both routers. To prevent black-holing traffic if Core-A loses its WAN connection, an interface tracking object is configured on Core-A to decrement its VRRP priority by 20 upon WAN uplink failure. Core-A is also manually configured as the Spanning Tree Protocol (STP) Primary Root Bridge for VLAN 10.

During a WAN link failure test on Core-A, tracking successfully decrements Core-A's priority to 90, and Core-B transitions to the VRRP Master state. However, telemetry shows that outbound host traffic from VLAN 10 experiences significant path latency and traverses the inter-switch trunk twice before leaving the network.

Which of the following root causes best explains why traffic is traversing the inter-switch trunk redundantly following the failover?

  1. Core-A remains the STP Root Bridge for VLAN 10, forcing access switches to continue forwarding Layer 2 traffic to Core-A first, which must then relay frames across the trunk to Core-B.Cevap
  2. B
    The host endpoints on VLAN 10 were statically assigned Core-A's physical interface IP address as their default gateway rather than the VRRP virtual IP address.
  3. C
    Core-B was configured with a higher numerical STP priority value than Core-A, causing Core-B to place its trunk interface into an STP Blocking state.
  4. D
    A native VLAN mismatch on the inter-switch trunk between Core-A and Core-B caused VRRP advertisement packets to be dropped, resulting in a dual-Master state.

Cevap

Core-A remains the STP Root Bridge for VLAN 10, forcing access switches to continue forwarding Layer 2 traffic to Core-A first, which must then relay frames across the trunk to Core-B.
First Hop Redundancy Protocols (such as VRRP or HSRP) manage Layer 3 gateway availability but do not automatically adjust Layer 2 Spanning Tree Protocol (STP) topologies. When Core-A loses its WAN uplink, interface tracking lowers its VRRP priority, allowing Core-B to become the active gateway. However, Core-A remains the STP Root Bridge for VLAN 10. Access switches continue forwarding frames to Core-A according to the STP active topology. Upon receiving these frames, Core-A must forward them across the inter-switch trunk to Core-B (the active VRRP Master), which performs the routing out to the external network. To prevent this 'traffic hairpinning' or 'tromboning', STP bridge priorities should be aligned or tracked alongside FHRP state changes.

Adım Adım Çözüm

1
Analyze the state of the First Hop Redundancy Protocol (VRRP) after the WAN link failure.
Core-A's priority decremented from 110 to 90 via interface tracking. Core-B (priority 100) successfully preempted and became the VRRP Master, taking ownership of the Virtual IP (VIP) and Virtual MAC address.
Tracking mechanism functions as designed at Layer 3 to transfer default gateway routing responsibility.
2
Analyze the state of Spanning Tree Protocol (STP) after the WAN link failure.
STP operates independently of VRRP. Core-A's bridge priority was not modified by the VRRP interface tracking event, so Core-A remains the STP Primary Root Bridge for VLAN 10.
FHRP protocol state changes do not dynamically update Layer 2 STP bridge priorities.
3
Trace the path of an outbound frame sent by a host on VLAN 10.
1. Host resolves the default gateway VIP to the VRRP Virtual MAC.
2. Access switch receives the frame destined for Virtual MAC and sends it toward the Layer 2 STP Root (Core-A).
3. Core-A receives the frame, recognizes it does not own the active Virtual MAC state, and bridges it over the trunk to Core-B.
4. Core-B receives the frame and routes it out its functional WAN interface.
Misalignment between the Layer 2 STP Root Bridge and Layer 3 FHRP Master creates hairpinned/tromboned traffic across the inter-switch trunk.

Anahtar Kavram

Alignment of Layer 2 Spanning Tree Protocol (STP) Root Bridge role with Layer 3 First Hop Redundancy Protocol (FHRP) Active/Master state
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