Get started

Before you can begin using Magic Transit, be sure to complete the onboarding steps below. Cloudflare can significantly accelerate this timeline during active-attack scenarios.

​​1. Scope your configuration

The onboarding process begins with an initial kickoff call where Cloudflare engages with your organization to confirm the scope and timeline for setting up Magic Transit.

After your call with Cloudflare, complete the prerequisites step.

2. Prerequisites

Before you can begin using Magic Transit, verify that you meet Cloudflare’s onboarding requirements.

Verify router compatibility

Magic Transit relies on anycast tunnels to transmit packets from Cloudflare’s global network to your origin network.

The routers at your tunnel endpoints must meet the following requirements to ensure compatibility with Magic Transit.

• Support anycast tunneling.
• Allow configuration of at least one tunnel per Internet service provider (ISP).
• Support maximum segment size (MSS) clamping.

Draft Letter of Agency

Draft a Letter of Agency (LOA) - sometimes referred to as a Letter of Authorization - that identifies the prefixes you want to advertise and gives Cloudflare permission to announce them. The LOA is required by Cloudflare’s transit providers so they can accept the routes Cloudflare advertises on your behalf. See this LOA template for an example.

If you are an Internet service provider (ISP) and advertising prefixes on behalf of a customer, an LOA is required for the ISP and for the customer.

If you are using a Cloudflare IP address, you do not need to submit an LOA.

Note

The LOA must be a PDF. Transit providers may reject the LOA if it is a JPG or PNG.

Verify IRR entries

Verify your Internet Routing Registry (IRR) entries match corresponding origin autonomous system numbers (ASNs) to ensure Magic Transit routes traffic to the correct autonomous systems (AS). For guidance, refer to Verify IRR entries.

If you are using a Cloudflare IP, you do not need to verify your IRR entries.

Optional: RPKI check for prefix validation

You can also use the Resource Public Key Infrastructure (RPKI) as an additional option to validate your prefixes. RPKI is a security framework method ↗ that associates a route with an autonomous system. It uses cryptography to validate the information before being passed onto the routers.

To check your prefixes, you can use Cloudflare’s RPKI Portal ↗.

Set maximum segment size

sequenceDiagram
accTitle: Magic WAN
accDescr: Maximum segment size
participant A as Client machine
participant B as Cloudflare Magic WAN/Transit
participant C as Origin router
A->>B: MSS = 1460 bytes <br> Protocol (20 bytes) <br> IP header (20 bytes)
Note left of A: SYN
B->>C: MSS = 1436 bytes <br> Protocol (20 bytes) <br> IP header (20 bytes) <br> GRE header (4 bytes) <br> IP header (20 bytes)
C->>A: MSS = 1436 bytes <br> IP <br> Protocol
Note right of C: SYN-ACK
A->>B: MSS = 1436 bytes <br> Protocol <br> IP
Note left of A: ACK
B->>C: Protocol <br> IP <br> GRE <br> IP

The SYN-ACK packet sent to the client during TCP handshake encodes the value for maximum segment size (MSS). Egress packets are routed via your ISP interface, and each packet must comply with the standard Internet routable maximum transmission unit (MTU), which is 1500 bytes.

Cloudflare Magic Transit uses tunnels to deliver packets from our global network to your data centers. Cloudflare encapsulates these packets adding new headers.

To accommodate the additional header data, you must set the MSS value to 1436 bytes at your physical egress interfaces — not the tunnel interfaces. For Magic Transit egress traffic, the MSS should be set via the tunnel’s interface for egress traffic.

Important

Refer to your device documentation to check if it sets IPsec MSS clamping automatically. If that is not the case and you are using IPsec inside GRE, you have to set MSS clamp manually.

Set MSS clamp at the IPsec tunnel interface and subtract 24 bytes from your current MSS value, which may be 1360 bytes or lower. This is because the physical interface will see IPsec-encrypted packets, not TCP packets, and MSS clamping will not apply to those.

Standard Internet Routable MTU	1500 bytes
- Original IP header	20 bytes
- Original protocol header (TCP)	20 bytes
- New IP header	20 bytes
- New protocol header (GRE)	4 bytes
= Maximum segment size (MSS)	1436 bytes

Unless you apply these MSS settings at the origin, client machines do not know that they must use an MSS of 1436 bytes when sending packets to your origin.

Clear Don’t Fragment (DF)

If you are unable to set the MSS on your physical interfaces to a value lower than 1500 bytes, you can choose to clear the don't-fragment bit in the IP header. When this option is enabled, Cloudflare fragments packets greater than 1500 bytes, and the packets are reassembled on your infrastructure after decapsulation. In most environments, enabling this option does not have significant impact on traffic throughput.

To enable this option for your network, contact your account team.

Follow router vendor guidelines

Instructions to adjust MSS by applying MSS clamps vary depending on the vendor of your router.

The following table lists several commonly used router vendors with links to MSS clamping instructions:

Router device	URL
Cisco	TCP IP Adjust MSS ↗
Juniper	TCP MSS – Edit System ↗

3. Configure tunnels

Configure the tunnels on both the Cloudflare side and your router side to connect to your origin infrastructure.

4. Configure static routes

Configure static routes to route traffic from Cloudflare’s global network to your locations.

5. Run pre-flight checks

After setting up your tunnels and static routes, Cloudflare validates tunnel connectivity, tunnel and endpoint health checks, Letter of Agency (LOA), Internet Routing Registry (IRR), and maximum segment size (MSS) configurations. Configurations for Cloudflare global network are applied and take around one day to rollout.

6. Advertise prefixes

Once pre-flight checks are completed, Cloudflare will unlock your prefixes for you to advertise via the dashboard, API or BGP at a time of your choosing. Refer to Dynamic advertisement best practices to learn more about advertising prefixes.

If you are using a Cloudflare IP, you do not need to advertise your prefixes.

Important

You must put the appropriate MSS clamps in place before routing changes are made. Failure to apply an MSS clamp can result in dropped packets and hard-to-debug connectivity issues.

Also, when using Cloudflare Network Interconnect with Magic Transit you must set the following MSS clamp sizes to accommodate additional overhead:

• GRE tunnels over Classic CNI: 1476 bytes
• Express CNI / Classic CNI with a maximum transmission unit (MTU) size of 1500 bytes handoff does not require an MSS clamp.

MSS clamps are used to backhaul data from the data center where traffic is ingested (close to the end user) to the facility with the CNI link.