In the configuration example of the previous section the IP address 10.0.0.1 is the address of the GRE tunnel endpoint from the perspective of payload IP packets. This address belongs to the address space of the VPRN 1 service and is not exposed to the public IP network carrying the GRE encapsulated packets. An IP interface associated with a private tunnel SAP does not support unnumbered operation.
It is possible to configure the IP MTU (M) of a private tunnel SAP interface. This sets the maximum payload IP packet size (including IP header) that can be sent into the tunnel – for example, it applies to the packet size before the tunnel encapsulation is added. When a payload IPv4 packet that needs to be forwarded into the tunnel is larger than M bytes the payload packet is IP fragmented (before tunnel encapsulation) if the DF bit is clear, otherwise the packet is discarded. When a payload IPv6 packet that needs to be forwarded into the tunnel is larger than M bytes the packet is discarded if its size is less than 1280 bytes otherwise it is forwarded and encapsulated intact.