Introduction
GRE stands for Generic Routing Encapsulation. It is a tunneling protocol that is used to carry one kind of network traffic within another protocol.
In simple words, GRE takes the original packet and wraps it into another packet to ensure it can be transmitted across an IP network. This is beneficial in situations where two remote networks must communicate via a logical tunnel.
This guide will explain what GRE (Generic Routing Encapsulation) is, how GRE works, where it is useful, GRE tunneling, and why GRE is often used with IPsec in real network designs.
What is Generic Routing Encapsulation?
GRE is a protocol that encapsulates packets of one network protocol, such as IPv4, IPv6, IPX, etc., within the payload of another protocol, like IP. The term “encapsulate” refers to the process of wrapping one data packet within another data packet. GRE is a way of establishing a direct point-to-point connection across a network. It works with several network layer protocols.
The GRE protocol allows the utilization of protocols that are typically unsupported by a network. This is achieved by encapsulating the packets within other packets that employ supported protocols.
Let ‘s take an example to better understand the GRE.
This diagram shows how GRE changes traffic flow. Without GRE, routing and multicast may not pass cleanly over a plain IP path. With GRE, routers build a logical tunnel and carry the traffic inside encapsulated packets.
History of Generic Routing Encapsulation protocol
GRE was developed by Cisco Systems in 1994 as a way to simplify the interconnection of different networks. It was initially defined in RFC 1701 and RFC 1702, which described how to use GRE to encapsulate any protocol over any other protocol.
Later, the GRE header structure was streamlined and made more effective by RFC 2784.
Checksum, key, and sequence number are now optional fields in the GRE header, according to RFC 2890.
GRE is currently supported by a broad range of manufacturers and hardware, and it serves a number of functions, including multicast routing, mobility protocols, and VPNs.
Before getting into the working of Generic Routing Encapsulation protocol, let’s first understand why to use GRE protocol.
What is GRE Tunnelling?
GRE tunneling is when GRE is configured between two routers to create a virtual point-to-point tunnel across an IP network. The key idea is that the original packet is carried inside a new outer packet, so the middle network forwards it using only the outer details.
When a packet enters the GRE router, the router adds two layers around it.
- An outer IP header is added with the tunnel source and tunnel destination addresses
- A GRE header is added to describe the payload being carried inside the tunnel
- The original packet stays inside as the payload and is not examined by transit routers
The encapsulated packet crosses the network using the outer IP header. At the destination, the router removes the outer IP header and GRE header and then forwards the original packet normally. If security is required, GRE is commonly paired with IPsec.
GRE Packet Header
1) C bit or Checksum Present (bit 0)
If this bit is set to 1, then Checksum and Reserved1 fields are present, and the checksum contains valid information. A compliant implementation must accept and process this field; it cannot ignore it.
2) Reserved0 (bits 1 to 12)
This is a control space that is mostly reserved.
A receiver must discard a packet if any of bits 1 to 5 are non-zero, unless it supports older behavior (RFC 1701 style handling).
Bits 6 to 12 are reserved for future use. They must be sent as 0 and must be ignored on receipt.
Important practical note
If you use later extensions like K (bit 2) and S (bit 3), then bits 2 and 3 are no longer “random non-zero values”. They have meaning, but only if both endpoints support that extension. If an endpoint supports only base behavior, it may treat those as “non-zero in bits 1–5” and discard the packet. That is why both sides must match features.
3) Version Number or Ver (bits 13 to 15)
This must contain the value 0. If the version is not 0, it is not this GRE format.
4) Protocol Type (2 octets)
This field tells what the payload packet type is inside the GRE. These values follow EtherType style identifiers. If a device receives a protocol type it does not recognize, it should discard the packet. This is how the tunnel endpoint knows whether it is carrying IPv4, IPv6, and so on.
5) Checksum (2 octets) optional
This field exists only when C = 1. It is the IP one’s complement checksum of all 16-bit words in the GRE header and the payload packet.
While calculating the checksum, the checksum field itself is treated as 0. The goal is basic integrity checking for the GRE header and the carried packet.
6) Reserved1 (2 octets) optional
This exists only when C = 1, along with the checksum. It is reserved for future use and must be transmitted as 0. It is not used for normal GRE operations.
7) Key Present (bit 2)
If this bit is set to 1, it indicates the Key field is present. If it is 0, the Key field does not appear.
8) Sequence Number Present (bit 3)
If this bit is set to 1, it indicates the Sequence Number field is present. If it is 0, the Sequence Number field does not appear.
9) Key (32 bits) optional
This field appears only when K = 1. It is mainly used to identify a specific tunnel or traffic flow between the same endpoints. It helps separation and identification, but it is not encryption and should not be treated as strong security.
10) Sequence Number (32 bits) optional
This field appears only when S = 1. It is used for ordering and tracking. It helps detect packet loss or reordering, but GRE does not retransmit packets by itself, so it is mostly for visibility and control at endpoints.
Why use the GRE (Generic Routing Encapsulation) in Networking?
GRE may be used for many functions, including:
- Connecting networks that use several network layer protocols, including IPv4 and IPv6.
- Building VPNs (Virtual Private Networks) that can pass through NAT (Network Address Translation) devices and firewalls.
- Enabling broadcast and multicast traffic across networks that only support unicast.
- Transferring routing information among several routing domains or protocols.
- Used for large-scale deployments with mGRE and DMVPN in which several sites are managed more efficiently than point-to-point tunnels.
- Common in overlay connectivity and WAN designs and often paired with IPsec for secure tunneling.
- Supports routing protocols between remote routers including OSPF, EIGRP, and BGP, based on the network’s design.
How Does Generic Routing Encapsulation Protocol Work?
GRE encapsulates the original data packets within new packets that include a new IP header and a GRE header. The source and destination addresses of the tunnel endpoints are included in the new IP header, while the GRE header provides details on the kind of encapsulated data. At the opposite end of the tunnel, the encapsulated packets are delivered to the target location after being decapsulated and forwarded via the IP network as usual.
Below, we have shown the GRE encapsulation structure.
Let’s take an example for better understanding.
Consider a scenario where a business has to connect two IPv6-compatible offices together, but the intermediary network can only handle IPv4 traffic. The business may easily transport IPv6 packets via the intermediary network by encapsulating them within IPv4 packets using GRE. The IPv4 packets act as carriers for the IPv6 packets, which are only opened at the endpoints of the tunnel.
Configuration of GRE Protocol
GRE configuration depends on the router, firewall, switch, or platform being used, but the basic idea remains the same. The GRE tunnel is configured at the router level, and the setup will vary depending on the hardware and service being utilized. Setting up a tunneling interface of IPs and providing public IP addresses on both tunnel endpoints form the foundation of the GRE tunnel configuration.
It’s crucial to confirm that the provided IP address has been whitelisted in the firewall before setting GRE. Both tunnel endpoints should be used for the whitelisting.
Eliminating the tunnels before the firewall is one best practice to follow in this situation. In this manner, the inner packets may be examined.
The MTU must then be checked to ensure it is within the limit. The MTU must be predefined. 1,500 bytes is the standard MTU limit. However, if you’re using the GRE, you must make room for the 24-byte GRE header.
The recommended MTU value for a GRE tunnel is 1400 bytes, and the traffic flow inside one is symmetric.
We now understand the working as well as the configuration of Generic Routing Encapsulation Protocol; let’s discuss its features.
Does GRE Provide Encryption?
GRE does not provide encryption by itself. It is a tunneling protocol, not a security protocol. Many beginners think that a tunnel automatically means secure communication. With GRE, that is not true because GRE only wraps the packet and sends it through the tunnel.
GRE does not hide the data, encrypt the packet, or authenticate the tunnel by itself. So GRE should not be treated as a complete VPN security solution. GRE can be used in VPN designs, but GRE alone is not a secure VPN. It only provides encapsulation.
If security is required, the GRE should be used with IPsec. GRE provides tunnel flexibility, while IPsec provides encryption, integrity, and authentication.
Why GRE over IPsec is used in Real Networks?
GRE is often used with IPsec because both protocols solve different problems. GRE handles the tunneling part, while IPsec handles the security part.
This combination is useful because GRE can carry traffic that IPsec alone may not handle as easily in some designs. GRE can carry routing protocols, multicast traffic, broadcast traffic, and different protocol types. IPsec then protects this traffic with encryption and integrity checks.
In simple words, GRE creates the flexible tunnel, and IPsec secures that tunnel. This is why GRE over IPsec is commonly used for site-to-site VPNs, branch connectivity, and enterprise WAN designs. Without IPsec, GRE is only an encapsulation tunnel. With IPsec, it becomes a more secure tunnel design.
This is also why GRE over IPsec is important for practical networking. It combines the flexibility of GRE with the security features of IPsec.
How to Know If a GRE Tunnel Is Working?
GRE tunnels are simple, but they are also stateless by design. This means one tunnel endpoint does not always know the actual condition of the far endpoint automatically.
A GRE tunnel interface may appear up even when the remote side is not reachable. This can create confusion during troubleshooting because the tunnel may look active but still fail to pass traffic.
GRE keepalives help solve this problem by checking whether the remote tunnel endpoint is still reachable. This is useful in production networks because engineers need to know whether the tunnel is really working or only showing as active.
So, GRE is simple, but it still needs proper monitoring and verification.
Benefits of Generic Routing Encapsulation
Some of the benefits of GRE are:
- GRE is useful because it is simple and flexible, and it can carry different types of traffic over an IP network to create logical connectivity between remote networks.
- It supports routing protocols over tunnels and can also carry multicast traffic, which helps remote sites communicate through a virtual path.
- GRE can be paired with IPsec when the tunnel needs encryption and security protection.
- It is useful when one protocol must cross a network that does not support it directly, such as carrying IPv6 traffic over an IPv4-only network.
- GRE works well in enterprise setups where two networks need to behave like they are directly connected even when separated by another IP network.
- Another advantage is that GRE is not limited to one traffic type, since it can encapsulate multiple network layer protocols instead of relying only on basic IP forwarding.
Limitations of Generic Routing Encapsulation
GRE has limitations that should be understood before using it in a network design. Some of these are:
- The biggest limitation is that GRE does not provide encryption. It only encapsulates packets, so security must be added separately when sensitive data is involved.
- GRE adds overhead to packets, which can create MTU and fragmentation issues. If MTU is not handled properly, the tunnel may work, but performance can suffer.
- Plain GRE is usually point-to-point, so managing many tunnels becomes harder at scale. In such cases, mGRE or DMVPN may be a better option.
- GRE tunnels depend on proper routing and underlay connectivity. If the tunnel destination is not reachable, the GRE tunnel will not work correctly.
- GRE may also need keepalives or monitoring to detect tunnel failures, so it should not be deployed without planning for operational visibility.
Frequently Asked Questions
Q1. What is GRE in networking?
GRE stands for Generic Routing Encapsulation. It is a tunneling protocol that wraps one type of network packet inside another packet so it can travel across an IP network.
Q2. Why is GRE used?
GRE is used to create logical tunnels between networks. It can carry routing protocols, multicast traffic, IPv6 over IPv4, and other traffic types across an IP path.
Q3. Does GRE encrypt data?
No, GRE does not encrypt data. It only encapsulates packets. If encryption is required, GRE is usually combined with IPsec.
Q4. What is GRE tunneling?
GRE tunneling is the process of wrapping an original packet inside a GRE packet and then sending it through an outer IP packet between two tunnel endpoints.
Q5. What is GRE over IPsec?
GRE over IPsec combines GRE tunneling with IPsec security. GRE provides flexible encapsulation, while IPsec provides encryption, integrity, and authentication.
Q6. Is GRE still used today?
Yes, GRE is still used in networking, especially for routing over tunnels, multicast transport, IPv6 over IPv4, GRE over IPsec, and DMVPN based designs.
Conclusion
In this blog post, we have learned what a Generic Routing Encapsulation protocol is, how it works, and why it is useful for network communication. We have also looked at the GRE protocol’s history, benefits, drawbacks, and configuration.
It is essential to have a thorough understanding of GRE for CCNA Training and certification exam.
