GRE tunnel use-cases:
- Route multicast traffic on non-supported transport(IPSec/Internet)
- Route IPv6 traffic on IPv4 underlay
GRE tunnels:
Main components:
- Passenger protocol: the data which will be encapsulated
- Carrier protocol: GRE(4 bytes)
- Transport Protocol: The routable protocol which will carry the passenger protocol to other side
GRE Packet: (transport protocol+GRE header)(encapsulate(Original Payload + passenger protocol)
Configuration:
See below scenario:
R1:
interface Tunnel100
tunnel mode gre
ip address 102.1.1.1 255.255.255.0
ipv6 enable
tunnel source Loopback0
tunnel destination 2.2.2.2
R2:
interface Tunnel100
tunnel mode gre
ip address 102.1.1.2 255.255.255.0
ipv6 enable
tunnel source Loopback0
tunnel destination 1.1.1.1
Please note that 1.1.1.1 and 2.2.2.2 should be reachable from each other router.
A ping between 2 PCs:
Recursive Routing:
Recursive routing could happen when the tunnel destination is routed and learned through the tunnel itself, resulting in recursive routing.