Characteristic of Gre Tunnel

[libra_ali786]

Characteristic of Gre .Please correct.

Which option fall in Overlay Network and Underlay Network.

1. MTU must be increased to avoid fragmentation.
2. De-encapsulates the tunnel header before routing.

[Seb Rupik · Accepted solution]

1. MTU must be increased to avoid fragmentation.  --> Underlay Network
2. De-encapsulates the tunnel header before routing. --> Overlay Network

[omz]

Hi Seb 

Can you please explain or re-direct to a link :) ?

Thank you 

[Seb Rupik]

The underlay network transports the encapsulated GRE packets. The act of encapsulation increases the header size of the packet  (specifically 4 bytes GRE header and 20 byte IP header) therefore this GRE packet size will exceed the default Ethernet interface MTU of 1500 bytes. You can therefore set the GRE IP MTU to be 24 bytes less than the outgoing interface, or increase the underlay transport MTU size. Failure to make this adjustment will cause the packet to be fragmented prior to encapsulation. The receiving host will then have the additional task of reassembling these fragments on receipt.

The overlay network is the GRE tunnel itself. When a GRE packet reaches an endpoint it is de-encapsulated and then a routing decision is made on the 'inner' IP header.

cheers,

Seb.

[Giuseppe Larosa]

Hello libra_ali786,

the GRE tunnel itself is a form of overlay network that builds a logical point to point link between two network devices..

Regarding your question, the underlay network is the network between the two GRE tunnel endpoints.

The GRE header is 24 bytes and adds this overhead to the passenger protocol.

So I would say the following:

1) MTU must be increased to avoid fragmentation.  -> This is underlay network, in order to transport an IP packet of 1500 bytes and the GRE tunnel overhead without fragmentation MTU on physical interfaces of the underlay network has to be increased. I needed to do this in real world in a complex network integration project.

2) De-encapsulates the tunnel header before routing.

This is less clear the GRE Tunnel receiver performs this operation and this can be considered the last step in the overlay network. However, someone could say this is out of overlay network.

Hope to help

Giuseppe