JNCIS-SP: IP tunneling.

Communications channel between two different networks.

- have traffic traverse the Internet.
- make link redundant.
- hide traffic from firewall.

GRE: Generic Routing Encapsulation.

A GRE header and an outer IP header are added to the packet, adding 24-bytes of overhead. Inner packet is not modified except TTL-1.
GRE uses IP protocol 47 (RFC 1702).


Encapsulates an IP-packet in an IP-packet, adding 20-bytes of overhead. Only IP traffic is allowed (RFC 2003).

Routing requirements for tunnels;

1. tunnel endpoints must have a valid route to the remote endpoint.
2. intermediary devices must have a route towards tunnel endpoint.

By default, tunnels are stateless. Use a keepalive or BFP.
It is recommended to install static routes on tunnel endpoints towards tunnel endpoints to avoid problems with recursive routing.