GRE over IPsec

Point-to-Point Generic Routing Encapsulation over IP Security

 

gre network

 

Generic Routing Encapsulation

Generic Routing Encapsulation (GRE) is a widely used encapsulation protocol in computer networking. It allows the transmission of diverse network protocols over an IP network infrastructure. In this blog post, we’ll delve into the details of the GRE and its significance in modern networking.

GRE acts as a tunneling protocol, encapsulating packets from one network protocol within another. By creating a virtual point-to-point link, it facilitates the transmission of data across different network domains. This enables the interconnection of disparate networks, making GRE a crucial tool for securely building virtual private networks (VPNs) and connecting remote sites.

 

  • GRE Operation

GRE is a layer 3 protocol, meaning it works at the IP level of the network. It enables a router to encapsulate packets of a particular protocol and send them to another router, decapsulated and forwarded to their destination. This is useful for tunneling, where data must traverse multiple networks and different types of hardware.

GRE encapsulates data in a header containing information about the source, destination, and other routing information. The GRE header is then encapsulated in an IP header containing the source and destination IP addresses. When the packet reaches the destination router, the GRE header is stripped off, and the data is sent to its destination.

  • IPsec site-to-site

An IPsec site-to-site VPN, also known as a gateway-to-gateway VPN, is a secure tunnel established between two or more remote networks over the internet. It enables organizations to connect their geographically dispersed offices, data centers, or even cloud networks, creating a unified and secure network infrastructure. By leveraging IPsec, organizations can establish secure communication channels, ensuring confidentiality, integrity, and authentication of transmitted data.

 

 

Before you proceed, you may find the following posts helpful for pre-information:

  1. Dead Peer Detection
  2. IPsec Fault Tolerance
  3. Dynamic Workload Scaling 
  4. Cisco Switch Virtualization
  5. WAN Virtualization
  6. VPNOverview

 

GRE Network

Key Generic Routing Encapsulation Discussion Points:


  • Introduction to Generic Routing Encapsulation and what is involved.

  • Highlighting the details of a GRE network with Head-end architecture

  • .Critical points on the GRE over IPsec

  • Technical details on branch design guides.

 

  • A key point: Video on multi-cloud. An IPsec use case.

The following video you may find helpful is on the multi-cloud. Connecting to the cloud and in multi-cloud scenarios may call for protection with IPsec protection. What differentiates the hybrid cloud from the public & private cloud is the data flow between public and private resources. And Multi-Cloud is a particular case of hybrid cloud computing. One connection method to a multi-cloud could be IPsec tunnels.

 

Technology Brief : Cloud Computing - Introducing Multicloud
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  • A key point: Lab guide on IPsec site to site

In this lesson, we have two Cisco IOS routers to use IPSec in Tunnel mode. This means the original IP packet will be encapsulated in a new IP packet and encrypted before sending it out of the network. For this demonstration, I will be using the following three routers.

R1 and R3 each have a loopback interface behind them with a subnet. We’ll configure the IPsec tunnel between these routers to encrypt traffic from 1.1.1.1/32 to 3.3.3.3/32. R2 is just a router in the middle, so R1 and R3 are not directly connected.

Notice with information 1 we can’t ping the remote LAN. However, once the IPsec tunnel is up, we have reachability. Under the security associations, with have 4 packets that have been encapsulated and encapsulated. However, I sent 5 pings. The first packet is lost to ARP.

ipsec tunnel
Diagram: IPsec Tunnel

 

IPsec relies on encryption and tunneling protocols to establish a secure connection between networks. The two primary components of IPsec are the IPsec tunnel mode and the IPsec transport mode. In tunnel mode, the entire IP packet is encapsulated within another IP packet, adding an extra layer of security. In contrast, the transport mode only encrypts the payload of the IP packet, leaving the original IP header intact.

To initiate a site-to-site VPN connection, the IPsec VPN gateway at each site performs a series of steps. These include negotiating the security parameters, authenticating the participating devices, and establishing a secure tunnel using encryption algorithms such as AES (Advanced Encryption Standard) or 3DES (Triple Data Encryption Standard). Once the tunnel is established, all data transmitted between the sites is encrypted, safeguarding it from unauthorized.

 

Back to basic with GRE tunnels

What is a GRE tunnel?

A GRE tunnel supplies connectivity to a wide variety of network layer protocols. GRE works by encapsulating and forwarding those packets over an IP-based network. The authentic use of GRE tunnels provided a transport mechanism for non-routable legacy protocols such as DECnet and IPX. With GRE, we add header information to a packet when the router encapsulates the packet for transit on the GRE tunnel.

The new header information contains the remote endpoint IP address as the destination. The new IP headers permit the packet to be routed between the two tunnel endpoints, and this is done without inspection of the packet’s payload.

After the packet reaches the remote endpoint, the GRE termination point, the GRE headers are removed, and the original packet is forwarded from the remote router. Both GRE and IPsec tunnels are used in solutions for SD WAN SASE and SD WAN Security. Both of these solutions would abstract the complexity of configuring these technologies.

 

GRE Operation

GRE operates by encapsulating the original packet with a GRE header. This header contains information such as the source and destination IP addresses and additional fields for protocol identification and fragmentation support. Once the packet is encapsulated, it can be transmitted over an IP network, effectively hiding the underlying network details.

When a GRE packet reaches its destination, the receiving end decapsulates it, extracting the original payload. This process allows the recipient to receive the data as if it were sent directly over the underlying network protocol. GRE is a transparent transport mechanism, enabling seamless communication between disparate networks.

 

GRE Tunnel
Diagram: GRE tunnel example. Source is heficed

 

  • Topologies and routing protocol support

Numerous technologies connect remote branch sites to HQ or central hub. P2P Generic Routing Encapsulation ( GRE network ) over IPsec is an alternative design to classic WAN technologies like ATM, Frame Relay, and Leased lines. GRE over IPsec is a standard deployment model to connect several remote branch sites to one or more central sites. Design topologies include the hub-and-spoke, partial mesh, and full mesh.

Both partial and full-mesh topologies experience limitations in routing protocol support. A full mesh design is limited by the overhead required to support a design with a full mesh of tunnels. Following a complete mesh requirement, a popular design option would be to deploy DMVPN. Regarding the context of direct connectivity from branch to hub only, hub-and-spoke is by far the most common design.

 

  • A key point: Lab guide with DMVPN and GRE

The lab guide below shows a DMVPN network based on Generic Routing Encapsulation (GRE). So, GRE is the overlay. Specifically, we use GRE in point-to-point mode, which means deploying DMVPN Phase 1, a true VPN hub and spoke design, where all traffic from the spokes must go via the hub. With the command: show dmvpn, we can see that two spokes are dynamically registered over the GRE tunnel, notice the “D” attribute.

The beauty of using DMPVN as a VPN technology is that the hub site does not need a specific spoke configuration as it uses GRE in multi-point mode. On the other hand, the spokes need to have a hub configuration with the command: IP nhrp nhs 192.168.100.11. IPsec encryption is optional with DMVPN. In the other command snippet, we are running IPsec encryption with the command: tunnel protection ipsec profile DMVPN_IPSEC_PROFILE.

 

DMVPN configuration
Diagram: DMVPN Configuration.

One of the primary use cases of GRE is in creating VPNs. Organizations can securely transmit data across public networks such as the internet by encapsulating traffic within GRE packets. This provides a cost-effective solution for connecting geographically dispersed sites without requiring dedicated leased lines.

Another use of GRE is in network virtualization. By leveraging GRE tunnels, it is possible to create virtual networks isolated from the underlying physical infrastructure. This allows for more efficient resource utilization and improved network scalability.

 

  • DMVPN (Dynamic Multipoint VPN)

DMVPN is based on the principle of dynamic spoke-to-spoke tunneling, allowing for dynamic routing and scalability. It also provides the ability to create a dynamic mesh topology, allowing for multiple paths between remote sites. This allows for increased redundancy and improved performance.

DMVPN also offers the ability to establish a secure tunnel over an untrusted network, such as the Internet. This is achieved with a series of DMVPN phases. DMVPN phase 3 offers better flexibility by using IPSec encryption and authentication, ensuring that all traffic sent over the tunnel is secure. This makes DMVPN an excellent choice for businesses connecting multiple sites over an unsecured network.

Dynamic Multipoint VPN
Diagram: Example with DMVPN. Source is Cisco

 

  • A key point: Video on DMVPN configuration

In this technical demonstration, we will start with the core block of DMVPN, GRE. Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint links over an Internet Protocol network.

 

Cisco DMVPN Configuration
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GRE Network: Head-end Architecture 

Single-tier and dual tier

Head-end architectures include a single-tier head-end where the point-to-point GRE network and crypto functionality co-exist on the same device. Dual-tier designs are where the point-to-point GRE network and crypto functionality are not implemented on the same device. Dual tier designs, the routing and GRE control planes are located on one device while the IPsec control plane is housed on another.

what is generic routing encapsulation
Diagram: What is generic routing encapsulation?

 

Headend

Router

Crypto

Crypto IP

GRE  

GRE IP

Tunnel Protection

Single Tier

Headend

Static or Dynamic

Static

p2p GRE static

Static

Optional 


Branch

Static

Static or Dynamic

p2p GRE static

Static

Optional 

Dual Tier

Headend

Static or Dynamic

Static

p2p GRE static

Static

Not Valid


Branch

Static

Static or Dynamic

p2p GRE static

Static

Not Valid

 

“Tunnel protection” requires the same source and destination IP address for the GRE and crypto tunnels. Implementations of dual-tier separate these functions, resulting in the different IP addresses for the GRE and crypto tunnels. Tunnel protection is invalid with dual-tier mode.

 

GRE over IPsec

GRE (Generic Routing Encapsulation) is a tunneling protocol that encapsulates multiple protocols within IP packets, allowing the transmission of diverse network protocols over an IP network. On the other hand, IPSEC (IP Security) is a suite of protocols that provides secure communication over IP networks by encrypting and authenticating IP packets. Combining these two protocols, GRE over IPSEC provides a secure and flexible solution for transmitting network traffic over public networks.

Benefits of GRE over IPSEC:

Secure Data Transmission:

By leveraging IPSEC’s encryption and authentication capabilities, GRE over IPSEC ensures the confidentiality and integrity of data transmitted over the network. This is particularly crucial when transmitting sensitive information, such as financial data or personal records.

Network Scalability:

GRE over IPSEC allows organizations to create virtual private networks (VPNs) by establishing secure tunnels between remote sites. This enables seamless communication between geographically dispersed networks, enhancing collaboration and productivity.

Protocol Flexibility:

GRE over IPSEC supports encapsulating various network protocols, including IPv4, IPv6, and multicast traffic. This flexibility enables the transmission of diverse data types, ensuring compatibility across different network environments.

 

Preliminary design considerations

Diverse multi-protocol traffic requirements force the use of a Generic Routing Encapsulation ( GRE ) envelope within the IPsec tunnel. The p2p GRE tunnel is encrypted inside the IPsec crypto tunnel. Native IPsec is not multi-protocol and lacks IP multicast or broadcast traffic support. As a result, proper propagation of routing protocol control packets cannot occur in a native IPsec tunnel.

However, OSPF design cases allow you to run OSPF network type non-broadcast and explicitly configure the remote OSPF neighbors, resulting in OSPF over the IPsec tunnel without GRE. With a GRE over IPsec design, all traffic between hub and branch sites is first encapsulated in the p2p GRE packet before encryption.

GRE over IPsec
Diagram: GRE over IPSec.

 

GRE over IPSec Key Points

Redundancy

Redundant designs are implemented with the branch having two or more tunnels to the campus head. The head-end routers can be geographically separated or co-located. Routing protocols are used with redundant tunnels providing high availability with dynamic path selection.

The head-end router can propagate a summary route ( 10.0.0.0/8 ) or a default route ( 0.0.0.0/0 ) to the branch sites, and a preferred routing metric will be used for the primary path. If OSPF is RP, the head-end selection is based on OSPF costs.

 

Recursive Routing

Each branch must add a static route to their respective ISP IP addresses for each head-end. The static avoids recursive routing through the p2p GRE tunnel. Recursive routing occurs when the route to the GRE tunnel source outside the IP address of the opposing router is learned via a route with a next-hop of the inside IP address of the opposing p2p GRE tunnel. Recursive routing causes the tunnel to flap and the p2p GRE packets to route into their p2p GRE tunnel. To overcome recursive routing, my best practice is to ensure that the outside tunnel is routed directly to ISP instead of inside the p2p GRE tunnel.

 

%TUN-5-RECURDOWN: Tunnel0 temporarily disabled due to recursive routing

 

Recursive routing and outbound interface selection pose significant challenges in tunnel or overlay networks. Therefore, routing protocols should be used with utmost caution over network tunnels. A router can encounter problems if it attempts to reach the remote router’s encapsulating interface (transport IP address) via the tunnel. Typically, this issue occurs when the transport network is advertised into the same routing protocol as the overlay network.

Routers learn the destination IP address for tunnel interfaces through recursive routing. First, the IP address of the tunnel’s destination is removed from the routing table, making it unreachable.

 

Split tunneling

If the head-end advertises a summary route to the branch, split tunneling is enabled on all packets not destined for the summary. Any packets not destined for the summary are split-tunneled to the Internet. For example, split tunneling is not used for the branch sites in a design where the head-end router advertises a default route ( 0.0.0.0/0 ) through the p2p GRE tunnel.

 

  • A key point: Additional information on Split tunneling.

Split tunneling is a networking concept allowing users to selectively route traffic from their local device to a local or remote network. It gives users secure access to corporate networks and other resources from public or untrusted networks. Split tunneling can also be used to reduce network congestion. For example, if a user is on a public network and needs to access a resource on a remote network, the user can set up a split tunnel to send only the traffic that needs to go over the remote network. This reduces the traffic on the public network, allowing it to perform more efficiently.

 

Control plane

Routing protocol HELLO packets initiated from the branch office force the tunnel to establish—routing protocol control plane packets to maintain and keep the tunnel up. HELLO, packets provide a similar function to GRE keepalives. Routing protocol HELLO operates at Layer 3, and GRE keepalives at Layer 2.

 

Branch router considerations

The branch router can have p2p GRE over IPSEC with a static or dynamic public address. The GRE and crypto tunnels are sourced from a static address with a static public IP address. With dynamic address allocation, the GRE is sourced from a loopback address privately assigned (non-routable), and the crypto tunnel is sourced from a dynamically assigned public IP address.

 

Generic routing encapsulation

Key design points

Summary of key design points

  • Deploy IPsec in tunnel mode for flexibility with NAT. The default Tunnel mode adds 20 bytes to the total packet size. However, the transport mode does not work when the crypto tunnel transitions to either a NAT or PAT device.

  • With GRE IPsec tunnel mode, the entire GRE packet ( which includes the original IP header packet ) is encapsulated and encrypted. No part of the GRE tunnel is exposed.

  • Use routing protocols for dynamic redundancy but scaling routing protocols can affect CPU overhead. Keep in mind a higher-end limit to the number of routing protocol peers.

  • Implement GRE for routing protocol support. For example, GRE requires that additional headers are added to the beginning of each packet; the header must also be encrypted, affecting the router's performance.

  • GRE is stateless and, by default, offers limited flow control mechanisms.

  • Configure redundant tunnels for high availability but with scalability limitations. For example, P2P GRE is limited to 2047 tunnels with VPN SPA and 2000 tunnels with SUP720.

  • Implement Triple DES ( 3DES ) or AES for encryption of transmitted data. Even considering AES has a wider key length, there is little or no variation in the performance of these.

  • Implement Dead Peer Detection ( DPD ) to detect loss of ISAKMP peer. Alternatively, IPSEC tunnel protection can be considered for single-tier head architecture.

  • Hardware-accelerated encryption is useful to protect the router's CPU from intensive processing.

  • Set the local Maximum Transmission Unity ( MTU ) and Path MTU to minimize fragmentation.

  • For a small number of sites, one can use pre-shared keys for tunnel authentication. However, large deployments look into Digital Certificates / PKI.

  • Keep CPU at head-end and branch sites around the 65% - 80% mark.

 

generic routing encapsulation