https://ttl-expired.net/tags/gre/ Recent content in Gre on TTL Expired Hugo en-us Wed, 15 Apr 2026 00:00:00 +0000 https://ttl-expired.net/docs/tunnels/ Wed, 15 Apr 2026 00:00:00 +0000 https://ttl-expired.net/docs/tunnels/ <h2 id="ip-tunnels">IP Tunnels</h2> <p>Tunnels encapsulate one protocol inside another, creating a virtual point-to-point link across a network that wouldn&rsquo;t otherwise carry that traffic. The encapsulating network is called the <strong>underlay</strong>; the encapsulated traffic and the logical topology it creates is called the <strong>overlay</strong>. Both GRE and IP-IP are <strong>stateless</strong> — they hold no session state and provide no encryption or reliability guarantees.</p> <p><strong>Common use cases:</strong></p> <ul> <li>Carry IPv6 traffic across an IPv4-only core (6in4)</li> <li>Carry IPv4 traffic across an IPv6-only core (4in6)</li> <li>Extend IGP adjacencies across a WAN that doesn&rsquo;t support multicast</li> <li>Tunnel MPLS across a non-MPLS network</li> <li>Bridge Layer 2 domains across a routed network</li> </ul> <hr> <h3 id="tunnel-concepts">Tunnel Concepts</h3> <p><strong>Underlay vs Overlay:</strong></p>