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352-001 · Question #495
352-001 Question #495: Real Exam Question with Answer & Explanation
The correct answer is C: Use HSRP millisecond timers between the CE routers.. HSRP millisecond timers reduce the hold-down detection window from the default 10 seconds to sub-second intervals, enabling rapid CE gateway failover when an Ethernet WAN access circuit fails.
Question
Refer to the exhibit. You have designed this network using a Layer 3 MPLS service from a service provide, with dual Ethernet WAN access circuits and dual CE routers at every site. All sites have a single LAN subnet with HSRP tracking the Ethernet WAN access circuit on each CE. Which step improves routing convergence times across the Ethernet WAN access if a circuit fails?
Options
- AReduce BGP hello and hold down timers.
- BReplace HSRP with a link state IGP on the LAN.
- CUse HSRP millisecond timers between the CE routers.
- DUse a link state IGP on the PE-CE circuit.
Explanation
HSRP millisecond timers reduce the hold-down detection window from the default 10 seconds to sub-second intervals, enabling rapid CE gateway failover when an Ethernet WAN access circuit fails.
Common mistakes.
- A. Reducing BGP hello and hold-down timers accelerates detection of a failed BGP session between PE and CE routers but does not address the LAN-side HSRP failover timing that governs how quickly the standby CE takes over as the active gateway.
- B. Replacing HSRP with a link state IGP on the LAN fundamentally redesigns the gateway redundancy model and adds complexity without directly addressing the WAN circuit failure detection and CE failover convergence time.
- D. Using a link state IGP on the PE-CE circuit improves routing protocol reconvergence between provider and customer edge routers but does not reduce the HSRP failover delay experienced on the LAN subnet when an access circuit fails.
Concept tested. HSRP millisecond timers for sub-second WAN circuit failover
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