Cisco GET VPN vs DMVPN: Key Differences

Cisco GET VPN and DMVPN are both overlay tunneling technologies, but they differ fundamentally: GET VPN encrypts traffic directly over the public network without a tunnel interface, while DMVPN builds dynamic IPsec tunnels on demand. This article explains the architectural, operational, and use-case differences, with a comparison table and guidance on choosing between them.

Background: Where Each Technology Fits

Cisco introduced DMVPN (Dynamic Multipoint VPN) in the early 2000s to solve the scalability problem of traditional site-to-site IPsec VPNs, which required a full mesh of static tunnels. DMVPN uses a hub-and-spoke model where spokes register with a hub and can dynamically build direct tunnels to other spokes, reducing latency and hub load. It is widely deployed for branch office connectivity, especially when sites need to communicate directly with each other.

GET VPN (Group Encrypted Transport VPN) emerged later, around 2004, to address a different problem: enterprises that already had a private MPLS or IP backbone but wanted to add encryption without redesigning their routing. GET VPN does not create tunnels; instead, it encrypts packets in transit using a group key shared among all members, while preserving the original IP headers. This allows the underlying network to route traffic normally, which is ideal for multicast and for networks where a full mesh of tunnels would be impractical.

How DMVPN Works

DMVPN combines three technologies: mGRE (multipoint GRE) for the tunnel interface, NHRP (Next Hop Resolution Protocol) for dynamic peer discovery, and IPsec for encryption. The hub has a static mGRE tunnel interface, and each spoke has a tunnel interface with a virtual IP. When a spoke wants to send traffic to another spoke, it queries the hub via NHRP to learn the public IP address of the destination spoke, then builds a direct IPsec tunnel to that spoke. This on-demand tunnel creation is the key feature.

Key characteristics of DMVPN:

• Hub-and-spoke or spoke-to-spoke topologies
• Dynamic tunnel establishment, no need for static peer configuration
• Supports multicast and broadcast traffic over the mGRE interface
• Requires a hub router that is always reachable
• Scales well for hundreds or thousands of spokes

How GET VPN Works

GET VPN uses a controller (the Group Domain of Interpretation, or GDOI) to distribute a common encryption key to all group members. Each member encrypts packets with this group key, but the outer IP header remains unchanged. The encrypted packet is then routed by the underlying network exactly as if it were unencrypted. This is often called “tunnel-less� VPN because there is no tunnel interface, no encapsulation, and no peer-to-peer IPsec sessions.

Key characteristics of GET VPN:

• No tunnel interface; packets are encrypted in place
• Preserves original IP headers, so routing and multicast work normally
• Uses a group key shared by all members, managed by a key server
• Requires a trusted underlying network (e.g., MPLS) because it does not hide internal IP addresses
• Ideal for networks that already have full IP connectivity, such as a private WAN

Comparison Table: GET VPN vs DMVPN

Feature	GET VPN	DMVPN
Tunnel interface	None (tunnel-less)	mGRE tunnel interface
Encryption model	Group key (GDOI)	Pairwise IPsec sessions
Packet header	Original IP header preserved	New GRE/IPsec header added
Routing	Unaffected; normal routing	Runs over tunnel; may need dynamic routing over mGRE
Multicast	Native support	Supported via mGRE, but can be complex
Spoke-to-spoke traffic	Direct, no hub required	Direct after NHRP resolution
Underlying network	Must be IP-routable and trusted	Any IP network, including Internet
Scalability	Very high, no tunnel state per peer	High, but hub can become bottleneck
Typical use	MPLS WAN encryption	Internet-based branch VPN

How to Choose Between GET VPN and DMVPN

The choice depends largely on your existing network and traffic patterns. If you already have a private MPLS or IP backbone where all sites can reach each other directly, GET VPN is often the better fit because it adds encryption without changing routing or requiring a redesign. It also handles multicast and broadcast traffic naturally, which is critical for applications like video conferencing or IPTV.

If you are connecting branch offices over the public Internet, DMVPN is the standard choice. It does not require a fully routable private network; spokes can be behind NAT and still establish tunnels. DMVPN also provides spoke-to-spoke direct tunnels, which reduces latency and hub load, but it adds encapsulation overhead and requires careful routing configuration.

Consider these factors:

• Do you need to encrypt traffic on an existing MPLS network? → GET VPN
• Are you building a VPN over the Internet? → DMVPN
• Do you have multicast traffic that must be encrypted? → GET VPN (simpler)
• Do you need to hide internal IP addresses from the WAN? → DMVPN (GET VPN does not hide them)
• Do you have a small number of sites? → Either works, but DMVPN is simpler to set up without a key server

Common Problems and Misconceptions

A common misconception is that GET VPN is “more secure� than DMVPN because it uses a group key. In reality, the security depends on key management and encryption algorithms, not the tunneling method. GET VPN’s group key is shared, so if one member is compromised, the attacker can decrypt traffic for the whole group. DMVPN uses unique pairwise keys, so a compromise affects only that one tunnel. For this reason, GET VPN is often used in trusted MPLS environments, not for Internet VPNs.

Another issue is that GET VPN does not hide internal IP addresses. Since the original IP header is preserved, an attacker on the underlying network can see the internal topology. This is acceptable on a private MPLS, but not on the public Internet. DMVPN encapsulates the original packet, hiding internal addresses.

Configuration mistakes are common with both. With DMVPN, a frequent error is misconfiguring NHRP network IDs or not allowing IPsec traffic through the firewall. With GET VPN, the key server’s ACL must correctly identify which traffic to encrypt; if it is too broad, you may encrypt routing protocols and break the network.

Costs and Operational Considerations

Both technologies are software features on Cisco routers; there is no additional license cost for the features themselves, but you need hardware that supports the encryption throughput. GET VPN may require more CPU because it encrypts every packet individually, but it does not have the overhead of GRE encapsulation. DMVPN adds GRE and IPsec headers, increasing packet size and potentially requiring MTU adjustments.

Operationally, DMVPN is more complex to troubleshoot because of the dynamic tunnel setup. GET VPN is simpler in that there are no tunnels to monitor, but you must manage a key server and rekeying. Both require careful planning of IP addressing and routing.

Frequently Asked Questions

Can GET VPN and DMVPN be used together?

Yes, they can be combined. For example, you might use DMVPN for Internet-based branch connectivity and GET VPN for encrypting traffic over an MPLS backbone. Some designs use DMVPN as a backup for GET VPN when the MPLS fails and traffic falls back to the Internet.

Which one has lower latency?

GET VPN generally has lower latency because it does not add encapsulation overhead and packets take the native routing path. DMVPN adds a GRE header and may route through the hub for initial traffic, though spoke-to-spoke tunnels reduce latency after establishment.

Does GET VPN support dynamic routing protocols?

Yes, GET VPN encrypts routing protocol packets like OSPF or EIGRP without changing their behavior. This is a major advantage over traditional IPsec VPNs, which often require special handling for multicast and broadcast. DMVPN also supports dynamic routing over the mGRE tunnel, but it requires careful configuration to avoid routing loops.

What is the maximum number of sites supported?

Both scale to thousands of sites, but DMVPN may hit a hub bottleneck if all spokes need to communicate through the hub (though spoke-to-spoke tunnels alleviate this). GET VPN scales better because there is no per-peer tunnel state, but the key server must handle rekeying for all members.

Is GET VPN secure enough for the Internet?

Technically, GET VPN can work over the Internet, but it is not recommended because it exposes internal IP addresses and uses a shared key. If a group member is compromised, the attacker can decrypt all group traffic. DMVPN is the safer choice for Internet VPNs.

Related

• Fix Encrypt iPhone Backup Option Greyed Out
• Check If Windows Firewall Is Blocking an App
• How to Use Guest Mode on Android