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Networking Fundamentals · Intermediate

Generic Routing Encapsulation (GRE)

A Cisco-developed tunneling protocol (IP protocol 47) that encapsulates almost any network-layer protocol in IP.

Definition

GRE adds a 4-byte (or larger) GRE header plus a new IP header, totaling at least 24 bytes of overhead. It supports multicast and routing protocols across the tunnel but provides no encryption, so it is often combined with IPsec. A GRE tunnel interface has a typical MTU of 1476 bytes.

Example

interface Tunnel0 with tunnel source and tunnel destination creates a GRE tunnel on Cisco IOS.

Exam tip

GRE is protocol 47, unencrypted; pair it with IPsec for security.

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