Generic Routing Encapsulation (GRE) is a tunneling protocol that encapsulates packets of one network protocol within another network protocol. This allows for the transmission of data across networks that may not normally support the original protocol.
Key Concepts:
Encapsulation: GRE wraps the original data packet (the inner packet) within a new header and trailer, creating a new, larger packet (the outer packet).
Tunneling: GRE creates a virtual point-to-point connection between two devices, even if they are not directly connected. This connection is called a “tunnel.”
Protocol Versatility: GRE can encapsulate a wide range of network layer protocols, such as IP, IPX, and AppleTalk.
Flexibility: GRE can be used to create various types of tunnels, including site-to-site VPNs, remote access VPNs, and network address translation (NAT) traversal.
How GRE Works:
Encapsulation:
The source device encapsulates the original data packet within a GRE header and trailer.
The GRE header includes information such as the source and destination IP addresses of the tunnel endpoints.
The outer packet is then typically encapsulated within an IP header for transmission over an IP network.
Transmission:
The encapsulated packet is transmitted over the network to the destination device.
Decapsulation:
The destination device receives the encapsulated packet and decapsulates it, removing the GRE header and trailer.
The original data packet is then processed and delivered to the intended recipient.
Use Cases:
Connecting Disparate Networks: GRE can connect networks that use different network layer protocols.
Site-to-Site VPNs: GRE can be used to create secure connections between two or more sites across a public network.
Remote Access VPNs: GRE can be used to allow remote users to access a private network securely.
NAT Traversal: GRE can be used to bypass network address translation (NAT) devices, which can block certain types of traffic.
Advantages:
Versatility: Supports a wide range of network layer protocols.
Flexibility: Can be used for various types of tunnels.
Simplicity: Relatively easy to configure and implement.
Disadvantages:
Security: GRE itself does not provide any security features, such as encryption or authentication.
Overhead: Encapsulation adds overhead to the data packets, which can reduce performance.
Limited Features: Compared to more advanced tunneling protocols like IPsec, GRE offers limited features.
In summary:
GRE is a versatile tunneling protocol that can be used to connect disparate networks and create various types of VPNs. While it offers simplicity and flexibility, it lacks security features and can introduce overhead. For more demanding security requirements, protocols like IPsec are generally preferred.
The current version of Cisco CCNA 200-125 is retiring on 23rd Feb 2020. Several other associate level exams, such as CCENT, CCDA, CCNA Security are also retiring along with CCNA. The new exam CCNA 200-301 will have completely revised syllabus with focus on hands-on experience and less coverage of TCP fundamentals. New areas included are Cisco Wireless Controllers (WLCs), Cisco Digital Network Architecture (DNA), and Networking Security (enhanced).