DMVPN Tunnels - Generic Routing Encapsulation (GRE) Tunnels

4 important questions on DMVPN Tunnels - Generic Routing Encapsulation (GRE) Tunnels

What is Generic Routing Encapsulation (GRE)?

It is a protocol providing connectivity to a wide variety of network layer protocols by encapsulating and forwarding those packets over an IP-based network.

What is important to note about GRE and mGRE tunnels and security?

They do not use encryption by default and require additional configuration for this.

What are the steps to configure a GRE tunnel?

  1. Create tunnel interface
    interface tunnel source tunnel-number
  2. Identify local source of tunnel
    tunnel source ip address | interface ID
  3. Identify tunnel destination
    tunnel destination ip address
  4. Set ip address on Tunnel interface
    ip address ip-address subnet-mask
  5. Optionally, define the tunnel bandwidt
    bandwidth 1-10000000
  6. Optionally, specify GRE tunnel keepalive
    keepalive seconds retries
  7. Optionally, define MTU
    ip mtu mtu
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How much encapsulation overhead is added to the packet size with GRE?

  • A minimum of 24 bytes for GRE without IPsec
  • A maximum of 77 bytes for GRE with AES and SHA-1

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