300-320 Question #688: Real Exam Question with Answer & Explanation

The correct answer is A: Source interface. In a multipoint GRE (mGRE) tunnel configuration, only a tunnel source (interface or IP address) is required. A tunnel destination is deliberately not specified because mGRE is designed to dynamically resolve destinations at runtime. The actual mapping between overlay (tunnel) IP

Question

When implementing multipoint GRE tunnels, which of the following do you need to specify?

Options

ASource interface
BDestination IP
CBoth a and b
DNHBP protocol mapping

Explanation

In a multipoint GRE (mGRE) tunnel configuration, only a tunnel source (interface or IP address) is required. A tunnel destination is deliberately not specified because mGRE is designed to dynamically resolve destinations at runtime. The actual mapping between overlay (tunnel) IP addresses and underlay (physical) IP addresses is handled by NHRP (Next Hop Resolution Protocol), not a static destination configured on the tunnel interface. This dynamic resolution is precisely what allows a single mGRE interface to serve as the hub for hundreds of spoke peers without requiring manual destination configuration for each one.

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